characterization of underwater sounds produced by

Characterization of underwater sounds produced by hydraulicand mechanical dredging operations

Kevin J. Reinea) and Douglas ClarkeEnvironmental Laboratory, Wetlands and Coastal Ecology Branch, U.S. Army Engineer Researchand Development Center, 3909 Halls Ferry Road, Vicksburg, Mississippi 39180

Charles DickersonBowhead Science and Technology Services, 3503 Manor Drive #4, Vicksburg, Mississippi 39180

(Received 22 January 2013; revised 25 November 2013; accepted 28 April 2014)

Sound recordings were made of two dredging operations at hydrophone depths of 3 and 9.1 m

at distances up to 1.2 km from the source in shallow waters (<15 m) of New York Harbor.

Sound sources included rock fracturing by a hydraulic cutterhead dredge and six distinct sources

associated with a mechanical backhoe dredging operation during rock excavation. To place sound

emitted from these dredges in perspective with other anthropogenic sounds, recordings were also

made of several deep-draft commercial vessels. Results are presented as sound pressure levels

(SPLs) in one-third octave versus range across the 20 Hz to 20 kHz frequency band. To address

concerns for protection of fishery resource occupying the harbor, SPL were examined at frequency

bands of 50–1000 Hz and 100–400 Hz, the ranges where the majority of fishes without hearing

specializations detect sound and the range of greatest sensitivity, respectively. Source levels (dB re

1 lPa-1 m rms) were back calculated using fitted regression (15LogR). The strongest sound sources

(180–188.9 dB) were emitted by commercial shipping. Rock fracturing produced a source level of

175 dB, whereas six distinct sources associated with rock excavation had source levels ranging

from 164.2 to 179.4 dB re 1 lPa-1 m (rms). [http://dx.doi.org/10.1121/1.4875712]

PACS number(s): 43.30.Nb, 43.50.Rq, 43.50.Cb, 43.50.Ed [MS] Pages: 3280–3294

I. INTRODUCTION

In recent years, the potential impact of underwater

sounds associated with dredging operations has come under

increasing scrutiny by regulatory agencies. Underwater

sound has previously been identified as a concern for poten-

tial negative impacts to fishes and other marine organisms

but has primarily been linked to high-intensity sounds from

seismic surveys, pile-driving, or military sonar (Richardson

et al., 1995; Woods et al., 2001; Popper, 2003; Popper et al.,2005; Popper et al., 2006; Nedwell et al., 2006; Popper

et al., 2007; Ruggerone et al., 2008; Popper and Hastings,

2009; Mueller-Blenkle et al., 2010). Woods et al. (2001)

examined fishes that died as a result of exposure to under-

water sounds from pile-driving operations. Mortalities were

observed in several species, attributed primarily to injury to

the swim bladders of fishes within 50 m of the sound source.

Sound pressure levels (SPL) ranged between 160 and 196 dB

re 1 lPa rms. Ruggerone et al. (2008) investigated the effects

of pile-driving sounds on caged yearling coho salmon

(Oncorhynchus kisutch). Although SPLs reached 208 dB re

1 lPa rms, no mortality was reported. Nedwell et al. (2006)

studied the effects of vibro-piling on brown trout (Salmotrutta). While brown trout showed no immediate reaction to

vibro-piling, both altered behavior and physical injury were

seen among individual trout exposed as far as 400 m from

the source. Pile driving can produce sounds that exceed

180 dB. Gas oscillations induced by high sound pressure

levels can cause the swim bladder to tear or rupture (Govoni

et al., 2003; Govoni et al., 2008). Engas et al. (1996)

reported a significant reduction in catch rates of haddock

(Melanogrammus aeglefinus) and Atlantic cod (Gadusmorhua) for up to 5 days after seismic surveys. Skalski et al.(1992) showed a 52% decrease in rockfish (Sebastes spp.)

abundance following a single airgun emission at 186–191 dB

re 1 lPa rms. The authors concluded that rockfish exhibited

a startle response to an SPL of 160 dB re 1 lPa rms, but this

sound level did not appear to affect catch rates. A compre-

hensive review on the effects of high-frequency anthropo-

genic sound on fishes can be found in Popper and Hastings

(2009).

In most cases, however, anthropogenic sounds are rela-

tively low in frequency, typically below 1000 Hz. These

sounds are within the hearing range of fishes, which tend to

prefer narrower bands and show greater sensitivity at lower

frequencies (Popper and Hastings, 2009). Therefore low-

frequency sounds have the potential to affect fishes in terms

of behavior, foraging, and communication. Fishes also create

low-frequency sounds (50–2000 Hz, most often

100–500 Hz), presumably for communication purposes. This

sound can be a significant component of local ambient noise

(Zelick and Mann, 1999). Likewise sounds generated by

whales and dolphins for echolocation and communication

can reach 180–200 dB in the 50- to 200-kHz frequency range

contributing to local ambient conditions (Richardson et al.,1995). Another contributing factor to background levels of

low-frequency underwater sound is sea state. Sea state, influ-

enced by prevailing wind speed, can produce ambient sounds

above 500 Hz (Knudsen et al., 1948).

a)Author to whom correspondence should be addressed. Electronic mail:

[email protected]

3280 J. Acoust. Soc. Am. 135 (6), June 2014 0001-4966/2014/135(6)/3280/15/$30.00

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In industrialized and urbanized ports and harbors, poten-

tially important sources of low-frequency underwater sound

include commercial and recreational vessel traffic and navi-

gation dredging, the latter being the focus of the present

study. Ship traffic can generate sounds that travel consider-

able distances, especially at frequencies ranging from 10 to

1000 Hz. Propulsion systems are a dominant source of radi-

ated underwater sounds at frequencies less than 200 Hz

(Hildebrand, 2009). In the present study SPLs (dB re 1 lPa

rms) were measured for deep-draft commercial vessels and

harbor ferries to place sounds produced by navigation dredg-

ing operations into context. Although the container ships

monitored in the present study are representative of a size

and class of vessel that frequently transit through the harbor

complex on a daily basis, a much wider variety of vessels

occupy New York and New Jersey Harbor. Leach (2011)

reported that 4811 ships entered the Harbor in 2010, carrying

over 32.2� 106 metric tons of cargo, making this harbor the

third busiest port in the United States based on total imports

and exports by weight (American Association of Port

Authorities, 2011). For example, tankers carrying crude oil

are almost a daily presence in the harbor. In addition to a

large number of high speed water taxis, the 310 ft (94 m)

Staten Island Ferries complete 35 000 trips annually servic-

ing St. George Ferry Terminal to the Battery in lower

Manhattan. Additional examples include tugs used to assist

ships in berthing, fuel barges, law enforcement, and U.S.

Coast Guard vessels. Few studies have characterized

low-frequency underwater sounds produced by ship/boat

traffic (Haviland-Howell et al., 2007) or dredging operations

(Central Dredging Association, 2011), and to the degree

such sounds may impact aquatic organisms. Studies by

Amoser et al. (2004) and Vasconcelos et al. (2007) con-

cluded that ship traffic can reduce the effective range of

communication signals and therefore the signaling efficiency

between individual fish. Other potential negative effects of

ship-generated noise include impaired hearing ability, dis-

placement of fishes from their physical habitat, and endo-

crine stress responses (Scholik and Yan, 2002; Mitson and

Knudsen, 2003; Wysocki et al., 2006; Sara et al., 2007). The

degree to which fishes are impacted by commercial and rec-

reational vessel noise is a major concern for several reasons:

(1) Greater than 80% of global freight transport takes place

over water by motorized vessels; (2) millions of people

yearly utilize rivers, lakes, and harbors for recreational activ-

ities involving motorized watercraft; (3) millions of passen-

gers are transported across harbors at regular intervals by

ferries and water taxis; and (4) the global commercial fishing

feet includes approximately 1.2� 106 vessels (Slabbekoorn

et al., 2010).

Another source of low-frequency underwater sounds is

associated with navigation channel maintenance and deepen-

ing projects, which entail dredging and dredged material dis-

posal activities. It should be noted that the dredging fleet

represents only 0.5% of the world’s total shipping fleet

(Central Dredging Association, 2011). In the United States,

the U.S. Army Corps of Engineers (USACE) is responsible

for maintaining the navigability of Federal channels that pro-

vide access to port and harbor facilities. This requires the

excavation and handling of approximately 300� 106 cubic

yards (229.4 m3) of sediment annually while ensuring stew-

ardship of environmental resources in coordination with

Federal and state regulatory agencies. Consequently, the

USACE is engaged in consultations with multiple resource

agencies concerning underwater sounds and their potential

impacts on fishes or species with threatened or endangered

status. A concern cited by the National Oceanographic and

Atmospheric Administration-National Marine Fisheries

Service (NOAA-NMFS) involves potential blockage or

delay in the migration of anadromous fishes such as

American shad (Alosa sapidissima), blueback herring (Alosaaestivalis), and Alewife (Alosa pseudoharengus) through navi-

gable waterways. In particular, American shad stocks, which

are currently experiencing declines, are thought to be poten-

tially sensitive to dredging-induced sounds. Another issue of

concern regards dredging near spawning grounds. On the

Pacific coast, restrictive management practices have been

implemented to avoid or minimize potential impacts on fishes

due to underwater sound related to pile driving and other con-

struction activities, but only recently has the issue of under-

water sound been raised in association with dredging projects.

In the present study, underwater sounds were recorded

and analyzed for a hydraulic cutterhead suction dredge

(CSD) fracturing limestone rock and a mechanical backhoe

dredge (BHD) tasked with excavating fractured rock and

gravel while engaged in the New York/New Jersey Harbor

Deepening Project. Of particular interest was determining

(1) the frequency spectrum of the rock fracturing and exca-

vation process, (2) the received sound pressure levels (SPL)

at various distances from the source, (3) the calculated

source level (SL), and (4) ambient and other anthropogenic

sound sources in the study area. All received sound pressure

levels associated with both types of dredging operations and

commercial shipping in the current study are reported herein

as dB re 1 lPa, root mean square.

A. Dredge types and potential sound sources

Conventional dredges use either mechanical or hydrau-

lic means to excavate sediment. Major categories of dredge

plants include mechanical bucket or backhoe dredges, hy-

draulic cutterhead suction dredges (CSD), and trailing suc-

tion hopper dredges (TSHD), which remove sediment from

waterways in very different manners. Hopper dredges are

self-propelled seagoing vessels that hydraulically remove

sediment from the seafloor through dragheads [Fig. 1(a)].

The dragheads are “trailed” beneath the dredge and held in

contact with the substrate as the dredge advances. Thus

TSHDs are similar to large commercial ships. Much of the

sound produced by this type of dredge is associated with pro-

peller and engine noise in tandem with sounds emitted by

pumps and generators. Relatively muted sounds are pro-

duced by the draghead while in contact with the sediment

bed at least when the dredge is working in fine maintenance

sediments (grain size¼ 3.9–62.5 lm). Hopper dredge sounds

are relatively continuous rather than punctuated (Clarke

et al., 2002). In contrast to the highly mobile TSHDs, CSDs

[Fig. 1(b)] are often perceived to be stationary, as the

J. Acoust. Soc. Am., Vol. 135, No. 6, June 2014 Reine et al.: Characterization of underwater dredge sounds 3281


embedded, rotating cutterhead swings laterally across an arc

in front of the dredge. Consequently the rate of forward

advance, managed either by swiveling between anchor wires

or spuds, is much slower than that of the hopper dredge.

Spuds are heavy walled pipes that slide vertically in wells

located at the rear corners of the dredge hull. The spuds are

lowered into the sediment to anchor the dredge. A second

“type” of spud, called the walking spud, is used to advance the

dredge forward when all material has been removed from its

current digging location. Occasionally the cutterhead is raised

off the bottom to entrain water to flush the system or while the

dredge is repositioned by spud or tender vessel maneuvers.

The system is flushed periodically to clear the pipeline path-

way or to prime pumps. The duration of production “cuts”

depends on a number of factors, including depth of insertion of

the cutterhead, type of sediment being excavated, and width of

the navigation channel. CSD sounds are therefore largely con-

tinuous (Clarke et al., 2002). For hydraulic dredging opera-

tions, the major processes contributing to underwater sounds

include: (1) Dredged material collection sounds originating

from the rotating cutterhead in contact with the bed and intake

of the sediment-water slurry, (2) sounds generated by pumps

and impellers driving the suction of material through the pipes,

(3) transport sounds involving the movement of sediment

through the pipes, and (4) ship and machinery sounds, includ-

ing those associated with the lowering and lifting of spuds and

moving of anchors by dredge tenders. In contrast to hydraulic

dredges, much of the sound produced by mechanical bucket

dredges [Fig. 1(c)] is repetitive rather than continuous. The

processes that comprise sound sources associated with me-

chanical bucket or backhoe (excavator) dredges [Fig. 1(d)] fall

within several categories: Physical removal of sediment from

the substrate as the bucket is inserted into the bed, forced

through the bed in a “scooping” arc, and removed from the

bed produces grinding and scraping sounds. Lifting of the ma-

terial from the bed up through the water column can produce

sounds emanating from winches, hydraulic pumps, and the

articulated bucket support arm. Placing the dredged sediment

into a barge can produce sounds that are transmitted through

the hull of the barge, particularly during the early stages of the

barge-filling process. The duration of individual events with a

typical bucket deployment-and-retrieval cycle may range from

seconds to a few minutes. Each phase of the bucket cycle pro-

duces a repeated set of sounds, which can be identified within

the acoustic record. Onboard machinery will produce various

sounds throughout the dredging process, such as sounds

associated with winches, generators, and the power plant.

Periodically, sounds may be produced when the dredge advan-

ces, either by raising and lowering spuds or by swinging along

deployed anchor cables. The periodic maneuvering and

replacement of barges requires the assistance of tugboats and

tenders, which entails sounds associated with their power

plants.

II. METHODS

A. Study site

New York/New Jersey Harbor lies at the confluence of

three major bodies of water: (1) The New York Bight to the

FIG. 1. (Color online) Schematic overview of dredge types and poten-

tial sources of sound for (a) trailing suction hopper dredge, (b) cutter-

head suction dredge, (c) mechanical bucket dredge, and (d) mechanical

backhoe or excavator dredge. (Photo credit: Philip Spadaro, Intell

Group.)

3282 J. Acoust. Soc. Am., Vol. 135, No. 6, June 2014 Reine et al.: Characterization of underwater dredge sounds


southeast, (2) Long Island Sound to the northeast, and (3)

the Hudson River extending northward. Acoustic monitoring

of both dredging operations occurred in the New York

Harbor Anchorage Channel. The Anchorage Channel is

located in the Upper Bay of New York Harbor, east of the

entrance to the Kill van Kull Waterway and south of the

Global Marine Terminal (Fig. 2).

B. Dredge plants

The CSD Florida and backhoe dredge New York are owned

and operated by the Great Lakes Dredge and Dock Company.

Vessel specifications are summarized in Table I. Both dredges

are atypical in that they represent the largest size class for hy-

draulic CSD and mechanical type dredges. While fracturing

consolidated substrate the Florida used an ESCO 54D cutter

with an 11 ft (3.3 m) diameter, rotating at 26 rpm. The dredge

New York is a BHD equipped with a 25-yd3 (18-m3) bucket.

C. Data acquisition

Sound data were collected using a Sound Technologies

ST1400ENV mobile audio data recorder and Reson TC 4032

hydrophones. The system is a self-contained unit designed

specifically to record underwater sounds while simultaneously

monitoring and logging sound pressure levels (SPL in dB)

and other (e.g., sound exposure levels or SELs) sound level

parameters. Continuous sound recordings were taken at a

sample rate of 48 kHz. The ST1400ENV records digital

WAV format audio files, which can be post-processed to

produce calibrated sound spectra. Sound data were recorded

with MDR_SLM software provided by Sound Technology

Incorporated. The MDR_SLM data collection parameters used in

this study were: (1) Gain¼ 0 dB, (2) filtering¼ off (none), (3)

file sample rate¼ 48 000 Hz, and (4) file bit density¼ 24 bit.

The sound analysis system was powered by a deep-cycle ma-

rine battery, which allowed for the complete shutdown of the

monitoring vessel to a “quiet” mode.

Recordings were made from the M/V Hudson provided

by the USACE New York District. After selecting an appro-

priate recording location, the hydrophone cable was attached

to a lift line, which was attached to a 5-lb (1.87 kg) weight

and lowered into the water. A similar deployment configura-

tion was used by Robinson et al. (2011). Hydrophone depths

were 10 ft (3 m) and 30 ft (9.1 m). Dredging activity was

recorded using a Sony Model HDR-HR550V digital hand-

held video recorder. During recording sessions the survey

vessel was either anchored at a known distance from the

dredge or was allowed to drift freely carried by prevailing

tidal currents. Recordings were made at 14 fixed stations

located 55 m to 1.2 km from the backhoe dredging operation.

During monitoring of the dredge Florida, recordings were

made at 10 fixed stations located 90–700 m from the dredge.

Multiple drift transects were occupied at distances from 89

to 1050 m from the dredge. Sound recordings were also

FIG. 2. (Color online) Study site (filled square, Dredge New York; star,

Dredge Florida; circle with letters, ambient sound monitoring stations; green

circles, fixed recording stations during monitoring of Dredge Florid; red dia-

monds, fixed recording stations during monitoring of Dredge New York;

straight lines, drift transects during monitoring of Dredge Florida).

TABLE I. Summary of vessel specifications and sound results for commercial shipping vessels that frequent New York/New Jersey Harbor.

RLg> ambient

Name Wgt (kt) Lgth (m) Wa (m) Drb (m) Spdc (Kn) O-Pwrd (kW) RLe (dB) Df (m) P5th P50th P95th SLh (dB)

Dredge New York* – 61 17 2.1 n/a 2,565 *See Table III

Dredge Florida* 0.45 159 18 4.3 n/a n/a

Staten Island Ferry 2.9 94 21 4.1 16 7,457 144.5 298 37.3 27.4 30.9 181.3

142.0 352 34.8 24.9 28.4 180.2

NYK Constellation 49.9 294 32 35 23.5 41,129 150.1 123 42.9 33.0 36.5 181.3

Maersk Idaho 46.3 300 32 35 17.7 43,070 147.0 622 39.8 29.9 33.4 188.9

CSAV Licanten 36.2 260 32 12.6 24.5 36,529 141.8 353 34.6 24.7 28.2 180.0

Zim Savannah 50.4 294 32 12.5 13.7 51,485 141.7 321 34.5 24.6 28.1 177.1

aVessel width.bVessel draft.cMaximum vessel speed.dOutput power.eMaximum received SPL in dB re 1 lPa rms.fDistance to sound source received level was obtained.gReceived levels exceeding the 5th, 50th, and 95th ambient percentiles.hSource level in dB re 1 lPa-1 m rms.



obtained for several ocean-going, deep-draft container ships

as well as the New York Staten Island Transient Ferry.

Distances from the monitoring vessel to the sound source

were measured at regular intervals using a Bushnell Elite

model 1500 laser range finder, capable of measuring distan-

ces up to 1500 m. Distances were confirmed during data

post-processing by plotting positioning information of the

dredge and each anchored monitoring station. When employ-

ing the “drift transect” method, distances to the sound source

were taken every 15 s by laser range finder. GPS coordinates

were logged automatically through the STV1400ENV,

which had an attached external GPS antenna mounted on the

roof of the survey vessel. Wind speed and sea state observa-

tions were recorded at regular intervals throughout the

recording sessions.

D. Hydrophone technical specifications andcalibration information

The TC4032 hydrophone has a high sensitivity, low

noise, and flat frequency response over a wide frequency

range, capable of measuring and detecting even weak signals

at levels below “Sea State 0.” The TC4032 incorporates

electrostatically shielded highly sensitive piezoelectric ele-

ments connected to an integral low-noise 10 dB preamplifier.

It has a usable frequency range of 5 Hz to 120 kHz. The hori-

zontal directivity is omnidirectional 62 dB at 100 kHz. The

vertical directivity is 270�6 2 dB at 15 Hz. The quiescent sup-

ply current is rated at � 19 mA at 12 VDC and � 22 mA at 24

VDC. Receiving sensitivity for the upper (3 m) and lower

(9.1 m) hydrophones as determined by Reson, Inc., A/S

Pistonphone test was �170.6 and �170.4 dB re 1 V/lPa.

E. Data analysis

Video files and underwater audio sound (WAV) files

were synchronized and reviewed in detail to link underwater

sounds to specific dredging-related processes. Detected

sounds were logged by time stamp and clipped from the

original calibrated WAV file recorded by the ST1400ENV.

Software used to clip the segments of interest included a

combination of Sony Sound Forge Audio Studio and

Syntrillium Cool Edit 2000. The newly created subsections

of the original WAV files were saved using the same input

parameters, thereby preserving the original file calibration

integrity. Data analysis was performed using Sound

Technologies SPECTRALAB 4.32 sound spectrum analysis soft-

ware using the following settings: (a) Decimation ratio¼ 1,

which resulted in a upper frequency analysis limit of 24 kHz

due to the Nyquist sampling theorem and the original file re-

cording parameter of 48 kHz, (b) fast Fourier transform

(FFT) size (samples)¼ 32 768, which resulted in a spectral

line resolution of 1.465 Hz, (c) FFT overlap¼ 50%, which

allowed a time resolution of 341.33 ms, (d) smoothing

window¼Hanning, (e) peak analysis¼ peak hold checked

(on) and average of 1, f) One-third octave analysis¼ peak

hold unchecked (off) and average all samples in the file

(Infinite), and g) Frequency weighting¼None (Flat).

SPECTRALAB uses FFT to convert the time-domain (amplitude

versus time) WAV files into the frequency domain

(amplitude versus frequency). Files were processed to gener-

ate an average sound spectrum and SPL across the entire file

from the time series values, and using one-third octave anal-

ysis averaged across the entire sound clip. Also noted during

analysis of each sound clip file were the peak frequency (in

Hz) and peak amplitude (dB re 1 lPA rms) for both the col-

lection of peaks and the one-third octave analysis. The one-

third octave analysis-infinite average-peak frequency is the

center frequency of the one-third octave band with the high-

est calculated decibels. In the majority of cases, particularly

involving dredging sounds of a non-impulse nature (e.g.,

engine/generator sounds); single peak values are not very

meaningful because they simply measure the peak amplitude

of the strongest single frequency observed throughout the

given sound clip. The total power would then be calculated

from all of the collective peaks and would exaggerate sound

levels for any sound event at any single instant during the

clip. The 1/3 octave analysis across the sound clip is a

more meaningful value for comparing one clip of a particular

sound type to another. Conversely, if the sounds were of a

more instantaneous, impulse type (e.g., pile driver strike) an

analysis of peak amplitudes and frequencies would be more

appropriate. Results from the 1/3 octave analysis are also

reported for the 50–1000 Hz band, the general frequency

range audible to most fishes, and the 100–400 Hz band, the

frequency range in which many fish species show greater

sensitivity (Popper et al., 2009). Source levels were back-

calculated by fitted regression (15LogR) and are sometimes

referred to as “practical spreading.” Practical spreading

occurs when the sound energy is not perfectly contained by

reflection and refraction resulting in the true spreading to lie

somewhere between the predictions of spherical (20LogR)

and cylindrical (10LogR) spreading.

Ambient data were collected in the upper (3 m) and lower

(6–9 m) portions of the water column. Dredge sounds were

compared to background data selected from files collected at

seven sites in the study area either prior to dredging or when

the dredge was shut down. Minimum, average, and maximum

ambient sound levels were determined for three frequency

bands along with selected percentile levels. Dredge sounds

exceeding ambient were made using the 50th percentile of am-

bient unless otherwise stated. For a file to be considered ambi-

ent, there had to be no vessel traffic transiting the immediate

area (typically line of sight) during the sound recording.

III. RESULTS

A. Ambient sound

Ambient file segments (146) were selected and analyzed

from seven monitoring sites (Fig. 2). Ambient 1/3 octave

SPL (dB re 1 lPa rms) by site and depth are summarized in

Table II. Representative examples of ambient SPL versus

frequency are presented in Fig. 3. SPLs were determined for

selected percentiles (5th, 50th, and 95th) for three frequency

bands. For all sites combined a SPL of 117.1 dB re 1 lPa

rms occurred across the broader frequency band (20 Hz to

20 kHz), which was 3.5 dB (50 Hz to 1 kHz) to 9.9 dB

(100–400 Hz) greater than the two narrower bands. Quieter

conditions (<100 dB re 1 lPa rms) occurred in the



100–1000 Hz frequency range at Sites A, B, and D at a depth

of 3 m, due in part to their somewhat sheltered locations and

comparatively slower current velocities (<0.5 m/s). Sites with

the highest ambient SPLs were C, F, and G, particularly in the

portion of the water column below 8 m, where SPLs were

approximately 15 dB higher. These sites, located at the entrance

to the East River and within the Kill van Kull Waterway, had

considerably higher current velocities (0.75–1.5 m/s).

B. Hydraulic cutterhead dredge sounds

During the present study, the dredge Florida was not

actively entraining water by pumping but using the mechani-

cal forces of the rotating cutterhead to fracture limestone

rock. The fractured material was allowed to settle in place

for later removal by the backhoe dredge New York.

Therefore pump sounds were not present, and sounds associ-

ated with material movement within pipes were also absent.

Sound pressure waveforms clearly indicated the continuous

nature of sounds measured immediately in front of the oper-

ating cutterhead. These sounds could not be partitioned into

discrete components attributable to individually identifiable

sound sources. Sound intensity did vary within the sound re-

cord, likely reflecting changes in hardness of the material

and depth of penetration of the cutter into the bed. Thus

characterizing the cutterhead sounds collected in this study

was constrained to analyses of cumulative sources. Received

SPL (dB re 1 lPa, rms) versus range from the sound source

is depicted in Fig. 4. This graph illustrates the variation in

SPLs at increasing distance and at two water depths. For

example, SPLs differed by as much as 10–15 dB at distances

less than 200 m from the source. This variation typically

decreased to less than 4 dB with increasing distance from the

sound source. Estimates of distances were based on ranges

to the wheelhouse of the dredge plant and compensated for

the distance between the wheelhouse and the cutter in rela-

tion to orientation to the hydrophone position.

One-third octave SPLs peaked at 151 dB re 1 lPa rms

(SL¼ 175 dB re 1 lPa-1 m rms) at both listening depths

(Table III), although peak occurred nearly 50 m closer to the

sound source at the lower listening depth. SPLs remained

above 132 dB (range, 132–135) as far as 740 m from the

source. At 740 m from the source, SPLs exceeded ambient

by as much as 8.7 dB (95th percentile) to 18.9 dB (50th per-

centile). Data collected at two additional anchored stations

located 900 and 1040 m from the source were corrupted by

hydrodynamic noise, which prevented detection of dredge

sounds at these distances. Attenuation to maximum

(131.2 dB) ambient levels was estimated to occur at 800-

850 m from the source and to the 95th percentile of ambient

by 2 km, based on the fitted regression equation (15LogR).

Peak SPLs in the two narrower frequency bands were 2 dB

(50 Hz–1 kHz) to 6 dB (100–400 Hz) lower when compared

to the broader frequency spectrum. When adjusted for the

lower ambient sound levels (all sites and depths combined)

in the two narrower frequency bands (Table II), peak SPLs

from the CSD exceeded the 50th percentile of ambient by

35 dB (50 Hz to 1 kHz) to 39 dB (100–400 Hz), an overall

increase from the broader to narrowest frequency band by

1–5 dB. The most common 1/3 octave peak center bin fre-

quencies were 300 Hz with additional peaks centered around

800 and 1000 Hz.

FIG. 3. (Color online) Representative examples of ambient SPL (1/3 octave)

in dB re 1lPa rms at monitoring sites throughout New York/New Jersey

Harbor.

TABLE II. Summary of ambient 1/3 octave SPL in dB re 1 lPa rms at

selected percentile levels.

20 Hz to 20 kHz 50–1000 Hz 100–400 Hz

Site Depth P5th P50th P95th P5th P50th P95th P5th P50th P95th

A 3 108.9 110.2 112.4 103.7 105.7 110.0 95.6 97.9 100.2

B 3 98.3 108.6 116.6 95.2 98.9 109.7 82.3 90.2 105.9

C 3 121.7 121.9 122.1 120.6 120.8 120.9 118.1 118.1 118.2

C 9 122.2 124.9 130.4 118.6 120.7 123.9 116.8 119.2 122.9

D 3 116.8 120.5 123.3 111.6 117.1 122.2 102.2 113.2 119.8

D 9 110.7 111.6 113.7 109.9 110.1 111.3 96.5 99.4 101.8

E 3 115.5 117.5 119.2 111.8 113.4 116.4 107.5 109.8 113.6

F 3 118.3 119.1 122.9 115.5 117.2 121.1 109.1 110.3 115.6

F 8 124.2 126.6 129.3 118.8 122.9 125.5 112.2 117.1 119.4

G 3 120.4 120.9 123.4 118.2 118.5 119.4 112.7 113.3 114.8

G 9 121.1 121.7 123.4 119.2 119.8 120.2 114.1 115.1 115.7

Alla – 106.4 117.1 127.3 98.5 113.6 123.8 89.1 107.2 119.7

aAll sites and water depths combined.

FIG. 4. (Color online) Analysis results for 1/3 octave SPLs (dB re 1lPa,

rms) versus range for the CSD Florida.



Given the complexities of characterizing ambient sound

in a major harbor, using a single median (P50th) ambient

value to determine the degree in which background levels

are exceeded by dredged-induced sound may be an

over-simplification. For example, Fig. 5 displays the SPLs

(dB re 1 lPa rms) obtained at selected distances and

depths for CSD sounds compared to ambient results for sites

A and F, which represent, respectively, the quietest and loud-

est locations monitored. Ambient SPLs for sites A and F in

the 100 Hz to 1 kHz range differed by as much as 20–25 dB.

SPLs of the CSD measured at 200 m from the source

exceeded ambient in the range of fish hearing by much as

38–45 dB at the shallow and deep listening depths, respec-

tively (Table IV). Based on a single ambient value for all

sites and depths combined (50th percentile), SPLs would

exceed ambient by 35 dB (50 Hz to 1 kHz).

C. Mechanical backhoe dredge sounds

1. Engine/generator sounds

The most frequently detected sound source was engine/

generator noise. The onboard engine/generators produced a

relatively strong and continuous sound, which was trans-

ferred through the ship’s hull to the water. Sound was

recorded at distances ranging from 55 to 680 m from the

source. A peak SPL of 134 dB re 1 lPa rms (SL¼ 167 dB re

1 lPa–1 m rms) occurred in the 20 Hz to 20 kHz frequency

band (peak frequency¼ 125 Hz) at a distance of 135 m from

the sound source. SPLs decreased by 1 dB (50 Hz to 1 kHz)

to 4 dB from the broader to the narrower frequency bands

(Table III). Peak SPLs in the two narrower bands exceeded

the 50th percentile of ambient (all sites and depths com-

bined) by 19 (50–1000 Hz) to 23 dB (100–400 Hz). Using

site-specific ambient results (summarized in Table IV), SPLs

exceeding ambient in the narrower frequency bands could be

as low as 10 dB if applying ambient results obtained at site

F, and as high as 39.8 dB at site B. At some frequencies,

TABLE III. Summary of 1/3 octave SPL (in dB re 1 lPa rms) for the CSD Florida and excavator dredge New York.

20 Hz to 20 kHz 50–1000 Hz 100–400 Hz

HDa¼ 3 m HD¼ 9 m HD¼ 3 m HD¼ 9 m HD¼ 3 m HD¼ 9 m

DEb FSc SPLd De SPL D Sf>A SPL D SPL D S>A SPL D SPL D S>A SLg PFh

FLi 377 151 152 151 100 34 149 89 149 100 35 145 89 146 100 39 175 0.3

Dredge New Yorkj

Ek 27 133 75 134 135 17 132 75 133 135 19 129 75 130 135 23 167 0.125

Bl 15 137 60 148 60 31 135 75 148 60 34 133 75 147 60 40 179 0.315

Lm 9 139 60 139 60 22 134 60 137 60 23 131 60 136 60 29 166 0.1

Rn 13 133 60 138 60 20 132 60 134 60 20 129 60 128 60 22 164 2.5

Po 6 140 60 – – 23 140 60 – – 26 138 60 – – 31 167 0.25

Ap 5 136 75 138 220 21 132 75 134 220 20 130 75 128 75 23 173 1.2

Wq 4 147 75 137 210 30 147 75 136 210 33 147 75 136 210 40 176 0.2

aHydrophone depth.bDredge or dredge event.cNumber of file segments analyzed.dPeak 1/3 octave SPL in dB re 1 lPa rms.eD ¼ distance in meters from sound source peak RL was obtained.fPeak SPL exceeding the 50th percentile of ambient (117 dB).gSource level in dB re 1 lPa-1 m (rms).hPeak frequency (kHz).iCSD Dredge Florida.jExcavator Dredge New York.kEngine/generator noise.lBottom impact.mBarge loading.nHydraulic ram.oPopping noise.pAnchoring spuds.qWalking spuds.

FIG. 5. (Color online) Representative examples of 1/3 octave SPLs (dB re

lPa, rms) versus frequency (Hz) for selected distances for the CSD Florida.



however (e.g., 400 Hz), engine/generator sounds differed

from ambient levels at Site F by only 0.6 dB. These differen-

ces are illustrated in Fig. 6. The greatest distance at which

engine/generator sounds could be positively identified as

coming from the dredge New York was 330 m. At this dis-

tance (330 m), SPLs had fallen by 12.5–121.5 dB at the deep

(9.1 m) listening depth (SPL> ambient¼ 5 dB). Sounds

resembling engine noise were detected in two file segments

at 680 m from the source but were absent at monitoring sta-

tions between 350 and 600 m from the source. It is unclear if

the engine/generator noise detected at 680 m was being emit-

ted from the Dredge New York or from some other source in

the area.

2. Bottom impact sounds

During monitoring, the backhoe dredge was removing

limestone rock previously fractured by the CSD Florida.

The material being excavated consisted of relatively uniform

pea gravel (4–8 mm). A peak 1/3 octave SPL of 148.4 dB re

1 lPa rms occurred at a peak frequency of 315 Hz at a dis-

tance of 60 m from the source at the deep hydrophone. This

SPL exceeded the 50th percentile of ambient by 31 dB. At

the same distance, SPLs were 11–14 dB lower at the shallow

listening depth. There was considerable variation in SPLs

(as much as 5 dB at 3 m and 6–12 dB at 9.1 m) between

file segments (n¼ 15) recorded at the same distance

from the sound source. In the narrowest frequency band

(100–400 Hz), SPL differed by as much as 4 dB at the shal-

low hydrophone when compared to the broader spectrum.

Peak SPLs exceeded ambient (50th percentile) by 40 dB in

the narrowest frequency band. Bottom impact sounds were

the loudest of all sources recorded from backhoe dredging

operations with a SL of 179.4 dB re 1 lPa–1 rms.

A comparison of 1/3 octave SPLs between bottom

impact sounds and lowest and highest ambient SPLs (sites A

and F) is depicted in Fig. 7 and summarized in Table IV.

Using 200 Hz as an example, ambient would differ by as

much as 16 dB between these two ambient monitoring sites.

At this frequency, bottom impact sounds exceeded ambient

by 33.5 dB (site F) to 49.3 dB (site A) re 1 lPa rms. Bottom

impact sounds were not detected beyond 75 and 175 m at the

shallow and deep listening depths, respectively.

TABLE IV. Summary of one-third octave SPL (dB re 1 lPa, rms) at selected frequencies in the audible range of most fish species.

Freq.

(Hz)

Amba

(dB)

site A

Ambb

(dB)

site F

Engine/generator

Dc¼ 75 m

HDd¼ 3 m

Hyd. ram

D¼ 60 m

HD¼ 9 m

Popping noise

D¼ 60 m

HD¼ 3 m

Barge loading

D¼ 75 m

HD¼ 3 m

Bucket impact

D¼ 60 m

HD¼ 9 m

Anchoring spuds

D¼ 75 m

HD¼ 3 m

Walking spuds

D¼ 75 m

HD¼ 3 m

CSD Florida

D¼ 200 m

HD¼ 3 and 9 m

100 90 101 111 112 116 112 130 113 113 120 121

200 88 103 117 119 127 118 137 118 145 124 128

300 88 110 115 120 131 114 143 116 121 125 133

400 90 116 116 121 133 115 141 117 125 124 128

500 89 114 118 120 121 118 137 119 119 124 129

600 87 110 117 123 130 116 135 117 116 125 127

800 87 110 115 119 131 116 132 117 116 127 130

1000 91 108 115 119 129 116 131 115 115 127 129

aAmbient SPL (dB re 1 lPa, rms) obtained from the quietest monitoring site (site A, HD¼ 3 m).bAmbient SPL (dB re 1 lPa, rms) obtained from the loudest monitoring site (site F, HD¼ 8 m).cDistance from sound source,dhydrophone depth.

FIG. 6. (Color online) SPLs (dB re 1 lPa, rms) produced by engine/genera-

tor, hydraulic ram, and popping noise, compared to the upper (site F) and

lower (site A) ranges of ambient.

FIG. 7. (Color online) SPLs (dB re 1 lPa, rms) produced by bottom impact

and barge loading sounds compared to the upper (site F) and lower (site A)

ranges of ambient.



3. Hydraulic ram sounds

A hydraulic ram is used to extend and retract the exca-

vator arm of the dredge. A peak SPL (1/3 octave, 20 Hz to

20 kHz) of 138 dB re 1 lPa rms was recorded 60 m from the

sound source at a depth of 9.1 m; this exceeded peak SPL at

the shallow listening depth by 5 dB. Peak frequency was cen-

tered around 620 Hz with a harmonic frequency of 2500 Hz.

Differences in SPL (1/3 octave) between the shallow and

deep listening depths within the two narrower frequency

bands were less than 2 dB. Hydraulic ram sounds exceeded

the 50th percentile of ambient (all sites and depth combined)

by 22 dB in the 100–400 Hz frequency band and by 20 dB in

the two broader frequency bands. Hydraulic ram sounds

were not detected beyond 170 m from the source.

A comparison by frequency (100–1000 Hz) in 100 Hz

increments indicated that hydraulic ram sounds could exceed

ambient at a quieter site (site A) by as much as 30 6 2 dB

and by as little as 10 dB for most frequencies in the range at

a louder ambient location (site F, Table IV, Fig. 6). SLs

were back-calculated to 164 dB re 1 lPa–1 m rms.

4. Popping sounds

Unidentified “pops” were detected in six file segments

collected at a distance of 60 m from the source at a depth of

3 m. This sound event reoccurred consistently at the same

point in the dredge cycle at or near the point when the fully

loaded excavator bucket was breaking the water’s surface.

The unidentified “pops” (peak frequency¼ 250 Hz) were

clearly related to the dredging operation, but the exact source

of the sound was undetermined. Peak SPLs (1/3 octave) are

summarized in Table III. SPLs peaked at 140.4 dB re 1 lPa

rms (SL¼ 167 dB re 1 lPa–1 m rms), exceeding the 50th

percentile of ambient (all sites and depths combined) by

23.3 dB. The most intense sounds occurred between 250 and

400 Hz where 1/3 octave SPLs exceeded 130 dB. At frequen-

cies between 100 and 1000 Hz, popping sounds exceeded am-

bient sound at the quietest monitoring site (site A) by

42–45 dB and at the loudest monitoring site (site F) by

16–26 dB (Fig. 6). Pops were not detected beyond 60 m from

the source either due to rapid attenuation or possibly due to

corrective equipment maintenance aboard the dredge. Because

this sound did not appear to be a permanent component of the

dredging process, it is discussed only briefly herein.

5. Barge loading sounds

Characteristics of sounds associated with the excavated

gravel being dumped into the attending barge were depend-

ent on the volume of material in the barge at the time the

measurements were taken. Material placed into an empty or

partially full barge would have the highest probability of

transmitting sound through the barge’s hull into the sur-

rounding waters, whereas material placed upon previously

piled material may or may not produce a detectable sound

due to buffering provided by the “softer” receiving surface.

Received SPLs (dB re 1 lPa, rms) did not exceed 139.5 dB

for any of the nine file segments recorded at distances less

than 100 m from the source. Lowest SPL was recorded at the

75 m anchored station at 130.1 dB re 1 lPa rms. Barge load-

ing sounds were not detected at the 170 m anchored listening

station. SL was estimated by back-calculation to be 166.2 dB

re 1 lPa–1 m, rms.

One-third octave peak SPLs were 139 dB (peak

frequency¼ 100 Hz) at both the shallow and deep listening

stations within the 20 Hz to 20 kHz frequency band. From

the broadest to narrowest frequency band, peak SPL

decreased by 3 dB at a depth of 9.1 m to 8 dB at 3 m. Peak

SPLs exceeding the 50th percentile of ambient are summar-

ized in Table III and in the range of greatest fish hearing sen-

sitivity (100–1000 Hz) in Table IV. Barge loading sounds

typically exceeded ambient results from site A, which had a

relatively stable SPL in the 100–1000 Hz frequency range by

22–30 dB (Fig. 7). When compared to site F, barge loading

sounds exceeded ambient by 10–15 dB at frequencies of 100

and 200 Hz but exceeded ambient by only 3.5–7 dB from

500 Hz to 1 kHz.

6. Anchoring spud sounds

Some of the more intense sounds recorded during the

present study were associated with maneuvering of the

dredge by spuds. Results of underwater sound measurements

for the two spud types (anchoring and walking) were some-

what distinct and are therefore discussed separately.

Underwater sounds produced by the lifting or lowering of

anchoring spuds were detected as far as 220 m from the

source. It should be noted that unlike other dredging sound

events, spud sounds are not a part of the bucket deployment

and retrieval cycle. Spud descending and raising maneuvers

are necessary for the dredge to advance beyond the newly

dredged area and occur much less frequently than bucket

digging cycles. Consequently at most listening stations, this

activity did not occur during the monitoring period. The rais-

ing and lowering of spuds did occur at the 330-m listening

station, but spud sounds were not detected during the moni-

toring session at either hydrophone depth. A peak SPL of

138 dB re 1 lPa rms occurred at a distance of 220 m from the

source and a depth of 9.1 m (SL¼ 173 dB re 1 lPa–1 m rms),

exceeding ambient by 21 dB (20 Hz to 20 kHz frequency

band). Spud re-positioning created a resonant sound centered

around 1.2 kHz. In the narrowest frequency band (100–400

Hz), peak SPL were 7–10 dB lower than the broadband spec-

trum, exceeding ambient (P50th) by 23 dB. SPLs by fre-

quency band and depth are summarized in Table III.

SPLs (1/3 octave in dB re 1 lPa, rms) versus frequency

are compared to ambient levels, representing the lowest to

the highest recorded in the present study in Fig. 8.

Anchoring spud sounds exceeded ambient by 23–30 dB

when compared to relatively quiet conditions found at site

A. On the upper end of the ambient range (site F), anchoring

spud sounds exceeded ambient by 12–15 dB (100–200 Hz)

and from 5 to 10 dB from 500 Hz to 1 kHz (Table IV).

7. Walking (advancing) spud sounds

At three distances (75, 210, and 330 m) from the source

sound, recording sessions captured the dredge while actively

“walking” on its spud. Peak SPL was 147 dB re 1 lPa rms



(SL¼ 176 dB re 1 lPa–1 m, rms) at 75 m from the sound

source at a depth of 3 m. In comparison to anchoring sounds

(resonance at 1.2 kHz), sounds generated by spud walking

created a resonance at a much lower frequency (200 Hz).

Advancement of the dredge on its walking spud did not

occur again until the survey vessel was located 210 m from

the source. At this distance, the SPL was 136.6 dB re 1 lPa

rms at a depth of 9.1 m, approximately 20 dB above ambient.

Unlike the other backhoe dredging sound sources, spud

walking SPLs did not differ greatly between the broader and

narrower frequency bands (Table III).

SPLs versus frequency for spud walking sounds are

depicted in Fig. 8. At 200 Hz, the SPL was 144.6 dB,

exceeding ambient at the loudest ambient monitoring site

(site F) by 41.3 dB. Utilization of the walking spud produced

the largest difference in SPL (57 dB above ambient at site A

at a frequency of 200 Hz) among all other dredging events.

With the exception of SPLs at 200 Hz, SPLs exceeding am-

bient (100 Hz to 1 kHz) were approximately 13 and 12 dB at

the lower and upper ranges (Table IV).

D. Underwater sounds associated with commercialvessels

Underwater sound measurements in the 20 Hz to 20 kHz

were recorded for several commercial vessels transiting nav-

igation channels in the harbor. A summary of vessel specifi-

cations can be found in Table I. For the Staten Island ferries,

received levels were recorded from either the port or star-

board side of the listening platform depending on the ferry’s

direction of travel at distances ranging from 298 to 830 m.

Received levels were lowest when the ferry was approaching

the listening platform. For example, at 750 m from the

source, received levels were 136 dB when the ferry was

approaching the listening platform and 139.6 dB when the

vessel was moving away. A maximum receiving level of

144.2 dB (SL¼ 181.3 dB re 1 lPa–1 m rms) occurred 298 m

from the source at a depth of 3 m.

During a second monitoring event, received levels at a

depth of 9.1 m were recorded as the ferry departed The Battery

and concluded when the ferry arrived at the St. George

Terminal. Received levels increased from 125.2 dB at departure

(930 m from the source) to a peak at 142 dB (SL¼ 180.2 dB re

1 lPa–1 m rms) as the vessel passed within 352 m of the listen-

ing platform. Lowest received level (125.2 dB) exceeded aver-

age background by 8.1 dB at 1 km from the source. Received

level versus distance for the two monitoring events revealed

differences in SPL as the ferry moved toward and away from

the listening platform as depicted in Fig. 9.

The container ship NYK Constellation was monitored as

it entered Newark Bay, New Jersey. Received levels were

measured at distances ranging from 122 to 1442 m at a depth

of 3 m as shown in Fig. 10. As the vessel approached the lis-

tening platform at a distance of 1400 m the received level

was 134 dB re 1 lPa rms, which exceeded average back-

ground by almost 17 dB. A maximum received level of

150 dB re 1 lPa rms (SL¼ 181.3 dB re 1 lPa–1 m rms) was

recorded at 122 m astern of the vessel, exceeding average

background by 33 dB.

The container ship Maersk Idaho was monitored during

its departure from the South Elizabeth Terminal in Newark

FIG. 9. (Color online) Received SPLs (dB re 1 lPa, rms) versus range pro-

duced by the Staten Island Ferry (*start of monitoring).

FIG. 10. (Color online) Received SPLs (dB re 1 lPa, rms) versus range for

the container vessels NYK Constellation and Maersk Idaho (*start of

monitoring).

FIG. 8. (Color online) SPLs (dB re 1 lPa, rms) produced by anchoring and

“walking” spuds sounds compared to the upper (site F) and lower (site A)

ranges of ambient.



Bay. Two tugs were used to assist the Maersk Idaho maneu-

ver from her berth. A maximum received level of 147 dB re

1 lPa rms occurred at a distance of 622 m from the source

(Fig. 10). During this phase of the departure, underwater

sounds were generated by both the cargo ship and the tugs

assisting the vessel. Maximum received levels exceeded av-

erage ambient levels by nearly 30 dB. The calculated SL

reached 188.9 dB re 1 lPa–1 m rms.

The container ship Zim Savannah was monitored pass-

ing through the KVK waterway into Anchorage Channel in

the upper harbor. Received levels were recorded at distances

ranging from 230 to 1269 m at a depth of 9.1 m. A maximum

received level of 141.7 dB re 1 lPa rms (SL¼ 177.1 dB re

1 lPa–1 m rms) was recorded 321 m from the sound source,

exceeding the average background level by 24.6 dB. Lowest

received level (129.2 dB) was recorded 645 m astern of the

vessel, exceeding ambient by 12 dB. Received levels versus

ranges are plotted in Fig. 11.

The container ship CSAV Licanten was monitored as it

passed through Anchorage Channel, New York Harbor into

the KVK waterway. SPLs were recorded at distances ranging

from 353 to 900 m at a depth of 9.1 m. A maximum received

level of 141.8 dB re 1 lPa rms (SL¼ 180 dB re 1 lPa–1 m

rms) was recorded at a distance of 353 m from the source,

exceeding the average background level by 24.7 dB. The

received level (133.76 dB) still exceeded background by

approximately 16.6 dB at 900 m from the source (Fig. 11).

IV. DISCUSSION

Ambient noise can be defined as sounds present in the

environment without distinguishable sources. Ambient noise

is continuous but with considerable variation on different

time scales, varying by as much as 10–20 dB from day to

day (Richardson et al., 1995). To fully characterize ambient

underwater sound, repeated measurements must be taken on

appropriate temporal and spatial scales under varying envi-

ronmental conditions, such as varying tidal and storm-

associated hydrodynamic conditions. Knowledge of average

ambient sound levels and the variability surrounding these

levels is fundamental to interpretation of sounds produced

by anthropogenic sources and assessment of responses of

organisms to those sounds. Under ideal research conditions,

a comprehensive characterization of ambient sound would

require long-term deployment of acoustic data-logging sen-

sor arrays. Long-term records would be collected either con-

tinuously or at predetermined intervals. This approach,

however, is extremely labor-intensive and costly. For the

purposes of the present study, the adopted approach used

site- and time-specific measurements. Although the obtained

ambient sound levels do not represent the acoustic sound

field for the entire harbor over an extended period of time,

site-specific measurements provide an accurate baseline for

comparison to sounds emitted by dredges during this study.

Several potentially significant sources of ambient sound

present in New York/New Jersey Harbor include sounds of

biological and anthropogenic origin. In this highly industrial-

ized and urbanized setting, many transient sounds emanating

from a plethora of sources comprise a complex and con-

stantly changing underwater sound environment.

Bassett et al. (2010) measured average ambient SPL

(15.6 Hz to 30 kHz) of 117 dB re 1 lPa over the span of a

year in Admiralty Inlet, Washington. Minimum and maxi-

mum ambient SPLs recorded there were 94 and 144 dB re

1 lPa, respectively. Ambient SPLs exceeding 100 dB re

1 lPa occurred 99% of the time. Admiralty Inlet and New

York/New Jersey Harbor are similar in that both locations

experience frequent passages by passenger ferries and com-

mercial ship traffic. However, peak tidal velocities in

Admiralty Inlet (3 m/s) are nearly twice that in New

York/New Jersey Harbor (0.75–1.5 m/s). Blackwell and

Greene (2002) reported ambient SPLs at six locations iso-

lated from industrial activities in Anchorage Harbor and the

Knik Arm of Cook Inlet, Alaska. The authors reported that

ambient sound levels ranged from 95 dB re 1 lPa in the Knik

Arm to 120 dB re 1 lPa near Point Possession, Alaska, on an

incoming tide. SPLs in Anchorage Harbor averaged 113 dB

re 1 lPa (10 Hz to 20 kHz). These ambient SPLs are compa-

rable to those recorded in the Canadian Beaufort Sea, aver-

aging 99 dB (90th percentile¼ 117 dB re1 lPa), by Greene

(1987) and off Barrow Alaska (1/3 octave to 115 dB re

1 lPa) by Richardson et al. (1995). In the present study, am-

bient conditions in New York/New Jersey Harbor (50th per-

centile SPL¼ 117.1, SPL range¼ 97.5–131.2 dB re 1 lPa

rms) had similar median values when compared to the pre-

ceding studies. The maximum ambient SPL (131.2 dB re

1 lPa rms) reported in this study did exceed those reported

in the preceding studies; however, maximum values

accounted for less than 1% of all measurements. This may

simply reflect the fact that the previous studies were con-

ducted in open-water environments away from major indus-

trial activities. Other factors that may contribute to observed

differences in ambient sound levels among sites may be ba-

thymetry, geotechnical properties of the bottom substrate,

and prevalent concentrations of total suspended solids in the

water column. The bottom substrate of Cook Inlet is a mix-

ture of cobbles (65–256 mm), pebbles (4–64 mm), silt

(3.907–62.5 lm), and clay (<3.906 lm). In Anchorage

Channel, New York Harbor, the sediment being excavated

FIG. 11. (Color online) Received SPLs (dB re 1 lPa, rms) versus range for

the container vessels CSAV Licanten and ZIM Savannah (*start of

monitoring).



consisted of pea-sized limestone rock (4–16 mm), although

this does not represent the undisturbed sediment. Sites with a

mixture of cobble and gravel/rock may have increased ambi-

ent sound levels when compared to sites dominated by silt

and consolidated clay, particularly at sites with moderate to

strong current flow, which would serve as a driving force to

move the loose material along the seabed. Lower ambient

SPL in the Beaufort Sea (median SPL¼ 99 dB compared to

117 dB for New York Harbor) may be related, at least in

part, by the presence of consolidated clay layer (Spofford

et al., 1983 as cited in Richardson et al. 1995).

Sound transmission in shallow water is highly variable

and heavily influenced by site-specific factors, including

acoustic properties of the seabed, salinity regime, stratifica-

tion of the water column, sea state at the surface, and the

speed of sound in the water column. Shallow water depths

generally do not allow for most types of channeling effects

of sound that are known to occur in deeper water

(Richardson et al., 1995). Refraction of sound in shallow

water can result in either reduced or enhanced sound trans-

mission. For example, SPLs of low-frequency sounds

(<100 Hz) measured in the present study at a hydrophone

depth of 3 m were lower than those measured simultaneously

at a depth of 9.1 m. This is explained by the Lloyd mirror

effect (Urick, 1983). When sound is refracted upward, bot-

tom reflections and bottom losses are reduced. When sound

is reflected from the sea bottom, the transmission path or the

route the sound takes from the source to the hydrophone (re-

ceiver) is affected. This can result in multiple transmission

paths, where sound is not moving in a single straight line.

Sound moving along multiple transmission paths from the

source to the receiver may also result in enhanced or dimin-

ished transmission loss. Irregularities in bathymetry can

exert a strong influence on sound transmission in shallow

water, as slopes alter effective surface areas and bottom

reflections. Sound transmission from deep to shallower water

depths may result in a reduction in spreading loss, although

in certain conditions attenuation from bottom loss may be

high as a consequence of reflections in shallow water.

In the present study, seabed characteristics, shallow

water depths, and geomorphology all contributed to reflec-

tion, refraction, and multipath loss, thereby affecting how

sound was locally propagated. These factors obviously influ-

enced detection ranges of dredge-induced sounds. Given the

complex configuration of shoals intersected by navigation

channels with relatively steep side slopes in the harbor,

sound propagation did not conform to the assumptions of ei-

ther spherical (20LogR) or cylindrical (10LogR) spreading

but was determined by fitted regression to fall between the

two models at 15.77LogR. The geomorphology of the study

site also affected sound propagation in the study area. The

presence of a broad navigation channel as deep as 60 ft

(18.3 m) MLW, boarded by shoals with water depths as deep

as 22 ft (6.7 m) and as shallow as 5 ft (1.5 m) created a com-

plex physical setting in which sounds reflected off side

slopes. In addition, comparatively high suspended sediment

loads in estuaries can shorten sound attenuation distances in

comparison to offshore “blue” water settings (Richards

et al., 1996). Ambient suspended sediment concentrations in

New York/New Jersey Harbor frequently surpass 20 mg/l or

higher during periods of storm passage or high river dis-

charges. The New York/New Jersey Harbor sound field is

constantly changing as a myriad of sound sources come into

play at any given time or location. A combination of these

factors contributed significantly to the relatively short maxi-

mum detection distance of 850 m for the dredge Florida.Sound attenuation to background levels for the dredge

Florida was estimated based on a maximum ambient SPL

threshold. Dredge sounds attenuated within an estimated

2 km at the 95th percentile of ambient. Sound attenuation

distances could not be determined for individual components

of the backhoe dredging operation due to the small number

of anchored stations in which dredged-induced sound could

be detected. Multi-path loss may explain why dredge sounds

that exceeded ambient by 10 dB (95th percentile) to 20 dB

(50th percentile) at an anchored station located 220 m from

the sound source could not be detected at an adjacent station

at a distance of 320 m.

The present study represents the first characterization of

sounds produced by a hydraulic cutterhead and mechanical

backhoe dredge engaged in rock fracturing and excavation

within a major harbor. Source Levels obtained for the CSD

(170–175 dB re 1 lPa–1 m rms) were within the range of pre-

vious sound measurements obtained by Greene (1987) in an

open-water environment. Greene (1987) measured 1/3

octave sounds emitted by two hydraulic cutterhead dredges,

the Aquarius and Beaver Mackenzie, to be 178 and 167 dB,

respectively. Attenuation rates were much shorter in New

York/New Jersey Harbor, falling to background at approxi-

mately 1 km from the dredge sources as compared to 25 km

in the Beaufort Sea. Peak frequencies were centered around

160 and 100 Hz (Greene, 1987), and 300 Hz in the present

study. Source Levels obtained for the backhoe dredge

(164–179 dB re 1 lPa–1 m rms) were generally below these

ranges with the exception of bottom impact sounds and dur-

ing the use of anchoring and walking spuds.

Source levels measured in the present study can be com-

pared to other types of dredging operations. Greene (1987)

reported SPLs for three large (hopper capacity¼ 6000–9000

m3) TSHDs, the Geopotes X, Cornella Zanen, and W. D.Gateway, during gravely sand excavation. SLs ranged from

179 to 187 dB re 1 lPa–1 m, considerably higher than meas-

urements for the cutterhead and backhoe dredges working in

the present study. Reine et al. (2013) also reported SPLs

(1/3 octave) for three medium size class (hopper

capacity¼ 835–1521 m3) TSHDs, the Padre Island, DodgeIsland, and Liberty Island, during offshore sand mining

where SLs ranged from 161 to 178 dB re 1 lPa–1 m rms.

SPLs for the smaller size class TSHDs during sand mining

operations fell within the range of SPLs measured for the

dredge Florida and New York during rock fracturing and

excavation. Parvin et al. (2008) measured the source levels

of a 2700–m3 hopper dredge (The City of Westminster) oper-

ating in the English Channel and calculated broadband

source levels of 186 dB re 1 lPa1–m (20 Hz to 80 kHz),

which was consistent with SPLs reported by Greene (1987)

for TSHDs during the removal of gravelly sand. Robinson

et al. (2011) reported source levels (1/3 octave



range¼ 155–185 dB re 1 lPa–1 m rms) for six hopper

dredges (capacity range from 1418 to 4832 m3) during ma-

rine aggregate dredging. The authors reported that noise

radiated at frequencies less than 500 Hz. Miles et al. (Miles

et al., 1986; Miles et al., 1987) measured sounds produced

by a bucket dredge, noting that the most intense sounds were

in the one-third octave at 250 Hz ranging from 150 to 162 dB

re 1 lPa rms. The authors reported that the loudest sounds

measured in their study were produced during winching of

the loaded bucket up through the water column.

Sounds produced by diverse commercial vessels in New

York/New Jersey Harbor were found to have SLs that ranged

from 180 to 189.9 dB re 1 lPa–1 m rms. SLs from commer-

cial shipping exceeded the SLs for both the cutterhead

dredge Florida and the backhoe dredge New York by

10–15 dB and by as much as 33 dB for the CSD monitored

by Greene (1987). SLs were 9–17 dB lower for three me-

dium capacity TSHDs monitored by Reine et al. (2013) but

only 1–3 dB lower for the larger capacity TSHDs monitored

by Greene (1987). The larger size class hopper dredges were

similar to a container ship traveling at a modest speed.

Overall, bucket dredging operations monitored by Miles

et al. (Miles et al.,1986; Miles et al.,1987) and Clarke et al.(2002) were relatively quiet with SLs 28–34 dB lower than

shipping and ferry vessels monitored in the current study.

Regulatory guidelines for determining protective SPL

thresholds for fishes and marine mammals are in the initial

stages of development (National Marine Fisheries Service,

2011). Based on a few existing studies, the NOAA-NMFS is

tentatively recommending thresholds for determining

impacts to marine mammals that are based on root-mean-

square (RMS) received levels between 180 and 190 dB re

1 lPa rms for potential injury to cetaceans and pinnipeds

(Level A criteria), respectively, 160 dB re 1 lPa rms for be-

havioral disturbance/harassment (Level B criteria) from an

impulsive sound source (e.g., impact pile driving) and

120 dB re 1 lPa rms for behavioral disturbance and/or har-

assment from a continuous sound source. The latter criterion

would apply to most dredging scenarios. Most categories of

sounds generated by TSHD or CSD operations can be con-

sidered continuous. However, in the case of mechanical

bucket or backhoe dredges, many components of sound pro-

duction could best be described as repetitive with time inter-

vals of several seconds to minutes between events. None of

the received levels (1/3 octave) produced by the dredges

Florida or New York exceeded 151 dB re 1 lPa rms, which is

well below Level A and B criteria (180 and 190 dB re 1 lPa

rms). Therefore the dredging operations did not violate the

tentative criteria for potential injury for cetaceans and pinni-

peds. SLs did exceed 160 dB re 1 lPa-1 m, rms in a zone that

extended less than 20 m from the dredge Florida and less

than 10 m from the dredge New York, even for the most

intense sounds produced by bottom impact during sediment

excavation. SLs generated by commercial shipping did

exceed 180 dB and in some cases were at or near 190 dB re

1 lPa-1 m rms; however, maximum received levels were

typically less than 145 dB re 1 lPa rms. The 120 dB re 1 lPa

rms proposed threshold for behavioral disturbance/harassment

from a continuous non-pulse noise source such as certain

components of dredging operations or vibratory pile driving

would be exceeded routinely in many situations. Of interest,

however, is the fact that this level was reached and frequently

exceeded during recording of ambient conditions in New

York/New Jersey Harbor in the absence of dredging activities.

Herring and shad species of the family Clupeidae are ca-

pable of hearing both low-and high frequencies (Mann et al.,1998). Clupeids are the most frequently cited key target spe-

cies in terms of potentially negative impacts from dredging

operations. Auditory sensitivity of the American shad

spanned 200–800 Hz in the sonic range and 25–130 kHz in

the ultrasonic range. Because most sounds produced by

dredges are at frequencies less than 1 kHz, American shad

could potentially be affected by dredge sounds in the sonic

range. A behavioral response to sound in the ultrasonic range

has been observed for some clupeids and has been used to

prevent fish entrainment by repelling them from power plant

intakes (Dunning et al., 1992). Behavioral responses to low-

frequency sounds generated by dredging operations are not

well documented, although the concern is frequently cited

by resource agencies as having potentially negative impacts

on anadromous fish migrations. Mann et al. (2001) demon-

strated that Gulf menhaden (Brevoortia patronus) can detect

sounds in the ultrasonic range. Bay anchovies (Anchoamitchilli), scaled sardines (Harengula jaguana), and Spanish

sardines (Sardinella aurita) may be able to detect sounds to

4 kHz. A critical issue in assessing dredging-induced sound

effects on fish behavior is not only whether the sound is

within the hearing frequency range of a fish species but

whether the sound is loud enough to be detectable above am-

bient thresholds. Hearing data exist for about 100 of the

29 000 known fish species (Popper et al., 2009).

Currently the NOAA-NMFS interim criterion for physi-

cal injury and or mortality to fishes is based on SEL, not

SPL, with a maximum allowed at 206 dB regardless of fish

size. For cumulative SEL, the criterion is 187 dB re 1 lPa

per unit of time for a fish weighing greater than 2 g and

183 dB re 1 lPa per unit of time for a fish weighing less than

2 g. There is a general consensus among underwater sound

experts to use SEL as a more appropriate measurement;

however, obtaining accurate SEL measurements for both a

moving dredge and a moving target species poses severe

technical obstacles. The SELs reported in the preceding text

are based upon exposures of caged fishes to sounds gener-

ated by pile driving. There are currently no estimates of

SELs for key target fish species in the presence of any dredg-

ing operation. Based on reviews by Popper et al. (2006) and

Southall et al. (2007), it is unlikely that underwater sound

from conventional dredging operations can cause physical

injury to fish species. In theory, temporary hearing losses

could occur if fishes remained in the immediate vicinity of a

dredge for lengthy durations, although the risk of this out-

come is low (Central Dredging Association, 2011).

V. CONCLUSION

Although there has been an increase recently in the

number of studies attempting to document the effects of

anthropogenic sounds on the behavior and fate of fishes,



future research is necessary to address the issue of low-

frequency sounds, such as those produced by dredges, on

fish behavior. Given the complexities of sound propagation

in predominantly shallow water harbors with intricate geo-

morphologies, and the fact that dredging is a frequent activ-

ity within most harbors, the need for investigations of

behavioral responses of fishes to encounters with dredges is

emphasized. In addition, there remain significant data gaps

involving fish hearing thresholds (audiograms) for key target

species that would most likely encounter a dredging opera-

tion in an estuarine environment. Insufficient data are avail-

able with respect to the masking of sounds, which may

negatively impact communication and alter predator-prey

relationships, and the consequences of long-term exposures

of fishes to noisy environments. Whether or not underwater

sound represents a meaningful source of stress that could

negatively impact reproductive success and exacerbate

declines in stocks of commercial and recreationally impor-

tant species remains unknown. The degrees to which differ-

ent fish species would habituate to sounds emitted by

individual sound sources against a background of harbor-

wide sounds of natural and anthropogenic origin are also

largely unknown. Nevertheless, SPLs measured in the pres-

ent study were well below levels that would cause physical

injury to any fish species in the harbor. The same was true

for SPLs measured for the narrower frequency ranges

(50–1000 Hz and 100–400 Hz), which are known to be audi-

ble by the majority of fish species.

ACKNOWLEDGMENTS

This study is a joint effort sponsored by the USACE

New York District and the U.S. Army Engineer Research

and Development Center, Vicksburg, MI, supported by the

Dredging Operations and Environmental Research (DOER)

Program. The authors wish to express thanks to Kate Alcoba

and Ann Marie Dilorenzo of the Estuaries Section, Planning

Division, New York District; Timothy Lafontaine, Chief of

Caven Point Operations, New York District; and Captain

Mike Marcello and First Mate Ray Ryan, crew of the M/V

Hudson, for providing logistical and technical support for

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