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Acoustics and Biology
Acoustics• loudness (intensity) and pitch (frequency)• How to read a spectrogram
Use of sound by marine animals• Predation and defense• Echolocation• Communication and social interaction
Signal-to-noise ratio
Man-made sounds and their effects on animals
T
a
Anatomy of a sound wave
a = amplitude of waveT = period of wavef = frequency = 1/Tλ = wavelength (= cT = c/f, where c is sound speed)
Amplitude determines sound level pressure, this determines loudness
These waves have the same frequency and wavelength but different amplitude
aa
a = amplitude
Loudness (Amplitude, sound level)
SL(dB)=20 log10(P/Pref)SL(dB)=10 log10(I/Iref)
Chart shows loudness in dB of some familiar sounds
Sound levels in air and water have different reference levels, so 0 dB (air) ≈ 26 dB (water)
Frequency determines “pitch”
T
T
These waves have the same amplitude but different period, frequency, and wavelength
Frequency f = 1/T, wavelength λ= c/f
Larger instruments produce lower frequencies
Pitch (frequency)
Instrument dBBass drum 35-115Piano 60-100Trumpet 55-95Violin 42-95Voice 40-90
Marine animal sounds are made up of multiple frequencies
The sound spectrum gives the pressure level at each frequency
Intensity pressure2
SL [dB] = 10 Log10(I/I0)SL [dB] = 20 Log10(P/P0)
Worcester & Spindel 2005
Spectrogram shows how sound spectrum changes over time
Some fish use sound for courting and as a fright response
A fish example of sound use: Atlantic Croaker
Snapping shrimp make noise to stun their prey.They create a cavitation bubble that “snaps” as it collapses.
http://stilton.tnw.utwente.nl/shrimp/
claw crab
An invertebrate example: snapping shrimp
Toothed (odonticete) whales• Smaller (1.5 to 17 m long)• Social• Most are not migratory • Chase and capture individual
fish, squid, crabs• Echolocate, communicate
Baleen (mysticete) whales• Larger (15 to 30 m long)• Often solitary• Long annual migrations• Feed on aggregations of
krill, copepods, small fish• Communicate long-distance
Toothed (odonticete)
whales
Baleen(mysticete)
whales
Toothed whales Baleen whales
Baleen whales
Toothed whales
Dolphins live in social groups that stay together 5-10 years. They have “signature whistles” that can be used to recognize individuals at distances of >500 m.
Time (s)
Fre
quen
cy (
Hz)
Social calls - sound for communication
Whale can determine distance, angle, size, shape, etc. from sound echoes
Echolocation using echoes from sound pulses or clicks
Mellinger 2007
Echolocation frequencies
Toothed whales
Baleen whales do not echolocate. Why not?
1. They don’t produce high enough frequenciesBaleen whales produce low-frequency sounds with longwavelengths. Wavelength determines the minimumecho detection distance.
Frequency f (Hz) Wavelength λ (m)
10 150
100 15
1,000 1.5
10,000 0.15
100,000 0.015
Minimumecholocation
frequency
Food too far away
OK
Toothed whale prey:•Squid and large fish•More likely to be solitary•Good acoustic targets(squid pens and fish swim bladders have density different from water)
Baleen whale prey:•Plankton and small fish•More likely to aggregate•Poorer acoustic targets (density similar to water)
Baleen whale prey
Toothed whale prey
2. Baleen whale prey (krill, copepods) are poor acoustic targets
-Hydrophones and 3D accelerometers in a waterproof, pressure-resistant case with suction cups-Sneak up on whale, attach D-Tag-Record audio, pitch, roll, heading, depth-Tag pops off, floats to surface 18 h laterMark Johnson with D-Tag
A cool invention for listening to whales: acoustic whale tag
Toothed whale foraging:Beaked whales dive deep to find prey
Natacha Aguilar de Soto
Peter Tyack et al.Yellow indicates echolocation
Baumgartner and Mate 2003
Colors: copepod concentration (#/m3)
—: whale trajectory
--: bottom of mixed layer
: Times of visual contacts
: Times of CTD+OPC cast
(OPC = Optical Plankton Counter)
Baleen whale foraging: Right whales dive to bottom of the mixed layer where plankton are most concentrated
Blue whales migrate and need to communicate over long distances
High-frequency sounds are absorbed more quickly
Absorption of sound in SOFAR channel
Because baleen whales have long, solitary migrations, they need to use low frequencies to stay in communication.
Because toothed whales move in groups, they can use high frequencies without losing communication.
Transmission loss: Sound signal loss of intensity due to cylindrical spreading, spherical spreading, and absorption
Bluewhale
Dolphins
Signal-to-noise ratio (SNR)
SNR in decibels indicates how much of the signal can actually be heard over the background noise level.
For communication, need a minimum SNR of 3 to 5 dB.A good SNR is 20 to 30 dB.
A negative SNR(dB) indicates no signal gets through.
Baleen whales
Toothed whales
Seals, sea lions, and walruses
Manatees and dugongs
Echolocation (toothed whales)
earthquake rainfall
Marine mammal sound levels are generally between 100 and 200 dB
Airgun10 to 500 HzUp to 232 dB
Outboard engine6,300 Hz
Commercial Ship10 to 20,000 Hz
Low-Frequency Active Sonar100 to 500 Hz230 to 240 dB
These are loud enough to damage tissues and cause hearing loss
These add constant background noise
Man-made noise in the ocean
Before motors~30 dB
After motors~75 dB
Since the invention of propeller-driven motors (~150 years ago),• Background noise level in the ocean has increased by ~45 dB• Lowest background noise f has dropped from ~100 Hz to ~7 Hz
Before motors~100 Hz
After motors~7 Hz
Blue whale song20 Hz, ~155 dB
Pre-motor noise level30 dBWhale song stays above ambient noise level for ~2,000 kme.g. San Diego to Seattle(area ≈10,000,000 km2 )
Current noise level75 dBWhale song stays above ambient noise level for ~60 kme.g. New Brunswick to NYC(area ≈10,000 km2)
Bluewhale
Can use transmission-loss curves to calculate the effective communication range
Range of effective communication for blue whale singing at 20 Hz and 155 dB
Range before mid-1800s
Current range(yes, that tiny speck)
Potential effects of man-made sounds on marine mammals
• Disruption of feeding, breeding, nursing, acoustic communication and sensing
• Psychological and physiological stress
• Temporary or permanent hearing loss or impairment
• Death from lung hemorrhage or other tissue trauma
Noise-induced mass strandings
Mass strandings associated with Navy sonar activity The Bahamas (2000):14 beaked whales, 1 spotted dolphin, 2 minke whalesBleeding in ears
The Canary Islands (2002):14 beaked whalesGas bubbles and bleeding in multiple organs
Mass strandings associated with air gunsTasmania and New Zealand (2004): 208 whales and dolphins
Senegal and Madagascar (2008): > 200 pilot whales and melon-head whales
Humans use acoustics to understand whales. Are the whales doing the same to us?
Captive beluga imitates human voice!
A great source of information on sound in the ocean:http://www.dosits.org/