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Acoustics in HVAC

HVAC Clinic

Table Of Contents

Introduction ............................................................................................................................................ 3 Fundamentals ......................................................................................................................................... 3 Sound Rating Methods .......................................................................................................................... 9 Acoustical Analysis ............................................................................................................................. 16 Equipment Sound Ratings .................................................................................................................. 21 Testing Equipment with Multiple Paths ............................................................................................. 24

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WN Mechanical Systems Acoustics of 25

Introduction

Acoustics has become an increasingly important topic for designers of heating, ventilating and air conditioning systems. Numerous studies have shown the importance of ensuring proper acoustics with respect to occupant comfort in a space. ASHRAE states in their guide “A Practical Guide to Noise and Vibration Control for HVAC Systems” that “after temperature problems, excessive HVAC system noise levels are responsible for more complaints than any other aspect of the office environment.” LEED, Leadership for Energy Efficient Design, recently published that noise problems are the NO.1 complaint in buildings. Additionally, LEED allows designer achieve points for commercial buildings, hospitals and schools by complying with specific noise standards.

Fundamentals Sound is the audible emissions that result from the vibration of molecules within an elastic medium. The medium may be either a solid or a fluid. With respect to building HVAC systems, the elastic medium may be either the structure or airborne pathways. For structural sound to become audible sound, it must become airborne at some point. This is generally referred to as transmission or breakout noise. Both of these concepts will be discussed in more detail later. Noise is distinct from sound. Noise is the unwanted sound. People generally object when noise interferes with concentration, speech or sleep. Sound

Audible emissions from a vibrations of molecules within an elastic medium

Generated by a vibrating solid (surface) or fluid (air)

In HVAC applications, may be either structural or airborne

Noise is undesirable sound

Airborne sound is transmitted away from a vibrating medium through the transfer of energy from one air molecule to the

adjacent air molecule. The vibrating medium alternately compresses and expands the air molecules. The alternating

compression and expansion of the air molecules takes the form of a sine wave (figure 1). The amplitude, or height,

represents pressure. As the pressure increases, the sound increases.

Figure 1. Sound and Frequency

The amount of time it takes for a single cycle of compression and expansion of the air molecules is called the frequency.

Frequency is measured as:



Equation 1. Frequency

The wavelength of the sound is the measure of the amount of linear distances a single cycle measures (figure 2). The

wavelength is determined by:

Equation 2. Wavelength

The speed of sound is a function of the physical property in which the sound is traveling. For air, density will fluctuate

with temperature and altitude. However, the change is speed as a function of temperature and altitude is relatively

minimal. Thus the speed of sound is generally assumed to be 1,127 feet/sec in air.

Figure 2. Wavelength

For example, if the frequency of a given sound source is 250hz, the wavelength is:

Sound generally does not consist of a single frequency. A sound source that consists of only a single frequency, as

shown in figure 2, is called a pure tone. Generally, sound is made up of several different frequencies and amplitudes, all

generated simultaneously. This type of sound is far more common and is referred to as broadband sound (figure 3).



Figure 3. Broadband Sound

When analyzing broadband sound, it can be useful to plot frequency versus amplitude (figure 4). This type of chart

allows the designer to see if any of the broadband sound sources stand out, in terms of amplitude, compared to adjacent

sound sources. When a sound source creates a significantly higher amplitude sound at any given frequency compared

to the adjacent sound, that frequency is said to produce a tone. Inspecting figure 4, the sound source produces a tone

at 100 hz.

Figure 4. Tones

Tones, due to their nature, are generally considered objectionable sound. Sounds produced by HVAC equipment are

generally both broadband and nature and often produce tones. Thus, an understanding by HVAC designers of

broadband sound and tones is essential.

The human ear can discern sounds in the range of 20 to 16,0000 Hz. However, HVAC designers generally are

concerned with sounds in the range of 45 to 11,200 Hz. While this range is slightly smaller than what the human ear can

discern, this range is still very broad. For example, if we were to measure every frequency between the range of 45 to

11,200 Hz, the sample would require 11,155 data points. Clearly, this would vastly complicate the process of evaluating

the sound performance of HVAC systems.

To make this process more manageable, the sound data is generally broken down into distinct ranges or bands. These



smaller ranges are called octave bands. An octave band is defined such that its higher frequency is exactly two times its

lower frequency. The octave band is defined by its center frequency, which is calculated by taking the square root of the

lowest frequency times the highest frequency. These bands are divided into the range most relevant to HVAC

designers, 45 to 11,200 Hz. The result shown in figure 5. Eight distinct octave bands are created, with center

frequencies of 63, 125, 250, 500, 1000, 2000, 4000 and 8000 Hz.

Table 1. Octave Bands

Octave Band

Center Frequency

(Hz) Frequency Range

(Hz)

1 63 45 to 90

2 125 90 to 180

3 250 180 to 355

4 500 355 to 710

5 1000 710 to 1400

6 2000 1400 to 2800

7 4000 2800 to 5600

8 8000 5600 to 11200

Each octave band is the logarithmic sum of the sound level at each frequency within the band. This results in a

compressed representation of the values within the band into a single value.

The primary downside to an octave band analysis is that you can lose important characteristics of the broadband sound.

For example, in figure 4, the tone show at 100 Hz is lost. This tone could produce objectionable which would otherwise

be lost in an octave band analysis.

Sound is measured using two distinct methods. The first is sound pressure, or the sound heard by the human ear. The

second is sound power, which is the sound produced by sound generating equipment. Sound pressure is measured in

pascals (Pa). Sound power is measured in watts (W). While both measures are distinct, they are commonly confused.

Both measures commonly use the term decibel (dB). However, sound pressure is very distinct from sound power.

Sound power is not affected by the environment, whereas sound pressure is.

A good analogy to explain the difference between sound power and sound pressure is the common light bulb (figure 5).

A light bulbs power output is measured in watts. It is the actual power produced by the device. However, the

environment can have a dramatic effect on the perceived brightness of the light bulb depending on the environmental

conditions. For example, a light bulb in a small, reflective, lightly colored room is going too observed as being much

brighter than the same light bulb in a large, non-reflective, darkly colored room. The difference in perceived brightness

is exactly analogous to the sound pressure heard by a sound producing piece of equipment. The nature of the

environment, such as distance, hardness of adjacent surfaces, objects in the space, and a number other variables will

have a dramatic effect on the actual sound pressure the user experiences.



Figure 5. Sound Analogy

The human ear is a very sensitive device. The highest amplitude of noise the human ear can withstand without damage

is about 1,000,000,000 times greater than the lowest amplitude it can discern. Due to this incredibly high range, a

logarithmic scale is generally used to measure the amplitude of sound. The unit of measure, for both sound power and

sound pressure, is the decibel (dB). The decibel is a dimensionless scale. It is defined as ten times the logarithm to the

base ten of the highest measured quantity divided by a reference quantity. The reference quantity is different for both

sound power and sound pressure.

Equation 3. Decibel

A logarithmic scale of 10 times to the base ten converts what would be a very unmanageable range (1,000,000,000 to 1)

into a very manageable range (table 2). For example, 10xlog1010 = 10. However, 10xlog101,000,000,000 = 90.

Table 2. Base Ten Log Scale

Ratio Log10 10 x Log10

1 0 0

10 1 10

100 2 20

1,000 3 30

10,000 4 40

100,000 5 50

1,000,000 6 60

10,000,000 7 70

100,000,000 8 80

1,000,000,000 9 90

10,000,000,000 10 100



The reference values are established based on the threshold values for hearing. This equates to a value of one

picowatt, or 10-12 watts, for sound power (equation 4).

Equation 4. Sound Power

Similarly, a value of 20 micropascals (µPa) or 2x10-5 pascals is used as the reference value for sound pressure

(equation 5).

Equation 5. Sound Pressure

Note that a multiplier of 20 is used in the sound pressure equation and a multiplier of 10 is used in the sound power

equation. The disparity between the two multipliers exists because sound power is proportional to the square of the

sound pressure.

As discussed earlier, sound is generally broadband in nature. That is to say, sound generally consists of multiple

sources with multiple waves. However, adding sound power or sound pressure levels is somewhat complicated due to

the logarithmic scale used to measure sound sources. In order to mathematically add sound sources, the logarithmic

scale must first be converted back into the ratios of sound intensity, adding the values, and then converting back into a

logarithmic scale. Figure 6 was created to simplify the task of adding sound sources.

Figure 6. Addition of Sound



For example, if two sound sources differ by 6 dB at 250 Hz, the logarithmic sum of the two sound sources will add 1 dB

to the overall sound level. Note that the logarithmic sum of two identical sound sources adds 3 dB to the overall sound

level. This is a useful characteristic to remember whenever two identical sounds producing pieces of equipment (i.e.

chillers) are placed together.

Sound Rating Methods

The response of the human ear to sound is critical to the understanding of acoustical rating methods. All rating methods

attempt to replicate the ears response to sound levels. However, the sensitivity of the human ear varies by magnitude

and frequency. In addition, sound is function of perspective. One person’s hearing can vary dramatically from anther

person’s perception of the same sound.

Loudness is a function of not only sound pressure, but frequency. The human ear, as a sensor organ, is more sensitive

to higher frequency sounds than lower frequency sounds. Also, the human ear’s sensitivity at varying frequencies

changes with sound pressure. Figure 7 illustrates the loudness contours of the human ear. Loudness contours attempt

to map the level at which the human ear interprets a constant sound level over varying frequencies.

Figure 7. Loudness Contour

For example, a sound level of 10 dB at 500 Hz is interpreted by the human ear as being of equal loudness as 30 dB at

100 Hz.

Note that the loudness contours decrease substantially between 20 Hz and 200 Hz. This demonstrates the increased

sensitivity of the ear to higher frequency sounds. In addition, the contours are flatter above 90 dB. This demonstrates

the ears decreased sensitivity at higher sound pressure levels.

In addition to the human ear responding to sounds as a function of frequency and amplitude, designers must also be

mindful of the ears response to tones. Recall that tones are sounds that stand out in terms of amplitude compared to the

adjacent sound sources. Tones can be challenging to represent using most sound rating methods.

Single Number Rating Methods

Considerable research has been done in an attempt to replicate, with electronic sound measuring equipment, the sound

perceived by the human ear. The human ear interprets sound in terms of pitch and loudness. Pitch is a subjective

quantity that is primarily based on frequency, but is also depended on sound pressure level and composition. Pitch is

not measured, but is described with terms like tenor, bass, and soprano. Electronic sound measuring equipment

measures sound in terms of pressure and frequency. The goal of all of the single number rating methods has been to

develop a system that expresses both the intensity and the quality of a sound. Utilizing single number rating methods,

sound targets can be established for different environments. These targets allow specifying Engineers to develop



targets based on space type. These targets then can be confirmed through measurement. For example, a designer of

a school can specify that the sound level must be within 35 dBA (as specified in ANSI S12.60 standard). The Engineer

can then require that the equipment that is provided meet the scheduled sound level and guarantee that level within the

occupied space.

The most commonly used single number rating descriptors are the A-weighting method, the noise criteria (NC) method,

and the room criteria method (RC). All three methods utilize octave band sound data. However, all three methods loose

valuable information regarding the character of the sound. Despite their shortcomings, the single number rating

methods described in this section can be a valuable tool utilized by designers to help ensure proper acoustic

environment within a space.

Perhaps the most commonly used single number rating method is the A, B, or C weighting method. The weighting

curves shown in figure 14 allow the designer to compensate for the varying sensitivity of the human ear to different

frequencies as a function of volume. The A weighting method is used for low volume (or quiet) environments. The B

weighting method is used for medium volume environments. Finally, the C weighting method is used for high volume (or

loud) environments. Recall, that at higher sound pressure levels, the loudness contours are relatively flat. This

accounts for the flatter representation of the C vs B vs A contours shown in figure 8.

Figure 8. dB ABC Criteria

In order to determine the overall dBA, dBB or dBC value, the user first determines the sound level at each octave band

and then deducts the appropriate value from figure 14. The last step involves logarithmically summing the result of the

eight octave bands, resulting in the overall dBA, dBB or dBC value.

For example, assume we want to calculate the dBA value for the sound source represented in table 3.

Table 3. dBA Weighting Example

Octave Band

Center Frequency

Actual Sound Pressure (dB)

1 63 70

2 125 60

3 250 58

4 500 55

5 1000 50

6 2000 48

7 4000 45

8 8000 40



First, we add or subtract the values at each octave band from the appropriate contour shown in figure 14. The resulting

values are shown in the yellow column of table 3. Next, we need to solve the antilogarithm for each sound pressure

value. Recalling that a decibel is:

Equation 6

If we solve for the ratio of the measure value to the reference value, we find:

Equation 7

Thus, the logarithmic sum of each of the octave bands becomes:

Equation 8

Table 4. dBA Weighting Example

Octave Band

Center Frequency

Actual Sound Pressure (dB)

A Weighting Factor

A Weighted Sound Pressure 10dB/10

1 63 70 -26 44 25118.86432

2 125 60 -16 44 25118.86432

3 250 58 -9 49 79432.82347

4 500 55 -3 52 158489.3192

5 1000 50 0 50 100000

6 2000 48 1 49 79432.82347

7 4000 45 1 46 39810.71706

8 8000 40 0 40 10000

Sum 10dB/10 517403.4119

10 x log10(Sum 10dB/10) 57.13829288



The result is a total sound level of 57 dBA.

A-weighting ratings should only be applied to sound pressure readings, not sound power. A-weighting ratings are often

applied to define outdoor sounds measurements (lot line ordinances) and schools. For schools, many regions follow the

ANSI S12.60 standard which states that classrooms should be kept at 35 dBA. The reason that criteria is in dBA instead

of NC (a single number rating method to be discussed later in this clinic) is that dBA is used in areas where speech

interference reduction is important. dBA has the added advantage of being easily measurable using relatively

commonly available field sound measuring devices.

Noise Criteria Curves (NC) are probably the most commonly utilized of the single number rating methods. NC curves

are based on the loudness contours for the human ear. The graph in figure 9 represents several NC curves ranging

from NC 15 to NC 70. Note that the NC curves are very similar in shape and slope to loudness contours. NC curves, like

the loudness contours they represent, slope downward to reflect the increased sensitivity of the ear at higher

frequencies.

Figure 9. NC Curves

In order to determine an NC level in a space, the designer plots the octave band sound data on an NC chart. The point

that crosses the highest NC curve represents the NC level in the space.

Example 1.

Assuming the measured data in a space is represented by table 5, the data is superimposed on an NC chart (figure 10).

Table 5. NC Example

63 Hz 125 Hz 250 Hz 500 Hz 1000 Hz 2000 Hz 4000 Hz 8000 Hz

50 48 40 32 30 20 15 15

Examining the data on an NC chart, we see that at 125 Hz, the point intersect the purple NC 30 line. At all of the other

octave bands, the points are at a value less than NC 30. Thus, the sound level in the space corresponds to an NC level

of 30.



Figure 10. NC Plot Example

NC levels have been established for various types of commercial spaces (figure 11). Caution should be used whenever

following published NC guidelines for various occupancies. An owner should always be consulted whenever a space

requires stringent acoustic guidelines.

Figure 11. Recommended NC Levels

The RC single number rating method, much line NC curves, are based on the loudness contours experienced by the

human ear. However, the RC single number rating method gives some additional information regarding the character of

the sound.

A discussed earlier, single number rating methods loose valuable information regarding the character of sound. Too

much high frequency sounds results in a hiss while too much low frequency sound results in a rumble. The RC rating

improves upon the NC method by identifying this type of lost sound character. The RC single number rating method

comprises of two descriptors. This first descriptor is a number which represents the speech level interference level (SIL)

of the sound. The SIL, much like the NC method, are based on the loudness contours of the human ear. The second

descriptor identifies the character of the sound as the human ear would perceive it. There are four possible identifiers

for the second descriptor. Those are:



N identifies a neutral spectrum

R Indicates a rumble

H represents a hiss

RV denotes a perceptible vibration

Unlike NC curves, RC curves are based on octave band sound data from 31.5 hz to 4,000 Hz. An NC chart is shown in

figure 12.

Figure 12. RC Chart

The process for calculating an RC rating is:

1. Plot the octave band sound data from 31.5 Hz to 4000 Hz

2. Determine the SIL by calculating the average value between the 500 Hz, 1,000 Hz and 2,000 Hz octave bands.

3. Draw a line (C) with a slope of -5 dB per octave that passes through the calculated SIL at the 1,000 Hz octave

band.

4. Between 31.5 Hz and 500 Hz, draw a line (D) that is 5 dB above the reference line (C). Between 1,000 Hz and

4,000 Hz, draw a second line (E) that is 3 dB above the reference line (C). These two boundary lines (D & E)

represent the maximum permitted deviation to obtain a neutral (N) rating.

5. Judge the character of the sound by observing how the sound spectrum deviates from the boundary lines drawn

in step four. Use the following criteria:

a. Neutral (N) is if the sound level in each octave band between 31.5 Hzx and 500 Hz is below line D and

the sound level in each of the octave bands between 1,000 Hz and 4,000 Hz is below line E.

b. Rumble (R) is if the sound level in any octave band between 31.5 Hz and 500 Hz is above line D.

c. Hiss (H) is if the sound level in any octave band between 1,000 Hz and 4,000 Hz is above line E.

d. Perceptible Vibration (RV) is if the sound level in the octave bands between 31.5 Hz and 63 Hz falls in

the shaded regions (A and B).

i. Region A indicates that there is a high probability that noise-induced vibration levels in the

lightweight wall and ceiling structure will be felt. There is a high probability of rattles in light

fixtures, door and windows.

ii. Region B indicates noise induced vibration levels in lightweight walls and ceiling structures may

be felt. There is a slight probability of rattles in light fixtures, door and windows.

The RC rating is the combination of the SIL and the value calculated in the second step and the letter descriptor



determined in step five.

Example 2.

Determine the RC sound criteria based on the sound data shown in table 6,

Table 6. RC Example

31.5 Hz 63 Hz 125 Hz 250 Hz 500 Hz 1000 Hz 2000 Hz 4000 Hz

52 50 48 40 32 25 20 15

1. Plot the sound data between 31.5 Hz to 4000 Hz (figure 19 green line).

2. The arithmetic average of the sound at 500 Hz, 1,000 Hz and 2,000 Hz octave bands is 27 dB.

3. Draw a line with a slope of -5 dB per octve that passes through 27dB at 1000 Hz (yellow line in figure 19).

4. Between 31.5 Hz and 500 Hz, draw a line (D) that is 5 dB above the reference line (C) at 27.5 Hz (red line).

Between 1,000 Hz and 4,000 Hz, draw a second line (E) that is 3 dB above the reference line (C) at 27.5 Hz

(blue line). These two boundary lines (D & E) represent the maximum permitted deviation to obtain a neutral (N)

rating.

5. The sound will produce a rumble, being that the sound data at 125 Hz exceeds the line at D.

Figure 13. RC Example Plot

Thus, the RC rating for the example is a 27(R).

The RC rating method is used less frequently than other single number rating methods for the simple reason that sound

data below 63 Hz is hard to obtain accurately. However, the RC rating method does have the advantage of indicating

the character of the sound which is lost with the other rating methods.

Two other common single number rating methods descriptors are the phon and sone. The pone is a measurement,

expressed in phons, where the pressure level of a standard sound at 1000 Hz is considered equally loud (figure 14). For

example, a sound which produces 50 dB at 1,000 Hz is considered to have a loudness of 50 phones so long the sound

source does not produce any octave bands above that loudness curve. The loudness curves are the same as the

loudness contours of the human ear.



Figure 14. Phon

The phone scale is logarithmic. In contrast, the sone is the linear equivalent to the phon. The principle behind the sone

is that the sone scale is established to be linear when compared to the response to the human ear. Thus, two sone

would be twice as loud as a single sone.

Phons and sones are seldom used in modern acoustic analysis. However, some fan equipment is still rated in sons.

Caution could be used when comparing equipment based on sons. Multiple methods exist for calculating sons, many of

which provide differing results.

While single number rating methods simplify the process of implementing acoustical design goals, they still loose

important information regarding the character of the sound. For this reason, there is no substitution of a basic octave

band analysis. An octave band analysis, compared to the single number rating methods, preserves most of the

character related to the sound. While important information, like tones, are often still lost with an octave band analysis,

it remains the best alternate available for many designers.

Acoustical Analysis

The goal of any acoustical analysis is to obtain a background sound level that is appropriate for the applied space or

condition. For example, an acoustical analysis may specify a 35 dBA sound level at a lot line so as not disturb

neighbors. Or, an acoustical analysis may specify an NC 30 level for a classroom in order to maintain speech levels and

limit distraction.

Whatever the goal, the designer should consult the appropriate standards and local codes in order to maintain the

proper acoustic levels for the application. The most common methods are utilized by most designers today are the NC

and RC methods. ASHRAE recommends target RC ratings for various types of spaces (refer to ASHRAE handbook –

Applications Table 43 in Chapter 46 of the 1999 edition). In addition, the NC acoustical guidelines referenced earlier in

this clinic (Figure 11) are a good starting point.

Performing an acoustical analysis involves more than just picking a single number descriptor for the desired space.

Performing a proper acoustical analysis involves identifying the sources, converting the source sound pressure into

sound power, determining the paths various paths that the sound may travel in order to reach the space and finally, how

the space will influence the perceived noise that reaches the receiver. This type of analysis is known as a

source-path-receiver analysis (figure 15).



Figure 15. Sound Path Receiver Analysis

Figure 15 demonstrates a relatively simple source-path-receiver model. The source of the sound is a packaged unit on

the roof. In this particular example, one of the multiple paths is the airborne sound delivered by the supply fan through

the ductwork to the conditioned space (illustrated by the red arrows). This is generally referred to as an airborne path.

The noise waves reverberate within the conditioned air that is being delivered to the space. Other paths are present

between the source and the receiver. For example, the roof in figure 15 presents a likely path for which the sound to

travel.

The process of identifying the various paths between the source and receiver is perhaps the most challenging aspect of

performing an acoustical analysis. There are three different types of sound paths:

Airborne

Breakout

Transmission

The airborne path is the path where sound travels with or against the direction of airflow. In an HVAC system, sound

waves travel along this type of path through the supply ductwork, return ductwork, or an open plenum. The breakout

path is the path where sound breaks out though the walls of the duct into the space. Finally, the transmission path is the

path where the sound travels through solid surfaces such as walls, floors and ceilings.

While the system shown in figure 15 likely contains multiple paths, single source paths do commonly exist in HVAC

systems. For example, the airborne path between an outdoor chiller and a lot line or an indoor wall mounted fan coil and

an occupant in that same space would be examples of single path systems (figure 16).

Figure 16. Single Path Sound Sources



However, HVAC systems often present multiple paths between the source and the receiver. Reexamining the previous

example of an air handler placed in space adjacent to the receiver (figure 17), multiple paths can be identified.

Figure 17. Multiple Path Example

First, casing radiated sound from the packaged unit (1) will transit through the roof and ceiling into the occupied space.

Second, the supply fan discharge airborne noise is transported down the supply ductwork to the space (2). Next, return

duct will convey the inlet fan sound via the airborne path to the space (3). Finally, both the supply airborne and return

airborne, as it travels through its associated ductwork, may generate noise via transmission through the duct (4). This

path is referred to as duct breakout. These four paths, are typical of what is experienced with most centralized air

handling equipment.

There are three important considerations to remember when identifying the sources and paths when performing a

source-path-receiver acoustical analysis.

1. One piece of equipment may contain several sound sources. For example, a typical packaged rooftop unit will

have one or multiple fans, compressors and condenser fans.

2. Sound may travel from a source to a receiver along multiple paths. This was demonstrated in the previous

example.

3. The total sound heard by the receiver is the sum of all of the sounds from all sources and all paths. The noise

heard by the receiver is the logarithmic sum of all of the paths.

Figure 18. Multiple Path Sound Sources



Only when all of the paths have been identified, they then can be individually modeled to determine the contribution of

each path to the receiver. The components of the path are called the elements. Elements may be straight duct, elbows,

junction, diffusers and silencers. The path also includes the receiver sound correction. The receiver sound correction is

the correction for the amount of sound entering a room and the sound pressure at a given point in the room where the

receiver observes the noise (figure 19). The correction takes into account distance, the absorptive qualities of the room,

and the type and hardness of the surrounding surfaces.

Figure 19. Receiver Sound Correction

Elements may cause either attenuation or regeneration. Attenuation refers to the reduction in sound level as the sound

travels along the path as noise energy is absorbed and converted into heat energy. Straight ducts, elbows and silencers

are good examples of elements that attenuate sound (figure 20). Regenerated sound results from components of the

duct system that create turbulence and thus generate sound energy. Turbulence is caused by an abrupt change in

airflow direction or velocity with an associated pressure drop. Elbows, junctions, dampers and even silencers are all

elements that can produce regenerated sound.

Figure 20. Attenuation & Regeneration

An understanding of sound transmission is critical to properly performing a source-path-receiver analysis as transmitted

sound will invariably be a contributing factor to the total noise in the occupied space. As sound energy strikes a surface,

some energy is reflected, some energy will be absorbed through heat energy and finally some energy will be transmitted

through the material. Generally, the goal is to reduce the amount of sound transmitted through the material. The

amount of energy transmitted through the material is a function of the material itself, the thickness of the material and

the frequency of the sound. For example, materials that are thick (like masonry block) or dense (like concrete) are

generally better at reducing transmitted sound than materials that are lightweight and flexible. In addition, the frequency

of the sound impacts the transmissibility of the material. High frequency sound is easier to reduce than low frequency

sound. This is because more waves will be contained within the depth of the material than low frequency longer

wavelength sound.



Figure 21. Sound Transmission

The ability of a material or element to reduce sound is rated in terms of:

Insertion Loss

Noise Reduction

Transmission Loss

Insertion loss is measured as the difference in sound pressure measured in a single location with and without the

element located between the source and the receiver. Most sound attenuators are rated in terms of insertion loss.

Noise reduction is the difference between sound pressure measurements taken on each side of a barrier. Finally,

transmission loss is the proportion of the ratio of sound power level on the receiver side of a barrier to the sound

pressure on the source side of the barrier. Insertion loss and noise reduction are both rated in terms of dB reduction.

Once all of the paths and elements have been identified, the amount of noise that actually is transmitted to the space

can be calculated. ASHRAE collected and developed numbers prediction equations for path components in HVAC

systems and published their algorithms in the Algorithms for HVAC Acoustics. Similar information can be found in the

National Environmental Balancing Bureau (NEBB) publication titled Sound and Vibration Design and Analysis. Finally,

many manufacturers will publish sound data for various elements (VAV boxes, attenuators, etc).

Figure 22. ASHRAE Predictive Equations for Path Components

Finally, once all of the sources, paths, elements of the path and the receiver sound correction have been identified, the



total sound can be determined. Recall, the total sound will be the logarithmic sum of all of the sources contributing to the

noise in the occupied space (figure 23).

Figure 23. Total Sound

Graphically plotting each of the sound sources along with the total sound pressure can provide helpful information about

the character of the sound. In addition, it becomes easier to identify which sound source needs to be attenuated and at

which bands should the total sound pressure be unacceptable. Should a single number descriptor, such as NC level be

required, the total sound level can be determined to be acceptable or unacceptable.

Equipment Sound Ratings

Sound pressure can be directly measured, while sound power cannot. However, because the environment can

influence sound pressure data, equipment is rated in terms of sound power. Sound power levels are determined by

measuring the sound pressure in an environment with known acoustical characteristics. The known effects of the

environment on the sound pressure data is then added back to the readings. The result is a tangible and repeatable

sound power data for the measured piece of equipment.

A free field is a homogenous isotropic medium that is free of any boundaries. A free field generally cannot occur directly

next to the measured equipment (also known as a near field). This non-uniformity of sound within the near field is

attributable to the non-uniform shape of most equipment. In order to be considered a free field, the sound pressure

measured must be equal at equal distances from the center of the device (figure 24). A free field must be devoid of any

obstructions, such as a parking lot or meadow.

Figure 24. Free Field



Doubling the distance from the center of a free field would spread the sound energy over four times a much surface

area. This relationship between distance and sound energy can be described using the equation:

Where:

Lp2 = Sound pressure level at distance r2

Lp1 = Sound pressure level at distance r2

r1 = Distance from the source where Lp1 was measured

r2 = Distance from the source to where the sound pressure Lp2 is desired

Using the expression above, it can be shown that by doubling the distance from the sound source in a perfect near field

results in a 6 dB reduction in sound pressure level. This equation can be very useful when determining the lot line sound

adjacent to an installed piece of equipment.

The opposite of a free field would be an ideal reverberant field. An idea reverberant field is defined as a field in which the

sound pressure is equal in all directions, regardless of distance (figure 25). Reverberant fields exist in rooms with

reflective walls, floors and ceilings. As the sound source is placed in an enclosed room with hard reflective surfaces, the

sound waves bounce back and forth, creating an equal sound energy field. Reverberant rooms are designed and

qualified for the purposes of measuring the sound emitted by small to medium sized equipment.

Figure 25. Reverberant Field

In most equipment rooms, a semireverberant field exists. A semireverberant field is an area which exhibits both the

characteristics of a reverberant field and a near field. An example of this would be a chiller in a mechanical room next to

a wall (figure 26). The area directly between the chiller and the wall would act very similarly to a reverberant field.

However, the area on the other side of the chiller would act like a combination of a free field (the sound reduces with

distance) and a reverberant field.



Figure 26. Semireverberant Field

Understanding the concept of a semireverberant field is important when measuring sound in equipment rooms. Often,

there will be areas that act like a reverberant field an others that act like a semireverberant field. This can have a

dramatic effect of the sound pressure readings in the room. This should be taken into account by the designer if they

choose to later verify that the sound in the room conforms to the design intent.

The most common methods of measuring sound power are the reverberant room method and the free field method.

Formal written standard qualify such test facilities and methods in order to promote the uniformity of data between

different manufacturers. These organizations include the Air Conditioning and Refrigeration Institute (ARI), the Air

Movement and Control Association International (AMCA) and the American Society of Heating, Refrigerating and Air

Conditioning Engineers. These standards ensure objective comparisons between similar equipment.

As mentioned earlier, the reverberant room method uses a room with hard reflecting surfaces with known acoustical

characteristics. A reverberant room creates a uniform sound field in all directions by reflecting and mixing the sound

waves (figure 27). In a reverberant room, the sound pressure should be virtually the same in all locations within the

room.

Figure 27. Reverberant Room

The reverberant room method is often used to measure the sound power for small to medium sized equipment,

including water cooled chillers, fans, air handlers, terminal equipment and diffusers.

The free field method is commonly used for equipment which is too large for a reverberant room. The free field method

uses either a sound absorbing room (anechoic room) or a large parking lot. These environments approximate the

characteristics of sound in a free field. In a free field, sound pressure is equal in equal distances from the equipment.



Thus, sound pressure readings are taken in a hemispherical surface surrounding the equipment (figure 28).

Figure 28. Free Field Methods

The free field method generally includes cooled chillers, cooling towers and may types of rooftop air conditioning

equipment.

Testing Equipment with Multiple Paths

Measuring the sound power for air conditioning equipment that moves air through a duct system is relatively involved.

An air handler typically has a wide range of operating conditions, at various airflows and static pressures. In addition, a

multitude of configurations and sound generating components may exist for a single size air handler or packaged unit.

Finally, the tested equipment may contribute noise along multiple paths. The noise may come from a ducted return, an

un-ducted return, supply fan or casing radiated paths. All of these configurations, the airflow and static pressure

capacities must be tested.

Equipment that generates sound along multiple paths is generally tested under four difference conditions. First, the

equipment is located outside the reverberant room with the fan discharge ducted into the reverberant room (figure 29).

Figure 29. Measuring Discharge Sound

This test determines the discharge sound power of the equipment. Next, the equipment is tested with just the inlet

ducted to the reverberant room (figure 30).



Figure 30. Measuring Inlet Sound

This measures the ducted inlet sound power. Third, the equipment is placed in the reverberant room with both the

discharge and inlet ducted to the outside of the room (figure 31).

Figure 31. Measuring Casing Radiated Sound

This measures the casing radiated sound for the equipment. Finally, the inlet ducting is removed and the test is

performed again. This determines the inlet plus casing radiated sound pressure levels (figure 32).

Figure 32. Measuring Inlet + Casing Radiated Sound

Again, each of these tests needs to be repeated across the entire range of airflows, static pressures and possible air handler configurations. Clearly, this can be a daunting and expensive proposition for many air handler manufacturers. Nevertheless, the uniformity of these tests ensures accurate comparisons for designers among multiple manufacturers. ARI Standard 260 uses the reverberant room method and requires the entire air handler to be tested, not just the fan. All common configurations and components should be included. For this reason, ARI standard 260 ensures the most accurate, repeatable results for HVAC system designers.

acoustics in hvacwnmech.com/class/acoustics.pdf · ashrae states in their guide “a practical...

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