noise reduction in hvac duct systems

NOISE REDUCTION IN HVAC DUCT SYSTEMS

Correctly designed HVAC duct systems address natural sound attenuating mechanisms

and requirements for sound control

November 2013

Sponsored by Kinetics Noise Control, Inc.

By Karin Tetlow

Use the following learning objectives to focus your study while reading this months Continuing

Education article.

Learning Objectives - After reading this article, you will be able to:

1. Summarize the basic concepts underlying HVAC duct system acoustics

2. Explain the principles employed in calculating sound values in HVAC duct systems

3. Identify natural attenuation mechanisms and how they impact HVAC duct design

4. Discuss the purpose of acoustical analysis and how it determines the need for specifying

different types of noise control silencers

Continuous or intermittent noise from an insufficiently designed HVAC system is disruptive and

distracting. In many instances design professionals focus on delivering the proper airflow, but ignore

acoustical aspects, often assuming that white noise from mechanical systems is beneficial because it

helps mask other noises within the work environment.

A noise reduction/abatement program is essential for many industries, especially manufacturing

facilities, because of safety and potential claims for hearing damage. In offices, acoustic problems are

both a leading source of employee dissatisfaction and the least addressed in office designs (General

Services Administration (GSA) workplace research (Sound Matters, January 2012)). Today, owners,

designers, developers and insurers of all building types are increasingly aware that HVAC noise can

contribute to litigious health issues for both occupants and neighbors. Addressing the acoustical

aspects of air handling systems is equally critical as meeting air flow heating and cooling design

requirements.

In order to analyze, evaluate and remedyif necessaryHVAC acoustic issues, several steps are

necessary. The first is to analyze the entire duct system. This begins by evaluating the natural

attenuation that occurs in HVAC systems. If undesired sound persists despite natural attenuation

efficient and cost effective acoustical remedies need to be specified. Leading noise control product

manufacturers offer no-cost design and engineering assistance in analyzing duct systems (see

example of a web-based program below), but it behooves design professionals to have an

understanding of the concepts underlying HVAC noise control. Such an understanding will also help

during the design phase when natural attenuation strategies can easily be incorporated.

Sound attenuators for HVAC systems

Photos courtesy of Kinetics Noise Control, Inc.

Use the following learning objectives to focus your study while reading this months Continuing

Education article.

Learning Objectives - After reading this article, you will be able to:

1. Summarize the basic concepts underlying HVAC duct system acoustics

2. Explain the principles employed in calculating sound values in HVAC duct systems

3. Identify natural attenuation mechanisms and how they impact HVAC duct design

4. Discuss the purpose of acoustical analysis and how it determines the need for specifying

different types of noise control silencers

Credits: 1.00 HSW

Basic Acoustic Concepts

Knowledge of duct system acoustics is an essential first step.

Sound Power and Sound Pressure

While sounding similar, they are different. Sound power is the amount of acoustical power generated

by a sound source in all directions. It is expressed in watts and cannot be directly measured but can

be calculated based on sound pressure measurements. Sound power level (abbreviated as PWL or Lw)

is a logarithmic measurement of the sound power.

Sound pressure is the fluctuation of the ambient pressure generated by the vibration of a surface that

creates a sound wave. It is measurable and expressed in the sound pressure unit pa, or pascal.

Sound pressure varies according to the magnitude of the sound source, the location of where the

sound is measured, the density of the medium the sound travels through and the directional path the

sound travels. The reason why the sound of an electric drill operating in different locations varies is

because the sound pressure varies, despite the fact that the sound from the drill is constant.

The range of sound power is huge: a faint noise at the lower level of human audibility is 1x (10)-12

(0.000000000001) watts, while the sound power of a space shuttle launch is 1 x (10)8 (100,000,000)

watts. Normal speech is 1 x (10)-5 (.00001) watts. A soft whisper is 1 x (10-9) (0.000000001) watts.

Similarly, sound pressure can range from 2 x (10)-5 (0.00002) pascal to 1 x (10) (100,000)5 pascal.

Normal speech at 3 feet is 2 x (10)-2 (0.02) pascal. For convenience and the fact that we cannot hear

variations in sound unless there is a large difference in sound pressure, a logarithmic definition of

sound power and sound pressure is used. To accommodate such a large span in values, a decibel

(dB) is defined as each successive whole number being 10 times larger than the previous number

based on a reference point of 20Pascals (20 x 10-6 pascal). This reference is considered the low

threshold of human hearing. Sound pressures are directly measured by a pressure transducer or

microphone, which converts pressure to sound pressure levels. Sound Pressure Level (abbreviated as

SPL or Lp) is a logarithmic measurement of the sound pressure.

Sound Pressure Level is defined by the equation Lp = 20 log10 (p/pref) which results in a dB value with

(pref=20Pascals). Sound Power Level is defined by the equation Lw = 10 log10 (p/pref) which results in

a dB value with (pref=10-12 watts).

Note that sound pressure levels are a measurement of pressure where sound power is a

measurement of energy.

The unit for both sound power level and sound pressure level is the decibel (dB), which expresses our

subjective reaction to noise, or how we perceive sound. Because sound levels are expressed in

logarithmic terms, a small change in decibel level is significant in how we hear and perceive that

sound.

Calculating sound pressure levels is critical to acoustical analysis because that relates to what sound

people will hear.

Since sound power (energy measurement of the sound source) and sound pressure (pressure

measurement of the sound we hear) are different, their values in decibels are also different. See

Table of Typical Sound Pressure Levels (Lp).

Table 2. Typical Sound Pressure Levels (Lp)

Table courtesy of Kinetics Noise Control, Inc.

Important Points to Remember

Never specify acoustic treatment without first analyzing both the supply and return air

critical duct system paths.

Doing so will result in an improperly designed system that introduces unnecessary

costs to the owner.

Avoid over-designing acoustic treatment by considering natural attenuation.

Often natural attenuation is sufficient to reduce noise levels to acceptable limits.

* A proper acoustic design goal is to achieve a comfortable acoustical environment not

to achieve the lowest possible sound level in a critical space.

* Do not over-attenuate high frequencies. A rumble noise problem will be created.

Do not over-attenuate low frequencies. A hissing noise problem will be created.

* Some background noise levels can be helpful for speech privacy and masking other

noise sources. Identify the use of areas to determine appropriate acceptable noise levels.

Combining Sound Levels

Because sound levels are logarithmic, and based on different reference points sound power and

sound pressure cannot be directly combined.

A doubling or halving of a sound pressure level will not be perceived as a doubling or halving the level

of noise we hear. Rather, since the values are logarithmic it only takes a small change in a sound

pressure level to perceive a large change in sound subjective perceptions:

A change of 3 dB is not detectable

Most people notice a change 5 dB

An increase +10 dB is perceived as a doubling of sound

A decrease of -10 dB is perceived as a halving of noise

Instead of employing extensive calculations in order to compute sound levels, i.e., what sound will we

actually hear if we are exposed to more than one sound source at the same time, it is possible to use

short-cut, rule of thumb guidelines:

Adding a sound source that is 10 dB less than a primary sound source will have no effect on the

overall level of sound perceived. The louder sound source masks the quieter source.

The addition of two equal sound sources will result in a noise level increased by 3dB.

The addition of sound sources which differ by 2 dB to 4dB will increase the louder noise source by

2dB

The addition of sound sources which differ by 5dB to 9dB will increase the louder noise source by

1dB

(See Table Simplified Decibel Additions).

Table 3. Guidelines for Simplified Decibel Additions


Example: Two sound pressure sources producing 100 dB each do not produce 200 dB when heard

together. Rather, according to sound pressure equations, the subjective combined sound pressure

level is 103 dB.

Frequency

The frequency of a sound is determined by the number of pressure fluctuations (sound waves)

produced per unit of time. Fluctuation in pressure determines the pitch. Sound frequency can be

correlated to pitch and is measured in Hertz (Hz), cycles per second. Humans have the capacity of

hearing sounds that range from about 20 Hz to 20,000 Hz. Middle C on a piano has a frequency of

approximately 260 Hz. One octave above or below has double or half the frequency of middle C.

Noise control is a function of the frequency of the noise source. Frequencies are important because

they can be grouped together into octave bands. The overall decibel level from combining sounds

from 8 octaves, or bands, can be calculated using the rule of thumb table. See the following four

tables for Octave Band Linear Addition

Table 4. Octave Band Numbers and their Center Frequencies

Table 5 and 6. Octave Band Center Frequency Linear Addition

Example of octave band center frequency linear

addition combining two noise sources using rule of

thumb guidelines from Table 3 Simplified Decibel

Additions.

Tables courtesy of Kinetics Noise Control, Inc.

Wavelength

The wavelength of sound is the distance over which the wave shape repeats. Low pitch sounds have

long wavelengths. High pitch sounds have short wavelengths. Wave length is important as it directly

relates to frequency and helps determine the proper product required to control the noise. The best

noise solution is to select a product that allows for multiple wavelengths to pass through the product.

The short wavelengths associated with higher frequencies (i.e., 1K , 2K , 4K and 8K Hz bands) of

Table 7 make it easier to attenuate noise at these higher frequencies.

Table 7. Wavelengths of Octave Band Center Frequencies


By Karin Tetlow

Loudness

Loudness, a subjective measure, is an attribute of auditory sensation. It is a function of frequency

and sound pressure level. However, the frequency of a sound will have substantial bearing on how

loud the receiver perceives it to be. Equal loudness contours have been developed which show the

increase and decrease in sound level energy required at various frequencies for the average human

to perceive sound in a particular frequency as sounding just as loud, say, as that of a 20 dB sound

pressure level at 1000 Hz.

Loudness is a function of sound pressure and frequency.

Provided by Kinetics Noise Control, Inc.

A-Weighting

Weighting is a useful means of adjusting a linear noise spectrum to closely reflect the human ear

response. An A-weighting filter on the sound meter is commonly used to emphasize frequencies from

1000 to 8000 Hz where the human ear is most sensitive, while attenuating very high and very low

frequencies to which the ear is less sensitive.

Human beings are so sensitive to sound in the 2,000 Hz to 4,000 Hz levels that A-weighting, the most

common weighting system, actually increases those levels to more closely reflect the human ear

response. The basis is equal loudness contours. Adjustments are made in all frequencies except

1000Hz. Once adjustments are made, decibel addition can be used to yield a single A-weighted

sound pressure level (dBA).

Table 9. Example of A-weighting using rule-of thumb guidelines.

Since A-weighting accounts for the sensitivity of human hearing and it is a relatively simple metric,

Occupational Safety and Health Administration (OSHA) has adopted it to set limits on noisy work

environments. Specific requirements and guidelines may be found in OSHA Standard 29CFR, Part

1910, Subpart G Occupational Health and Environmental Control.

Table 10. Federal Noise Limits (OSHA)


Duct System Acoustics

In order to determine if a duct system requires additional attenuation, all the duct components along

the critical duct path must be analyzed. This analysis starts with the fan sound power levels (per

octave band) and ends with the sound absorption or reflection effects of the receiving room.

Design professionals should be aware that there are multiple noise propagation paths that should be

considered: the noise that travels through both the supply and return duct system paths, and the

airborne noise that radiates away breaks out through the walls of the duct into the neighboring

spaces.

Fan Noise

Fans generate the primary noise source in HVAC systems. Fan manufacturers can provide laboratory

test results for the total sound power levels of the fan that includes the fan inlet, discharge, motor

drive train and casing radiated noise. Testing is conducted according to Air-Movement and Control

Association (AMCA) Standard 300, Reverberant and Room Method for Sound Testing of Fans.

AMCA, a trade organization comprising fan, damper and silencer manufacturers, specifies test

methods and requirements so that all members use the same rating guidelines. Similar test methods

are published by the International Organization for Standardization (ISO) and the American Society

for Testing and Materials (ASTM).

Sound level data from fan manufacturers may be either total fan sound power levels or separated

into inlet sound power level and discharge sound power levels. The most important data concerns the

noise propagating inside and along the duct system paths. If the sound power level is only given as a

total sound power level the rule of thumb is to subtract 3 dB from the total fan Lw for each octave

band. This is true for both stand-alone fans as well as for packaged air handling units.

Noise from Packaged Air-Conditioning Equipment

Design professionals should obtain manufacturers noise data from tests conducted in accordance

with Air-Conditioning and Refrigeration Institute (ARI) Standard 260.

Controlling Fan Noise

Fans should be selected to operate near their maximum efficiency. Fans are noisier when they are

either oversized and operating under design speed, or undersized and operating above design speed.

Duct system components can both attenuate and generate noise as sound and air propagate through

the system. The following discusses some of these more prominent affects.

Natural Attenuation

Single-wall duct system components provide natural sound attenuation. Several mechanisms such as

duct wall losses, elbow reflections, sound power splits and terminal end reflections may serve to

provide significant natural attenuation sufficient to reduce noise levels to acceptable limits. Natural

attenuation could also eliminate the need for expensive energy consuming supplemental products and

should be the first step in acoustically analyzing a duct system.

In-duct Single Wall Duct Attenuation

When sound travels through a duct system, some of the sound energy is transmitted to the duct

surface. This will cause that duct surface to vibrate and dissipate some of the sound energy. The

amount of energy dissipated through the walls of a duct is a function of the shape, size, length and

the frequency of the sound.

The reduction of in-duct sound energy, which is dissipated through natural duct attenuation is

expressed in units of (dB/ft). Long duct lengths have significant amounts of attenuated sound energy.

Natural duct attenuation assumes that the duct walls are massive enough to contain most of the in-

duct noise. Also, that the sound energy is transferred to the duct surface and converted into vibration

of the duct wall material. See the following two tables for natural sound attenuation through round

and rectangular ducts.

Table 11. Natural attenuation-straight circular sheet metal duct.


Table 12. Natural attenuationunlined rectangular sheet metal duct.


Elbow Attenuation Reflection

When sound energy enters an elbow, part of the sound wave is attenuated through reflection. The

amount of attenuation is proportional to the elbow bend angle and frequency of the sound. It is

expressed in dB/elbow per frequency.

Table 13. Insertion Loss (attenuation) of Radiused Rectangular Elbows (dB)

Sound Power Splits

The most significant mechanism of natural attenuation is sound power splits. Airborne sound power

energy in watts behaves the same as air when approaching a divided-flow fittingit divides or splits.

The division of sound energy at a junction will be proportional to the cross-sectional area of the

downstream path of the flow divided by the total of all the cross-sectional areas of the downstream

flow paths.

For example, sound energy moving through a 12-in. common duct with a 5-in. branch and an 11-in.

straight-through will be split into the following ratios: branch duct: 0.17; main duct: 0.83. Meaning

that only 17 percent of the sound energy entering the fitting will propagate down the branch and the

balance will travel straight-through.

Table 14. Natural attenuation sound power reduction.


Ab is branch cross-sectional area

As is the straight-through cross-sectional area (downstream)

Lw is sound power level in dBs.

Terminal End Reflection

Terminal end reflection is effective for attenuating low frequency energy. At the termination of a duct

path, a portion of low frequency power wave energy is reflected back into the duct.

Table 14A. Duct end reflection loss when duct is terminated in free space.


End reflection attenuation is virtually negated if a variable air volume (VAV) system, diffuser/grille is

placed at the duct opening.

Airflow-Generated Noise

Air flowing over duct surfaces generates noise. The following airflow conditions should be avoided:

* High face velocities:

Greater than 2,000 ft/min for rectangular duct

Greater than 3,000 ft/min for round and flat oval duct

It is recommended that acoustic analysis calculations include airflow-generated noise for all duct

elements, along the critical path. This includes the sound power levels of fan powered VAV boxes.

Generated sound power levels should be compared to the resultant levels after subtracting all

pertinent attenuation. If they are within 10 dB of the resultant value of any octave band, they will

contribute to the overall noise level.

Radiated Break-out Duct Noise

Break-out noise is noise that radiates from the duct system elements to surrounding spaces. It is an

important design parameter whenever a duct system runs through or over an acoustically sensitive

space. This noise will most likely be a problem if the localized in-duct sound power level at any

frequency, minus the duct wall transmission loss, exceeds or is within 3 to 5 dB of the NC level of the

acoustically sensitive space. This is best addressed during the initial design of the duct system.

Break-in Noise

Ambient noise transmitted into a duct is known as break-in noise. This noise can be ignored if in-

duct noise is 10dB or greater than the break-in noise. But at places where the fan and

aerodynamically generated noise are minimal, significant levels of break-in sound can radiate from

surrounding areas to inside the duct and propagate to critical places along the duct path.

Room Acoustics

After logarithmically combining the attenuated sound power levels with the airflow generated sound

power levels, the next step is to determine the sound pressure levels within the critical space (i.e.

office, conference room, sanctuary, etc.). This is done by taking the sound power levels remaining at

the supply air diffuser or return air grille and converting them to sound pressuretaking into account

the acoustic effects of the room (i.e. sound absorption and sound reflection). The resulting sound

pressure levels will be used to determine whether HVAC sound is within acceptable/specified design

criteria, or if attenuating strategies need to be employed.

Air Terminal Noise

Duct system paths generally terminate at a register, diffuser, grille or other device. Because terminal

devices generate noise as a result of air passing across them, it is necessary to calculate the sound

power level of that noise before addressing the entire room duct system. Noise generated from

diffusers/grilles is critical because it is the last noise source that affects sound levels in the critical

space.

Acoustic and airflow test data are usually generated in accordance with ASHRAE Standard 70 Method

for Testing for Rating the Performance of Air Outlets and Inlets. If the diffuser sound power is within

10 dB of the residual sound power in the duct, it will increase the sound power level emitted in the

space.

ASHRAE Room Effect Equation

As mentioned, sound power levels (Lw) and sound pressure levels (Lp) cannot be directly combined.

Therefore, in order to calculate sound pressure levels resulting from sound power emanating from

HVAC duct terminals, a procedure endorsed by ASHRAE may be used. The procedure assumes that

rooms have normal sound-absorbing surfaces and furnishings. Other formulas are available for special

rooms (i.e., rooms that are more absorptive, recording studios or rooms that are more reflective,

gymnasiums).

Design Criteria

Having accounted for fan sound power levels, natural duct and fitting attenuations, generated duct,

fitting and terminal noise, and converting sound power levels to sound pressure levels for a specific

space, the next stage is to determine whether the sound pressure level meets the acoustic design

criteria. Various criteria have been established for different space occupancy situations.

Indoor Noise Criteria (NC)

NC curves establish the desirable background sound pressure levels in a critical space. Because higher

frequencies are perceived louder than lower frequencies of the same dB level, NC curves allow higher

dB levels at lower frequencies. The NC criteria consist of a family of curves that define the maximum

allowable octave-band sound pressure level corresponding to a chosen NC design goal. They primarily

apply to the noise produced by a ventilation system, but they may be applied to other noise sources.

Measured Lps within a critical space can be plotted against standard NC curves to determine

compliance with specification or to rate the noise within the critical space. The appropriate NC curve

for a given space is that lowest NC curve that is closest to the highest noise spectrum sound pressure

level at a particular frequency.

Table 15. Typical indoor design acoustical goals using NC curves


NC Curve Plot

Allowable Sound Pressure Level (dB) per Frequency vs. NC Level


Methods of Acoustical Treatment

Acoustic remedies are required if the sound pressure levels within the critical space exceed the sound

pressure levels per frequency of the corresponding NC or RC curve. The most common acoustic

attenuation method for HVAC duct systems is to install a silencer (i.e., sound attenuator or sound

trap).

Silencers

A silencer attenuates sound when it is directly inserted in the ducted air stream (path). The silencer is

basically a series of perforated sheet metal baffles (rectangular silencers) or bullets (circular silencers)

placed inside a silencer single or double wall outer solid shell. The baffles/bullets are usually filled

with sound absorbing material.

Acoustic performance of duct silencers is generally described in terms of insertion lossthe

measure of noise level reduction determined by comparing the noise level without a silencer to the

noise levels with silencer. Since the silencer itself can generate noise (because it disturbs the airflow),

its self-generated noise has to be added to the attenuated sound level. Baffle and bullet-type

silencers block a portion of the air stream and will cause additional pressure drop (PD). Manufacturers

should always list values for insertion loss, regenerated noise and pressure drop.

An absorptive silencer is the most common type of silencer. It uses absorptive fibrous material

within sound baffles or sound bullet cavity with perforated sheet metal facings that allow sound

energy to pass through and be absorbed by the fibrous fill.

Rectangular and circular silencers have solid rounded nose cones on the air inlet end and tapered tails

on the air discharge end of each baffle/bullet to minimize silencer pressure drop. The tapered end of

the silencer baffle/bullet allows for static regain to occur, thereby offering the lowest silencer pressure

drop for a particular level of attenuation. This is important because silencer pressure drop is in direct

relation to a duct systems lifetime energy costs.

Kinetic cut view of rectangular silencer showing internal acoustic media, nose cone,

perforated baffle face and outer solid silencer shell

Image courtesy of Kinetics Noise Control, Inc.

Manufacturers offer a selection of standard or custom engineered silencers that will satisfy the

requirements of project specific applications.

Circular silencer with sound absorbing bullet and outer shell supply and return air systems

Photo courtesy of Kinetics Noise Control, Inc.

Rectangular Silencer with circular end caps and absorptive baffles.


Circular silencer with absorptive outer casing and no sound absorbing bullet.


T-rectangular elbow silencer with absorptive side baffles and curved air passages.


Circular silencer with sound absorbing bullet and no absorptive outer casing


Circular silencer with internal sound absorbing bullet and absorptive outer shell.


Rectangular elbow silencer with extended body (casing)


Guidelines for selecting silencers

The following information is needed to determine silencer Insertion Loss (IL) and Pressure Drop (PD)

performance:

Insertion Loss (IL) requirements

Forward (supply) or reverse (return) airflow direction

Forward flow: airflow in the silencer is in the same direction as the noise propagation

Reverse flow: airflow in the silencer is in the opposite direction to the noise propagation

Maximum allowable silencer PD

Airflow (cfm)

Length, width and height restrictions for where the silencer is to be installed, connecting duct

dimensions.

The IL requirements, quoted for octave bands 1 through 8 may be determined by:

Comparison to an existing specification

Duct system analysis.

Application-dependent requirements, such as elimination of low frequency or tonal noise

Example of Web-based Silencer Analysis

By incorporating the most up-to-date design analysis algorithms recognized by ASHRAE, this

example from a leading manufacturer of a no-cost software service provides a complete

eight-octave band acoustic analysis The program produces a report that displays whether or

not a design meets the required noise level within the critical space. If the sound levels are

not met, the program will automatically choose a duct silencer that meets the required

attenuation while still meeting the user specified constraints such as the applications

allowable height, width, and length and pressure loss.

Conclusion

Proper acoustic analysis of an HVAC duct system is an important part of any design. Design

professionals should always set specific acoustic requirements and analyze the duct system design to

determine how much unwanted acoustic energy (noise) is produced by the system. A correctly

performed acoustic analysis will determine exactly how much noise treatment is required to provide a

quiet system with the lowest initial and operating costs.

Celebrating over 55 years, Kinetics Noise Control has extensive

experience in designing and manufacturing innovative products to

control sound and vibration. Established in 1958 as engineers

focusing on sound and vibration control, Kinetics pioneered

development of pre-compressed, molded fiberglass pad isolators

that would be incorporated into an innovative new floor isolation

system. Previous trade names of Kinetics Noise Control include

Consolidated Kinetics and Peabody Noise Control.

www.kineticsnoise.com

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