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Engineering GuideSilencers & Panels

Please refer to the Price Engineers HVAC Handbookfor more information on Noise Control.


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All Metric dimensions ( ) are soft conversion . Copyright Price Industries Limited 2011.Imperial dimensions are converted to metric and rounded to the nearest millimeter.M-2

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Silencers & PanelsEngineering Guide

Terminology

Near FieldThe sound eld immediately surroundingthe source where the sound pressure isinuenced by the radiation characteristicsof the sound source. In this eld the soundpressure and the particle velocity are outof phase. Negative Air FlowSee Reverse Air Flow.NoiseAny unwanted sound or meaningless soundof greater than usual volume.NR (Noise Reduction)A value that represents the difference insound pressure level between any twopoints along the path of sound propagation.The unit of measurement is decibels (dB).NRC (Noise Reduction Coefcient)A measure of the acoustical absorptionperformance of a material, expressed asa single value. This value is calculated byaveraging the materials sound absorptioncoefcients at 250, 500, 1000 and 2000 Hzand rounding to the nearest multiple of 0.05.OctaveThe interval between two sounds having afrequency ratio of 2. There are 8 octaves onthe keyboard of a standard piano.Octave Band

A segment of the frequency spectrumseparated by an octave.Pascal (Pa)A metric unit of measurement for pressurethat is equivalent to one Newton per squaremeter (N/m 2).Pink NoiseA sound with a frequency spectrum thatis at in logarithmic scale, and has equalpower in bands that are proportionallywide. When compared with white noise, thepower density decreases by 3 dB per octave.PitchA subjective sound quality that is determinedby the frequency of a sound wave, or acombination of waves, that is placed on ascale extending from low to high.Positive Air FlowSee Forward Air Flow.Pressure Attenuation (Pa) CodeA code used to dene the % of free areaof a silencer cross-section. A decrease infree area typically results in an increasedinsertion loss and increased pressure dropacross the silencer.Pressure DropThe difference in static pressure from theinlet to the outlet of a system component.

AcousticsThe science that is related to the production,control, transmission, reception, and effectsof sound.Acoustic MediaThe sound-absorbing material used insideabsorptive type silencers and plenum walls.Ambient NoiseAn all-encompassing noise that is presentin a given environment and is comprisedof many sounds from many sources, withno particular sound being dominant. Alsoknown as background noise.A-Weighting

A weighting network that is widely used,often abbreviated dBA, dB(a). It has aprescribed frequency response that bearsa close relationship to loudness judgmentsof the human ear.Background NoiseNoise sources other than the noise sourcethat is of interest or being measured. Alsoknown as ambient noise.Breakout NoiseNoise that travels through the wall of anenclosure or duct work into an occupiedspace.Broadband NoiseNoise with components over a wide rangeof frequencies.Decibel (dB)A unit for expressing the ratio of twoamounts of acoustic signal power equal to10 times the common logarithm (to the base10) of this ratio.Dynamic Insertion LossThe insertion loss at a given air ow directionand velocity. The insertion loss of a silencervaries depending on whether the sound istraveling in the same or opposite directionas the air flow. Silencer performancechanges with the absolute duct velocity.The unit of measurement is dB.Flanking

Is when the walls of the ductwork on theupstream side of a silencer are readilyexcited into vibration, which is thentransmitted through the silencer walls andinternal components and re-enters thedownstream duct path as noise.Forward Air FlowA condition that exists when airbornesound and air ow are moving in the samedirection. Also known as positive air owor supply air ow.Free FieldThe sound eld where only directly radiatedsound waves moving away from the soundsource are present. This condition exists

when a sound source is located a largedistance from reecting surfaces, or whennearby surfaces are highly absorbent.FrequencyThe number of times in one second thata passing wave repeats itself. The unit offrequency is Hertz (Hz), which correspondsto 1 cycle per second.Fundamental FrequencyThe lowest frequency in a harmonic series.Generated NoiseThe sound power produced by air owingthrough a silencer at a given velocity anddirection (forward or reverse). The unit of

measurement is decibels (dB).HarmonicAn integer multiple of the fundamentalfrequency.Hertz (Hz)The unit of measurement for frequency, orthe number or cycles per second.High Transmission Loss (HTL)Occurs when a heavier material orcombination of materials create a higherresistance to sound transmitting throughthem.Insertion Loss (IL)The decrease in sound pressure level orsound intensity level measured at a receiverwhen the silencer or a sound-attenuatingelement is inserted into the path betweenthe source and the receiver. The unit ofmeasurement is decibels (dB).IntensitySee Sound Intensity.Intensity LevelSee Sound Intensity Level.Media ProtectionA liner that is applied between the berglassmedia and the air stream of the silencer toprotect the media from erosion due to highvelocity air ow and airborne contaminates.Module Factor

A value that is used to dene the width ofa single module.NC (Noise Criteria)A weighted value obtained from a seriesof curves that covers a spectrum of octavebands with center frequencies ranging from63 Hz to 8000 Hz. Sound levels are plottedat each of these frequency bands, with thehighest point on the NC curve, regardless offrequency, being the NC rating for the pieceof equipment. These values are commonlyused by manufactures to rate equipmentbecause this system factors in the humansensitivity to each of the frequency bands.The single number value is known as theNC level.
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Terminology


Pure ToneA single frequency sound. A sound for whichthe sound pressure is a simple sinusoidalfunction of the time.Random NoiseAn oscillation whose instantaneousmagnitude is not specied for any giveninstant of time.RC (Room Criteria)A value obtained from a series of curvesthat cover a spectrum of octave bands withcenter frequencies ranging from 16 Hz to4000 Hz. This rating system was specicallycreated to establish design goals for HVACsystems, as it provides guidance formaintaining a certain level of backgroundsound for masking. The curves are used toprovide a rating of an occupied indoor spacewith a single number value indicated as theRC level.Return Air FlowSee Reverse Air Flow.Reverberant FieldThe sound eld that consists of both directlyradiated and reected sound waves.Reverse Air FlowA condition that exists when airborne soundand air ow are moving in the oppositedirection. Also known as return air ow ornegative air ow.SabinA unit of measurement for acousticabsorption in relation to the absorptionby one square foot of a perfect absorptivematerial.SIL (Speech Interference Level)The arithmetic average of sound levels(dB) in the 500 Hz, 1000 Hz, 2000 Hz, and4000 Hz octave bands. This rating dealswith the ability of speech to be heard andunderstood.Silencer BafeA silencer component that is comprisedof perforated metal and acoustic media orresonant chambers.Silencer BankA complete silencer assembly thatcan contain one or multiple silencercomponents. The bank dimensions are theoverall dimensions of the complete silencer,and typically match the duct dimensions.Silencer ComponentA factory assembly that contains one ormore bafes and may or may not be coupledwith other components in the eld to createa larger silencer bank.Silencer ModuleA portion of the silencer that consists ofone air path located between two acoustic

baffles. A silencer can have multiplemodules within one component.SoneA subjective unit of measurement forloudness for an average listener that isequal to the loudness of a 1000 Hz soundthat has an intensity 40 dB above thelisteners threshold of hearing.Sound(A) An oscillation in pressure, stress,particle displacement, particle velocity, etc.in an elastic or partially elastic medium,or the superposition of such propagatedalterations.(B) An auditory sensation evoked by theoscillation described above.Sound IntensityThe amount and direction of ow of acousticenergy at a given position (W/m 2).Sound Intensity LevelThe ratio of sound intensity, which is obtainedby taking ten times the common logarithmof a given sound intensity in reference to10 -12 W/m 2. The unit of measurement is dBre 10 -12 W/m 2.Sound PowerThe total sound energy radiated by a sourceper unit time. The unit of measurement isthe watt (W).

Sound Power LevelAlgebraically dened as LW=10log(W/Wref)where typically Wref = 10 -12 watts. Thisequation indicates that sound power levelis ten times the logarithm (to the base 10)of the ratio of a given sound power to areference power (typically 10 -12 watts). Theunit used to express sound power level isdecibels in respect to the reference soundpower (e.g. 84 dB re 10 -12 watts).Sound PressureThe instantaneous difference between theactual pressure produced by a sound waveand the average or barometric pressure ata given point in space. The unit of measureis typically Pascals (Pa).

Sound Pressure Level (L P )Algebraically dened as LP=20log(P/Pref)where typically Pref = 20 x 10-6 Pascal. Thisequation indicates that sound pressure levelis twenty times the logarithm (to the base10) of the ratio of a given sound pressureto a reference pressure (typically 20 x 10 -6Pascal). The unit used to express soundpressure level is decibels in respect to thereference sound pressure (e.g. 87 dB re 20x 10 -6 Pascal).Sound Transmission Class (STC)A single gure rating system designed togive an estimate of the sound insulation

properties of floors, walls, ceilings,windows, doors, etc.Static Pressure (P s )The pressure exerted equally in all directionson a system, regardless of air ow. This canalso be dened as the difference betweenTotal Pressure (P t) and the Velocity Pressure(P v), expressed as P s = P t-Pv.

Static Pressure LossThe difference in static pressure from theinlet to the discharge of a duct element.Static RegainAn increase in system static pressure thatoccurs in correlation with a decrease invelocity pressure caused by an increase inthe cross-sectional area of the ductwork.Supply Air FlowSee Forward Air Flow.System EffectA deviation from the cataloged performancefor a piece of equipment. This is caused bythe difference between how the productwas tested and how the product is actuallyinstalled in the system.ToneAn intense sound that is concentrated ata single frequency and typically containsharmonics at multiples (2x, 3x, 4x, etc.) ofthe fundamental frequency.Total Pressure (P t)The combined effect of velocity pressureand the static pressure. This can also bedefined as the algebraic sum of StaticPressure (P s) and Velocity Pressure (P v), P= P s + P v.Transmission Loss (TL)A reduction of sound levels as a result ofpassage through an obstruction such as awall, partition, or ductwork. These valuesare expressed with a unit of decibels (dB).VibrationA periodic motion or displacement in a solidelastic medium.Velocity Pressure (P v)The pressure exerted by a moving uidwhich acts only in the direction of uid ow.This can also be dened as P v = P t - P s.WavelengthThe linear distance between two successivepoints of a sound wave which are separatedby one period.White NoiseA sound with a at frequency spectrum inlinear scale. It has equal power in any linearband and at any center frequency having agiven bandwidth.


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Wavelength

Comparision of Wavelengths for Different Frequencies

Fundamental Concepts

What is Sound?Sound is a propagating vibrationaldisturbance or wave in an elastic medium(solid, liquid or gas). Sound is mostcommonly thought of as being transmittedin air and detected by a persons ears.What is Noise?Noise is defined as any unwanted orundesirable sound. While sound in generalis not necessarily a problem and may evenbe desired in certain situations, when itis unwanted or annoying, we refer to itas noise. The proper application of NoiseControl Products such as silencers andpanels are discussed in this engineering

guide.FrequencyFrequency refers to the number of cyclesper second of an oscillation. For example,middle C on a piano represents a pure toneat 250 Hz.The audible frequency range for humansis about 20 Hz to 20000 Hz. This largefrequency range is broken down intooctave bands to make measurement andanalysis more manageable. Each octaveband is identied by the center frequency.The eight commonly used octave bands forHVAC noise are 63 Hz, 125 Hz, 250 Hz, 500Hz, 1000 Hz, 2000 Hz, 4000 Hz, and 8000 Hz.For more detailed analysis, octave bands

can be sub-divided into 1/3 octave bands.Sound Pressure LevelsSound is propagated as a pressureuctuation above and below the ambientatmospheric pressure. The amplitude ofthese uctuations is proportional to howloud something is perceived to be.The range of pressure uctuations that canbe detected by the human ear is extremelylarge. The decibel (dB) scale is used todescribe sound pressure levels in acousticsbecause it reduces the scale of values toa more manageable range. Refer to theSound Pressure Level chart (right) to seecorresponding dB levels of common noises.Decibel level is dened as:

Eq.1

where:

LP = Sound pressure level p = Root mean square value of

acoustic pressure uctuation,Pa

p ref = Reference quantity definedas the threshold of hearing,20uPa

Subjective and Objective Sound Changes

Subjective Change Objective Change

Much Louder More than 10 dB

Twice as Loud 10 dB

Louder 5 dB

Just Perceptibly Louder 3 dB

10 20 30 40 50 60 70

dB Level

80 90 100 110 120 1300

T H R E

S H

O L D

O F H E A R I N

G

T H R E

S H

O L D

O F P A I N

140

200 2,000 20,000 200,000 2,000,000 20,000,000 200,000,000

Pa Level

20

F o r e

s t

L i b r a r y

O f

c e

H e a v

y T r u

c k

J a c k H

a mm

e r

J e t T a k

e - O f f

Sound Pressure Levels



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Heat Source Sound SourceSound Power LevelsThe sound power level of a source is ameasure of the acoustical energy emittedby a source. Sound power is independentof the environment and only depends onthe operating conditions of the equipment.Equipment is rated in terms of sound powerlevels to facilitate calculation of expectedsound levels due to the environment andallow for fair comparisons between differentsound sources. Sound power is dened as:Eq.2

where:

LW = Sound power level

w = Acoustic energy radiated by source,Watts

wref = Reference power dened as 10 -12WattsSound power levels cannot be measureddirectly. Values are typically calculatedusing sound pressure levels measured in alaboratory environment with known soundabsorption characteristics.There are several general techniques fordetermining sound power levels whichcannot be measured directly. Sound powerlevels are calculated from sound pressureor intensity measurements in a controlledenvironment.A good analogy for the distinction betweensound power and sound pressure is anelectric room heater. The heater emits aspecic amount of heat energy, measured inwatts. The amount of heat energy producedis a power rating that is independent of thesurroundings. However, the temperaturemeasured in the room will not only dependon the power rating of the heater, but also onthe distance from the heater, heat absorbedby the walls, and heat transfer through thewalls, windows, etc. Similarly, a soundsource has a certain amount of soundenergy or power level. The sound pressuremeasured in the room will be a function ofthe sound power generated by the source,as well as the distance from the source tothe measurement point and the amount ofsound energy absorbed and transferred bythe walls, windows, furnishings, etc.

Various HVAC Sound Sources

Fans Rooftop Mounted Air Handling Units VAV Systems Air Flow Generated Noise Room Air Devices (GRDs) Compressors, Chillers, and Air-Cooled Condensers Emergency Generators


Fundamental Concepts


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Source, Path, ReceiverA key approach to noise control is tobreak each problem into its fundamentalcomponents. This is the Source Path Receiver concept. Every noise controlproblem can be broken down into a sourcecreating the noise, a path transmitting thenoise, and a receiver hearing the noise.There are different options available forreducing the noise at each component.The most desirable and effective option isto treat the noise at the source. Selectionof quieter equipment can eliminate manynoise problems before they even begin.Treatment options along the path are thenext best option and can include silencers,barriers, absorption, lagging, or other

options. The last resort is typically treatmentat the receiver, with hearing protection forloud occupational exposure.Sound SourcesWherever possible, it is important toobtain source sound power levels from thesupplier. The sound power levels should bederived from tested performance accordingto a recognized standard applicable to thatequipment. Fans are commonly testedper AMCA 300 or 320, Air Handling unitsaccording to AHRI 260, terminal unitsper ASHRAE 130. Testing according to arecognized standard ensures that the resultscan be compared and evaluated betweenmanufacturers. Estimates of power levelsshould only be used as a last resort wheremanufacturer supplied data is not available.Common sound sources in an HVAC systemare fans, element generated ow noise, VAVcomponents, and mechanical equipment.PathOnce the sources are known, the position inrelation to the receiver can be determined.This will allow the path by which the noise istransmitted to be identied. It is importantto note that noise typically travels throughmultiple paths, both airborne and structural,so possible paths must be acknowledgedand evaluated appropriately. Additionally,once one path is treated, another path maybecome dominant and any further treatmentto the rst path will not be effective.The Sound Paths diagram (right) illustratespossible transmission paths for the soundand vibration from the source to thereceiver. In this example, the sound sourceis an air handler that contains both a fanand a compressor; the receiver is the humanoccupant in the adjacent room.Silencers are an effective and economicalmeans of incorporating noise control intoa system to reduce noise from duct-bornepaths. The main types of silencers, includingabsorptive or dissipative, lm lined, andreactive or packless, are discussed in thefollowing sections.


Sound Paths

Path A Structure-borne path, where the vibration of the air handler passesthrough the oor.Path B Airborne path, where noise from the equipment radiates directly to thereceiver.Path C Duct-borne path, where noise from the equipment radiates throughthe walls of the ductwork or passes through the supply/return ductwork into theoccupied space.

ReceiverAfter the source and path have beenidentied, it is a matter of assessing thereceiver and determining what sound levelsare considered acceptable so that the mosteffective and economical solution to thenoise problem can be selected. Calculationof the sound pressure level at the receiveris the final component to the source-path-receiver concept, and combinescontributions from all path types. Themain consideration for specifying a targetor design sound criteria is the intended

Approach to Noise Control

use of the space. The most commonlyreferenced source for typical designcriteria for an indoor occupied space is theASHRAE Applications Handbook. Outdoorsound criteria are typically specied by localordinance in terms of overall A-weightedvalue specied at a property line.


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Silencer Types and Applications

Absorptive Packless

Film Lined Absorptive

AbsorptiveAcoustic silencers, which are also known as dissipative silencers,utilize sound-absorbing media to attenuate sound levels. As thenoise inside the duct passes through the silencer, the acoustic energyenters the bafes through the holes in the internal perforated metalliner. This perforated metal liner protects the acoustic media frombeing eroded by the air at high velocities, but has a large enoughfree area to be acoustically transparent. Once inside the bafe, theacoustic energy interacts with the absorptive media, which typicallyconsists of strands of glass ber. The friction between the acousticenergy and the glass bers converts the acoustic energy into heat,thereby reducing the amount of acoustic energy and decreasingsound levels at the discharge of the silencer. For silencers thatwill experience high gap velocities when installed, a sheet ofberglass cloth can be placed between the perforated metal andthe glass ber media for protection against erosion with no effecton the acoustic performance of the silencer. Typical absorptiveapplications include rectangular, elbow, and circular ductwork, airhandling units, generator silencers, ventilation, fan plenums, andgeneral HVAC supply, return and exhaust.Film Lined AbsorptiveFilm lined absorptive silencers work on the same principle as thestandard absorptive silencers. However, these silencers have a thinpolymer lm liner wrapped around the acoustic media to protectthe media from airborne contaminants and moisture that may bepresent in the air stream of some systems. It is important to notethat the addition of the lm liner will negatively affect the soundabsorbing capabilities of the acoustic media. To minimize theseeffects, a thin acoustic standoff liner is placed between the lm andthe perforated metal. Typical lm lined silencer applications includelaboratories, cleanrooms, and hospitals where contaminants andmoisture in the air stream may be of concern.

PacklessPackless silencers, sometimes referred to as reactive silencers,contain no absorptive material and are constructed solely of solidand perforated sheet metal. Attenuation of sound is achieved byusing multiple resonant chambers of varying size located behindthe perforated metal liner. As sound passes through the silencer,the acoustic energy dissipates in a manner similar to the Helmholtzresonator concept, which results in decreased sound levels at thedischarge of the silencer. Since these silencers are tuned to anarrow band of frequencies, it is more difcult to achieve signicantattenuation over broadband frequencies. Packless silencers aretypically specied when glass ber media is not acceptable or whenit is necessary to sterilize the entire duct system. This may includelaboratories, cleanrooms, hospitals, or electronics manufacturing.Effect of Acoustic Media

The graph below is a comparison of the different silencer types.The graph shows that the standard absorptive silencer has thebest overall performance across the full range of frequencies. Theaddition of a lm liner such as Mylar signicantly decreases theeffectiveness of the absorptive silencer, but can be greatly improvedat the higher frequencies by placing an acoustic standoff betweenthe perforated metal and the lm liner. The graph also shows theunique performance characteristics of the packless silencer, whichutilizes tuned resonating chambers instead of acoustic media. Thestandard absorptive silencer provides a fairly predictable broadinsertion loss curve across the frequency ranges, while the packlesssilencer peaks in the mid frequency ranges. This peak is due to thesize and shape of the internal chambers that are tuned to removethat frequency.

Comparison of 5 ft Long Silencer Performance


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RectangularRectangular silencers are the standardfor silencing noise transmitted throughductwork. Their straightforward designand relative low cost make them the rstchoice for the highest sound attenuation andlowest pressure drop on the air distributionsystem. They are available with a numberof options that allow them to be integratedinto any HVAC system with ease. A widerange of designs with low to high pressuredrop and a large selection of media typescan be applied to systems ranging from0-2500 fpm.Elbow

Elbow silencers have many of the sameperformance characteristics of rectangularsilencers, but are extremely versatile and anexcellent choice for systems where straightlengths of ductwork are not available. Elbowsilencers perform at or above the levels ofrectangular silencers with a small increasein pressure drop on the system, and can becongured to suit most duct sizes withoutthe use of transitions.CircularCircular silencers are an excellent solutionwhen round ductwork is utilized in thesystem. They eliminate the need for square-to-round transitions that cause undesirablepressure drops and system effects. Circularsilencers are available in a wide range ofsizes which allow for varying ranges ofattenuation and pressure drop.Axial FanAxial fan silencers are designed to be closecoupled to an axial fan. These silencers areengineered to provide noise attenuationat the source and improve aerodynamicperformance at the inlet and discharge ofthe fan. The axial fan silencers center podis properly sized to help reduce the pressureloss over the fan hub. A discharge axial fansilencer decelerates the air to maximize thestatic pressure regain.CustomA standard silencer conguration cannot

always be adapted to work in everysituation. In these cases it is necessary todevelop a unique silencer that meets therequirements of the application. Whennoise control products must be appliedto a system with limited space or whenthe silencer is directly coupled to a fan, acustom silencer can be designed to providethe required performance. Typical customdesigns include transitional silencers, T or Zshaped silencers, and silencers that requirea size that is not available in the standardmodel line.


Rectangular

Elbow

Circular

Axial Fan

Custom


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Air Transfer ProductsAir transfer silencers allow air to move fromone area to another without compromisingthe acoustic integrity of the wall. Thesesilencers are typically not ducted and arepart of the wall construction.Cross-TalkCross-talk silencers are used to transferair to adjacent areas while providingthe necessary attenuation to preventtransferring unwanted noise. They arecommonly used to prevent speech intrusioninto adjacent rooms. Cross-talk silencers aremanufactured in a variety of congurationsthat can be used to solve many different

acoustic air transfer problems.Thin Line Return DissipaterThin line return dissipaters utilize multiplebafes lled with acoustic media that arestaggered in a frame that is only 4 in. thick.They are great for applications in place ofstandard transfer grilles to reduce soundtransmission between adjacent spaces orto attenuate return air noise.Acoustic LouversAcoustic louvers can be used for allowing airto ow through an opening while providingthe necessary sound attenuation. Acousticlouvers use acoustic media to absorb thesound energy and a perforated metal toprotect the media from erosion by the airow. Acoustic louvers are designed to beaerodynamic to help minimize the pressuredrop.Acoustic Panels and EnclosuresUsed for controlling excessive noise fromequipment, QUIET-LINE acoustic panelsand enclosures are available in many shapesand sizes to meet specic noise reductionrequirements. Typical applications includebarrier systems, custom enclosures,and inlet and exhaust plenums; withapplications ranging from large units for gasturbines to smaller units for pumps, motors,compressors or any other unwanted HVACor industrial noise source.

Cross-Talk

Thin Line Return Dissipater

Acoustic Louvers

Acoustic Panelsand Enclosures


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CasingThe outer shell of the silencer is constructedof solid sheet metal. This shell can befabricated from various materials and rangein thickness from 24 gauge to 10 gauge.A heavier gauge casing is often used toimprove the transmission loss propertiesof the silencer, which reduces the amountof noise that can breakout through the wallsof the silencer and enter into the occupiedspace. A heavier gauge material will alsomake the silencer more robust to provideresistance to the elements or other sources.Standard silencer casings are manufacturedfrom hot-dipped galvanized steel with acoating weight of G-90 (G-90 designatesa zinc coating of .90 oz/ft 2). Additionalavailable casing materials are 304 and 316stainless steel and aluminum.Perforated LinerThe perforated material on the interior ofthe silencer is specially designed to provideprotection to the internal acoustic media,but is acoustically transparent, allowingsound energy to penetrate the acousticmedia.Acoustic MediaIn absorptive silencers the acoustic mediais the internal brous material that absorbsenergy from the sound waves as they passthrough the silencer. The selection of thismaterial may be based on requirementsincluding local and international standards,the application parameters, and thecustomers preference.LinerVarious silencer liner materials can beinstalled depending on the application.Liners like fiberglass cloth are used toprotect the internal acoustic media fromerosion in applications with high velocityair ow. Polymer lm liners such as Mylaror Tedlar can be used to protect againstmoisture or other contaminants that maybe found in the air. It should be noted thata berglass cloth liner will not affect theacoustic performance of the media, whileliners such as Mylar will cause decreasedinsertion loss levels.Acoustic StandoffAcoustic standoff is required in silencerswith lm liners in order to improve theperformance of the unit. By providing a gapbetween the perforated liner and the lmliner, the acoustic standoff greatly increasesthe absorptive characteristics of lm linedsilencers.Construction StandardsAll components used in the constructionof Price acoustical products conform toASTM E84, UL723, and NFPA255 for a amespread classication of 25 and a smokedevelopment rating of 50.

Silencer Construction

Casing

Perforated Liner

AcousticMedia

AcousticStandoff

Liner


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Silencer Dimensional Considerations

The dimensions of a silencer depend on theavailable space, performance requirements,and dimensions of the ductwork upstreamand downstream of the silencer.Silencer width and height dimensions aretypically specied to match the ductwork.The main benets of matching the silencerdimensions to the ductwork are theelimination of costly transitions and theeffect of the transitions on the silencerperformance. Transitions at the inlet anddischarge of the silencer will create turbulentair ow that may cause the pressure dropand generated noise to increase.The length of the silencer is typicallydetermined by performance requirementsand the space available. A longer silencerwill provide greater insertion loss, but thismust be weighed against the increasedpressure drop and additional cost. The mostcost effective solution is to build silencersonly to the length necessary to meet theinsertion loss requirements.Due to construction constraints andperformance requirements, it is necessaryto have multiple silencer congurationsavailable. Rectangular and elbow silencersconsist of three variables that represent theconguration of the silencer. These threevariables are the module, component, andbank dimensions.Silencer ModuleA silencer module consists of two halfbafes and one air gap, and can vary inwidth. The module width dimension, bafegeometry, and length of the silencer will alldepend on a combination of performancerequirements, available space, and ductdimensions.Silencer ComponentDue to raw material size and shippingconstraints, large silencers need to bemanufactured in several pieces, calledcomponents. A component may consist ofone or more modules, depending on theperformance required. These componentsare assembled on-site to make up thesilencer bank.Silencer BankThe silencer bank dimensions are the overalldimensions of the silencer, and are typicallyequal to the duct width and height in whichthe silencer is installed. The silencer bankmay consist of one or multiple components,which are made up of one or multiplemodules.Varying silencer models may have differentdimensional constraints, therefore theindividual data sheets or silencer selectionprogram should be referenced for availabledimensions.

Air Flow Pattern when Silencer Matches Ductwork


Multiple Module, Single Component Silencer

Multiple Module, Multiple Component Silencer

Effects of Transitions on Air Flow


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Un-Nested ElbowWhen the component width is equal to thebank width, the elbow silencer will be anun-nested conguration. For this elbowconfiguration, the performance centerline length is equal to the bank center linelength. The model number will always showthe performance center line length as themodel number represents how the silencerwill perform.Nested ElbowThe elbow silencer conguration will benested when the bank width is made up ofmultiple components which are identicalin size and performance. Duct extensions

are used to make an even duct connectionfor the silencer inlets and outlets. When asilencer is nested, the performance centerline will be different from the bank centerline. The performance center line is thecenter line of each individual component,while the bank center line is the geometriccenter line of the entire elbow assembly.

Un-Nested Elbow Conguration

Bank (Geometric) Center Line = Performance Center Line = Leg A + Leg B Bank Width (or Component Width)

Nested Elbow Conguration

Bank (Geometric) Center Line = Leg A + Leg B Bank WidthPerformance Center Line = Bank Center Line Component Width


Silencer Dimensional Considerations


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Testing Noise Control Products


Test standards are published proceduresand requirements for performing a specicmeasurement or procedure. These areconsensus based documents developedand typically written by a committeeformed by people who are familiar withand interested in the specic measurementprocedure. These documents are typicallyadministered by independent, not-for-prot groups such as ASTM International,ASHRAE, ANSI, SAE, ISO, and others. Manyof these groups are open to any interestedparty who would like to participate in thestandards development process.Lab AccreditationThe National Voluntary LaboratoryAccreditation Program (NVLAP*) isadministrated by the National Instituteof Standards and Technology (NIST).This organization provides third-partyaccreditation to public and privatelaboratories based on evaluation of theirtechnical qualications and competenceto carry out specic calibrations or tests.Accreditation criteria are established inaccordance with the U.S. Code of FederalRegulations (CFR, Title 15, Part 285) and NVLAPProcedures and General Requirements, andencompass the requirements of ISO/IEC17025. NVLAP accreditation signies that alaboratory has demonstrated that it operatesin accordance with NVLAP management andtechnical requirements pertaining to qualitysystems, personnel, accommodation andenvironment; test and calibration methods;equipment; measurement traceability;sampling; handling of test and calibrationitems; and test and calibration reports.NVLAP accreditation does not imply anyguarantee or certification of laboratoryperformance or test/calibration data; it issolely a nding of laboratory competence.A laboratory may cite its accredited statusand use the NVLAP term and symbol onreports, stationery, and in business andtrade publications provided that this usedoes not imply product certication. Thismeans that when a laboratory is accreditedfor ASTM E477 (or any other test standard),the laboratory is capable of testing to thisstandard, but the data produced by thelaboratory is neither certied by NVLAPnor is the data itself veried in any way.The Price Sound Lab is NVLAP accreditedfor acoustical product testing accordingto current versions of ANSI S12.51Determination of Sound Power Levels ofNoise Sources Using Sound Pressure -Precision Method for Reverberation Roomsand ASTM E477 Measuring Acoustical andAir Flow Performance of Duct Liner Materialsand Prefabricated Silencers.

* All information pertaining to theseorganizations has been reviewed andapproved by each of the respectiveorganizations.

Product Certication

Performance data certication has beenwidely adopted within the HVAC industry.The distinction between data certicationand lab certication is the scope of what isbeing compared. A lab certication looks atthe internal processes and documentationfor the lab. Product certication involvesindependent third-party verification ofmanufacturer performance data. Sampleproducts are often randomly selected andtested to verify that catalog data is publishedin accordance with independent third-partyresults. Air-Conditioning, Heating, andRefrigeration Institute (AHRI) administersthe heating, ventilation, air conditioning andcommercial refrigeration (HVACR) industrysperformance certification programs forheating and cooling equipment andcomponents. Manufacturers who havehad their product performance claimstested and certied by AHRI can apply oneof the associations family of certicationmarks. The Air Movement and ControlAssociation (AMCA) International, Inc. is anot-for-prot international association of theworlds manufacturers of related air systemequipment - primarily, but not limited to:fans, louvers, dampers, air curtains, air owmeasurement stations, acoustic attenuators,and other air system components for theindustrial, commercial and residentialmarkets. AMCA International has beenserving the industry and the public since

1917, and is considered the worlds leading

authority in the development of the scienceand art of engineering as it relates to airmovement and air control devices. AMCAInternational publishes and distributesstandards, references, and applicationmanuals for designers, engineers, andothers with an interest in selection,evaluation, and troubleshooting of airsystem components. AMCA Internationalhas developed a certied rating program(CRP) for prefabricated acoustical ductsilencers which looks at dynamic insertionlosses, air flow generated noise, andpressure drop as a function of air owsobtained in accordance with ASTM E477-06a. The AMCA publication 1011-03 detailsthe test method and procedure for licensing

a product to carry the AMCA seal.


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1

2

3

Air Transfer Silencer Test Method

There is currently no test method establishedfor air transfer silencers. The performancedata for these products is typically reportedas noise reduction or transmission loss.These silencers are installed as part of alarger assembly, with the percentage of thetransfer opening determining the effect onthe overall transmission loss of the installedwall.Acoustic louvers are typically tested andrated by AMCA according to AMCA 500Lfor air leakage, pressure drop, and waterpenetration. Acoustical louvers are typicallytested according to ASTM E90 for soundtransmission loss. The performance is oftenreported as a noise reduction to accountfor the louver being installed in a free eldenvironment versus a reverberant testenvironment.

Silencer Test MethodDuct silencer performance data isobtained according to the proceduresand requirements specied by the latestversion of ASTM E477. A silencer (1) isinstalled in a straight length of ductworkthat joins a single source chamber (2) anda reverberation room (3). The ductworkupstream of the silencer entrance must be astraight run with no transitions or elbows fora minimum of 5 equivalent duct diameters,and downstream of the silencer dischargeno less than 10 equivalent duct diameters.The single source chamber (2) consists ofa heavy gauge enclosure that is lined withsound-absorbing material that exceeds 0.25NRC. Inside this enclosure are multiple loudspeakers that are electronically coupled toensure all speakers are acting in phase. Thespeakers are structurally isolated from thechamber and duct system to minimize thechance of sound entering the reverberationroom by paths other than through theductwork. These speakers produce a pinknoise that has a series of 1 /3 octave bandswith center frequencies ranging from 50 to10 000 Hz. The test signal in each band ismaintained within dB throughout thetest, and is monitored electronically bymeasuring the voltage at the loud speakervoice coil.With the silencer and ductwork in place,

measurements are taken in the reverberationroom (3) using a rotating boom microphone,ltering equipment and a sound analyzerthat meet the requirements outlined in ASTMtest method E90. The insertion loss valuesare obtained by taking sound measurementswith the silencer installed in the duct andthen again with an unlined piece of duct oflike construction in place of the silencer. Thedifference between the two measurementsis the insertion loss of the tested silencer.The measurements are taken several timesat various air ow rates, including no ow,and since the performance of the silencerwill change based on air ow direction, thetest is also run under conditions of forwardand reverse ow.

To obtain generated noise data, the samesetup is used, with the exception of thesingle source sound chamber, which isturned off. The sound level measurementsare taken with an empty piece of duct toestablish the system noise, including fans,ductwork, and laboratory surroundings,then measured again with the silencer inplace. The generated noise performanceof the silencer is the difference betweenthe two measurements with a correctionfactor that accounts for the effects of endreection.



Acoustic Panel Test Method

Acoustic panels are tested for transmissionloss and absorption. Transmission loss istested according to ASTM E90 StandardTest Method for Laboratory Measurementof Airborne Sound Transmission Loss ofBuilding Partitions and Elements. ASTM E90is a laboratory measurement of airbornesound transmission loss of buildingpartitions. The test specimen is installedbetween two adjacent reverberation rooms,with the rooms being isolated such thatthe only signicant sound transmissionpath is the test specimen. A sound sourcecreates a diffuse sound eld in the sourceroom, and the resulting sound pressuresin the source and receiving rooms aremeasured. The transmission loss of the

specimen is calculated using the measuredsound pressure levels, the receiving roomabsorption, and the specimen area. Theacoustic absorption of panels is measuredusing ASTM C423. Acoustic absorption isa measure of the sound energy reectedby a surface. Small absorption coefcientsmean that most of the acoustic energy isreected into space. A perfect absorber,where all acoustic energy is dissipated, hasa coefcient of 1.

Silencer Test Setup per ASTM E477


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Relevance of Test Standard VersionThe 1999 version of ASTM E477 introducedsignicant changes to the calculation ofinsertion loss. Versions of the test standardup to 1996 calculated insertion loss based onthe difference between octave band soundpressure levels for the empty and silencedduct. Insertion loss calculated according1996 and earlier versions assume a atsound spectrum generated by the soundsource. If the sound spectrum is not at, theinsertion loss can be biased to the highest1/3rd octave band value. Results from theold test standard could provide a higherinsertion loss value for an individual octaveband than would be realized in practice.Current versions of the test standard (1999and on) calculate the octave band insertionloss based on the logarithmic average ofthe 1/3rd octave performance. This methodof calculation is unaffected by the frequencyspectrum of the sound source and providesa realistic number for the entire octaveband. The other signicant change wasdening how to test elbow silencers. Priorto 1999, the test method for elbow silencerswas left to the discretion of the individualmanufacturer. Comparisons of silencerperformance data and acoustic calculationsshould always be performed with datatested according to the most current versionof the test standard.Price Sound LabFor over 30 years, Price has been generatingaccurate, reliable performance data forgrilles, registers, diffusers, and terminalunits using an extensive in-house test facility.Our laboratory was expanded and upgradedin 2007 to include the new Price Sound Labfor testing and development of the Pricenoise control product lines. By furtheringthe capabilities of the Price Laboratories toinclude noise control products, this world-class research and development facility withstate-of-the-art test equipment solidiesour commitment to noise control productinnovation and performance.In the Price Air Distribution Laboratories,6,500 ft 2 of space has been dedicated tothe testing and development of the PriceSound Lab. The Price Sound Lab includesa 21,000 ft reverberant Sound Chamberdesigned specically for testing commercialand industrial noise control products inaccordance with ASTM and ANSI teststandards, down to 50 Hz.Air ow to the reverberant sound chamberis supplied by a dedicated bi-directional,variable speed axial fan. This fan system iscapable of producing up to 30,000 cfm at4 in. w.g. in both directions, which allowsfor testing of supply and return applications.Utilizing a networked graphical dataacquisition system and NIST traceable

flow meters, the test parameters andconditions are monitored with the highestlevel of precision, while Bruel and KjaerMulti-Channel Analyzers ensure the highestaccuracy of sound measurement available.The entire noise control test facility wasdesigned and constructed utilizing thelatest AMCA test standards. This industry-leading facility also gives Price the ability toperform in-house testing on a wide array ofcustom products to the appropriate testingstandard.


Price Sound Laboratory

Independent TestingAt Price we believe that third-partyverification of certified test data is anexcellent way to verify that the publishedcatalog data and finished products aredesigned and performing correctly. That iswhy throughout our product developmentand catalog testing, we sought and willcontinue to seek independent testing forour noise control product line. Through thisthird-party testing we are able to verify thatour laboratory is conducting measurementsaccurately.

Insertion Loss


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PerformanceWhen selecting silencers or noise controlproducts for an HVAC system, the followingperformance parameters need to beconsidered.Air Flow Direction & VelocityThe direction and face velocity of the airow through the duct will directly affect theperformance of a silencer. Both of theseparameters must be known to properlydetermine the performance of a silencer.Dynamic Insertion LossThe foremost parameter in silencer selectionis the insertion loss. Insertion loss is thedecrease in sound pressure levels that canbe expected when a silencer is insertedinto the path between the source and thereceiver. The data is provided at each of theoctave band center frequencies typicallyranging from 63 Hz to 8000 Hz and at varyingduct velocities in forward and reverse ow.Pressure DropThe pressure drop is the differential staticpressure of the silencer. The upstreamlocation is a minimum of 5 equivalentdiameters upstream and the downstreamlocation is a minimum of 10 diameters fromthe silencer. When adding any component toan HVAC system, the pressure drop shouldalways be considered. With a commercialsilencer, pressure drop typically rangesfrom 0.01 in. w.g. (3 Pa) up to 1.00 in. w.g.(249 Pa). A poor silencer selection resultingin a high pressure drop can make the entiresystem operate inefciently or ineffectively.It is desirable to keep the pressure drop ofa silencer below 0.35 in. w.g. (87 Pa), butthis is not always possible when trying toachieve high insertion loss values.Generated NoiseGenerated noise is the sound power createdwhen air ows through a silencer at a givenvelocity and direction (forward or reverse).Since these values represent the amountof sound produced by the silencer, a lowervalue will indicate better performance. Datafor generated noise levels are provided at

each of the octave band center frequenciesranging from 63 Hz to 80 00 Hz. This data isbased on a standard cross-sectional area,and when this area increases or decreases,a correction factor that is logarithmicallyproportional to the cross-sectional areamust be applied to the data. It is important tonote that generated noise in a given octaveband does not contribute to the overallsound level if the value is 10 dB below theattenuated sound levels exiting the silencer.When the generated noise values are within10 dB of the attenuated levels, generatednoise values are logarithmically added toattenuated levels to predict the nal soundlevel.

Performance

Generated Noise

Dynamic Insertion Loss

Pressure Drop

Generated Noise Corrections (Based on a 4 ft 2 silencer)


Face Velocityfpm (m/s)

Dynamic Insertion Loss (dB) Measured at Octave Band Center Frequencies63 Hz 125 Hz 250 Hz 500 Hz 1000 Hz 2000 Hz 4000 Hz 8000 Hz

+1000 (+5.08) 6 10 23 36 38 28 16 14

+500 (+2.54) 7 10 24 37 39 29 16 14

0 (0.00) 7 11 26 38 40 30 16 14

-500 (-2.54) 7 12 28 39 41 31 16 14

-1000 (-5.08) 8 12 29 40 42 32 16 14

Length in. (mm)Pressure Drop (in. w.g. [Pa]) at a Specic Face Velocity

500 fpm 1000 fpm 1500 fpm

36 (914) 0.16 (40) 0.61 (152) 1.4 (348)

60 (1524) 0.16 (40) 0.66 (164) 1.48 (368)

84 (2134) 0.18 (45) 0.7 (174) 1.57 (391)

108 (2744) 0.17 (42) 0.7 (174) 1.57 (391)

Face Velocityfpm (m/s)

Generated Noise (dB) Measured at Octave Band Center Frequencies63 Hz 125 Hz 250 Hz 500 Hz 1000 Hz 2000 Hz 4000 Hz 8000 Hz

+1000 (+5.08) 63 56 51 46 46 51 49 42

+500 (+2.54) 61 50 37 32 33 31 28 31

-500 (-2.54) 63 52 42 45 46 43 33 29

-1000 (-5.08) 63 53 47 48 53 59 57 47

Silencer Face Area (ft 2) 0.5 1 2 4 8 16 32 64 128

dB Addition or Reduction -9 -6 -3 0 +3 +6 +9 +12 +15


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Factors Affecting Performance


Effect of Flow on Silencer AttenuationThe direction and velocity of air ow througha silencer in reference to the directionsound is traveling will have an affect on theinsertion loss performance that a silencerwill provide. When insertion loss valuesinclude the effects of air ow direction andvelocity it is known as dynamic insertionloss.The terms forward ow or positive (+) owindicate that the air is owing through thesilencer in the same direction the soundis traveling. These conditions occur in asupply air system or on the outlet of a fan.The terms reverse ow or negative (-) owindicate that the air is owing throughthe silencer in the opposite direction ofthe noise. Examples of these conditionsinclude a return air system or an exhaustapplication.In reverse flow conditions, when theair ows in the opposite direction of thesound wave the effective speed of sounddecreases slightly,causing the sound to takea longer period of time to travel the silencerpassages. This results in improved lowfrequency insertion loss performance. Athigh frequencies, however, the attenuationdecreases as the velocity prole in thesilencer passage tends to focus the soundtoward the center of the passage and awayfrom the acoustic media.

The opposite is true of forward flowapplications. When the air is owing in thedirection of sound propagation the resultis decreased acoustic performance at lowfrequencies and increased performance athigh frequencies.The direction of air ow and air ow velocitycan be important factors in making the bestdecision for the selection of a silencer. Forcritical noise control applications, selectionsoftware like Price All-In-One, which allowsthe exact air velocity to be entered andcorrects the insertion loss values for thedirection of ow, should always be used.

Forward Flow

Reverse Flow

Standard Ductwork

Exhaust Applications

Inside Air Handler


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1. Straight Unobstructed Duct 2. Free Air/Plenum with Smooth Inlet

The desired installation for a silencer willhave 3 to 4 duct diameters of straightductwork upstream of the silencer.This type of installation will result inperformance that is very similar to

cataloged data.

When a silencer is installed 3 to 4 ductdiameters downstream of a free airintake/plenum with smooth inlet, apressure drop factor of 1.05 shouldbe applied. This type of installation will

result in a slight deviation from catalogedperformance data.

3. Free Air/Plenum with Sharp Inlet

When a silencer is installed 3 to 4 ductdiameters downstream of a free airintake/plenum with a sharp inlet, apressure drop factor of 1.1 to 1.3 shouldbe applied. This type of installation will

result in a moderate deviation fromcataloged performance data.

5. Radius Elbow with No TurningVanes

When a silencer is installed 3 to 4 ductdiameters downstream of a radius elbowwith turning vanes, a pressure dropfactor of 1.05 should be applied. This willresult in a slight deviation from catalogedperformance data.

When a silencer is installed 3 to 4 ductdiameters downstream of a radius elbowwithout turning vanes, a larger pressuredrop factor of 1.1 should be applied. Thiswill result in a moderate deviation fromcataloged performance data.

When a silencer is installed 3 to 4 ductdiameters downstream of a miteredelbow, an even larger pressure dropfactor of 1.3 should be applied. This typeof installation will result in a moderatedeviation from cataloged performancedata.

4. Radius Elbow with Turning Vanes 6. Miter Elbow

When a silencer is installed 3 to 4 ductdiameters downstream of a fan, apressure drop factor of 1.3 should beapplied. This type of installation willresult in a moderate deviation fromcataloged performance data.

7. Fan Discharge

Installed Pressure Drop, System EffectElements in a duct system, such as elbows, tees, and transitions, can have a negative effect on the performance of a silencer if they arelocated too close to the silencer inlet or discharge. The effect of these elements on a silencer is known as system effect. The effect thatan element will have on performance depends on the type of element and its distance to the inlet or outlet of the silencer. The ASTMStandard E-477, to which silencers are tested, states that there shall be straight duct of no less than 5 duct diameters upstream of thesilencer and no less than 10 duct diameters downstream of the silencer. In actual applications these conditions are often not possible;however, a minimum of 3 to 4 duct diameters on both sides of the silencer should be a design goal. The ASHRAE Applications Handbookprovides correction factors that can be applied to the pressure drop of a silencer for various duct elements at a distance within 3 to4 duct diameters of the silencer inlet or outlet. The ASHRAE Handbook recommends the pressure drop across a silencer not exceed0.35 in. w.g., with the correction factors applied to ensure that a silencer performs as intended.Equivalent Duct Diameter = Square Root [(4 x Duct Width x Duct Height) / ]Silencer Inlet ConditionsBelow are common inlet conditions that a silencer may be installed downstream of, along with their corresponding correction factors.When a silencer is installed within 3 to 4 equivalent duct diameters of one of these seven elements, the actual pressure drop willincrease from the cataloged silencer performance. To obtain the installed pressure drop, multiply the cataloged pressure drop by thecorresponding system effect correction factor.


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Silencer Outlet ConditionsBelow are common outlet conditions that a silencer may be installed upstream of, along with their corresponding corrections factors.When a silencer is installed within 3 to 4 equivalent duct diameters of one of these seven elements, the actual pressure drop willincrease from the cataloged silencer performance. To obtain the installed pressure drop, multiply the cataloged pressure drop by thecorresponding system effect correction factor.

ExampleDetermine the actual pressure drop for a rectangular silencer installed with a mitered elbow 4 duct diameters upstream and an abruptdoubling of the duct area 2 duct diameters downstream. From the catalog or software selection program, a rectangular silencer modelshows a pressure drop of 0.21 in. w.g. at 2000 fpm. To determine the installed pressure drop of the silencer including the system effect,multiply the pressure drop by the corresponding correction factors.

Installed Pressure Drop = Cataloged Pressure Drop x Inlet Pressure Drop Factor x Outlet Pressure Drop FactorPressure drop of the rectangular Silencer = 0.21 in. w.g.Inlet correction factor for a miter elbow = 1.3Outlet correction factor for an abrupt doubling of duct area = 1.4

Therefore:Installed pressure drop = 0.21 x 1.3 x 1.4 = 0.38 in. w.g.



8. Straight Unobstructed Duct 9. Duct Doubles Area Abruptly

The desired installation for a silencer willhave 3 to 4 duct diameters of straightductwork downstream of the silencer.This type of installation will result inperformance that is very similar tocataloged data.

When a silencer is installed 3 to 4 ductdiameters upstream of a duct transitionthat abruptly doubles the cross sectionalarea of the duct work, a pressure dropfactor of 1.4 should be applied. This typeof installation will cause a large deviationfrom cataloged performance data.

10. Abrupt Expansion/Plenum

When a silencer is installed 3 to 4duct diameters upstream of an abruptexpansion/plenum, a pressure dropfactor of 2.0 should be applied. Thistype of installation will result in a largedeviation from cataloged performancedata.

12. Radius Elbow with No TurningVanes

When a silencer is installed 3 to 4 ductdiameters upstream of a radius elbowwith turning vanes, a pressure dropfactor of 1.5 should be applied. This willresult in a large deviation from cataloged

performance data.

When a silencer is installed 3 to 4 ductdiameters upstream of a radius elbowwithout turning vanes, a larger pressuredrop factor of 1.9 should be applied.This will result in a large deviation from

cataloged performance data.

When a silencer is installed 3 to 4 ductdiameters upstream of a mitered elbow,an even larger pressure drop factor of2.0 should be applied. This will resultin a large deviation from catalogedperformance data.

11. Radius Elbow with Turning Vanes 13. Miter Elbow

When a silencer is installed 3 to 4 ductdiameters upstream of a fan, a pressuredrop factor of 1.2 to 1.4 should beapplied. This type of installation willresult in a moderate deviation fromcataloged performance data.

14. Fan Inlet


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1. Inlet/Discharge of Air Handler

3. Duct Terminations

5. Close Coupled to Fan

2. Riser Branches

Location in System

Silencer LocationSilencers are an important part of an HVACsystem. If the silencer is installed in a poorlocation the desired results will not beachieved. Listed below are some typicalsilencer locations that are used in HVACsystems. The choice of location is determinedby the type of equipment being used, thelocation of the mechanical room with respectto the occupied spaces, and the requirednoise levels in the occupied spaces.1. Inlet/Discharge of Air HandlerSilencers placed at the inlet and/or dischargeof air handling equipment are typicallyused to attenuate noise right at the source,

reducing or eliminating the need for furthersound attenuation downstream. Attenuatingthe noise at the source can also help toeliminate both airborne and structure bornenoise from propagating through the rest ofthe building.2. Riser BranchesIt is often not practical to install silencersat the source of the noise, which createsthe need to attenuate the noise furtherdownstream. When silencers are placed inriser branches, the noise concern can still bedealt with before the noise moves into theoccupied space of each respective oor inthe building.3. Duct Terminations

Sometimes noise control products arerequired closer to the occupied space.This can be due to space constraints or toattenuated noise generated by duct elementsfar from the mechanical room, such asdampers, terminal units, fan powered boxes,control valves, or exhaust fans.4. Speech / Equipment NoiseDuct silencers may also be used inapplications where they are not part of atypical supply or return air ducting system.Instead, the silencers are used to reducesound transfer in ductwork that is dealingwith pressure differentials between twospaces. It is often necessary to eliminatespeech transfer between adjacent ofces or

meeting rooms, or to prevent the noise of amanufacturing plant from entering adjacentofce spaces.5. Close Coupled to FanIn some cases there will be a need to silencefan noise directly before or after an axial fan.In these instances, it is important to create asilencer design that corresponds directly tothe fan itself, rather than utilizing a silencerthat is off the shelf. These custom silencerdesigns not only greatly reduce fan noise,but can also improve the performance of thefan by providing static regain at the outlet ofthe discharge silencer. There are several othertypes of fan inlet and discharge silencers thatcan be used with centrifugal fans as well.

4. Speech / Equipment Noise


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Location in System


Breakout NoiseSound that is allowed to pass through thewalls of the ductwork and the silencer casingis called breakout noise. Many things willcontribute to the amount of sound that isable to break out of the silencer casing,including the shape and size of the silencer,the gauge of material used to construct thecasing, and the amount of acoustic mediain the walls. The transmission loss table tothe right shows the transmission loss valuesof different rectangular silencer casingconstruction classes. The HTL2 casing hasthe highest transmission loss performance,therefore less noise will break out of thecasing of a HTL2 silencer as compared toa CL1 silencer.When trying to limit breakout noise in anHVAC system the silencer should be locatedas close as possible to the noise source.This will allow the silencer to attenuatethe sound before it has a chance to travelthrough the duct system to an area wherenoise breakout through the duct walls willbe a concern.Mechanical RoomWhen removing airborne sound createdby fans and other equipment located in amechanical room, the ideal location for thesilencer is straddling the mechanical roomwall. This location removes the unwantedsound created by the equipment as it leavesthe mechanical room, which eliminates theconcerns of breakout noise in occupiedspaces. However, re dampers are typicallylocated at the wall of the mechanical room.When this is the case, the best location forthe silencer is inside the mechanical room,

just before the mechanical room wall. Ifadequate space inside the mechanical roomis not available, the silencer may need to belocated outside the room. In this situationthe silencer should again be located at themechanical room wall, but a heavier gaugematerial may be required to prevent noisefrom breaking out of the silencer before itis fully attenuated.

Transmission Loss

Construction Transmission Loss (Rectangular Silencers)Class Gauge 63Hz 125Hz 250Hz 500Hz 1KHz 2KHz 4KHz 8KHz

CL1 22 25 26 28 30 33 37 40 40

CL2 18 27 28 30 32 35 38 41 41

HTL1 16 28 29 31 33 36 39 42 42

HTL2 10 31 33 34 36 38 42 45 45

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