Engine Exhaust Noise Control

Jerry G. Lilly, P.E.JGL Acoustics, Inc.Issaquah, WAjerrylilly@aol.com

Return to www.enoisecontrol.com

ASHRAE TC 2.6

Engine Exhaust Noise ControlnReactive MufflersnAbsorptive SilencersnReactive/Absorptive MufflersnTail Pipe DesignnTuned ResonatorsnProject ExamplesThe above are the subjects that we will discuss.Some data will also be presented from field tests:One an example of a project failure and the other abig success.

ASHRAE TC 2.6

Engine Exhaust ConsiderationsThe exhaust system of a generator hasseveral inherent design problems that mustbe considered. These characteristics imposesevere limitations on what can be done tosilence the engine exhaust noise:nVery High Noise (100 to 120 dBA @ 1 m)nHigh Temperatures (950 to 1050

ο F)

nHigh Velocities (5,000 to 15,000 fpm)nCombustion By-Products (soot & corrosion)nPipe Thermal Expansion

ASHRAE TC 2.6

Performance Characteristicsn Insertion Loss (dB)

depends on design, size and frequency

n Pressure Drop (inches H2O or Hg)depends on velocity & design

n Self-Generated Noise (dB ref. 1 picowatt)depends on velocity & design

Insertion loss (IL) is defined as the reduction of noise levelthat occurs when a silencing element is inserted into thesystem. Because engines generate strong tonalcomponents, the IL of any one muffler will not be the samewith different engines, different loads, or different pipingconfigurations. Pressure drop is more predictable, however.Specific data on self noise is generally not available.

Engine exhaust noise varies significantlywith loading. Typically the noise level atfull load is about 10 dB higher than theno-load condition. The next slide showstypical engine exhaust sound levels atdifferent loads.The curves also show that the majority ofthe engine exhaust noise is at lowfrequencies.

ASHRAE TC 2.6

Unsilenced Exhaust Noise

n Unlike engine blocknoise, exhaust noiseincreases significantlywith engine load

n Graph at right showsnoise level vs. loadfor a 16-cyl. 2000 KWengine (1800 RPMdiesels)

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ASHRAE TC 2.6

The overall noise level from most unsilencedengine exhaust systems varies from about 110dBA to 120 dBA, when measured 1 meter fromthe pipe outlet.

The noise level does not always align with thepower rating of the generator as you can see bythis next graph.

Exhaust noise can also be affected by engineturbochargers and after-coolers. It is best toobtain exhaust noise data from the enginemanufacturer.

ASHRAE TC 2.6

Unsilenced Exhaust Noise

n Unsilenced engineexhaust noise isbroad-band withhighest levels at lowfrequencies

n Data compares 6-cyl.150 KW and 500 KWengines with a 16-cyl.2000 KW engine (all1800 RPM diesels)

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150 KW (114 dBA) 350 KW (110 dBA)500 KW (113 dBA) 2000 KW (120 dBA)

ASHRAE TC 2.6

The exhaust noise spectrum will always containstrong tones associated with the rate of cylinderfirings. In 4-cycle engines each cylinder firesonce every other revolution of the drive shaft.

Cylinders fire once every rev in 2-cycle engines.The lowest tone is always the CFR, which is thefiring rate for any one cylinder.

The engine firing rate is generally the strongesttone in the exhaust spectrum.

ASHRAE TC 2.6

Engine Exhaust Tones

n Cylinder Firing Rate (CFR)

CFR = RPM/60 for 2-cycle enginesCFR = RPM/120 for 4-cycle engines

n Engine Firing Rate (EFR)EFR = N(CFR) where N = # cylinders

n Harmonics of CFR and EFR

ASHRAE TC 2.6

This graph shows a narrow band spectrum of theexhaust noise of a 6 cylinder diesel enginerunning at 1800 RPM in a 500 kW generator. Thedata was collected with the microphone placed 1meter from the exhaust outlet with the enginerunning at full load.

Note the strong tone at 90 Hz, which is the EFR.Note that the second and third harmonics arealso prominent in the spectrum.

ASHRAE TC 2.6

500 KW Engine Exhaust Tones

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No Silencer, 6" Pipe (113 dBA)

90 Hz (1xEFR)

60 Hz(4xCFR)

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270 Hz(3xEFR)

30 Hz(2xCFR)

ASHRAE TC 2.6

The most common element used to silencegenerator exhausts are reactive mufflers.Reactive mufflers are available in a wide range ofcost and performance. The noise is reduced byforcing the exhaust air to pass through a seriesof tubes and chambers.

Each element in the muffler has sound reductionproperties that vary greatly with acousticfrequency, and it is the mixing and matching ofthese elements that constitutes muffler design.

ASHRAE TC 2.6

Reactive Mufflers

n 2, 3 or 4-chamberdesigns

n All metal constructionwith no soundabsorptive materials

n Maximize ratio of bodydiameter to pipediameter & volume

ASHRAE TC 2.6

Over the years a series of muffler grades haveevolved to describe the approximate insertionloss performance for engine exhaust mufflers.The words do not necessarily imply where themufflers should be used. Note that better quality(e.g. higher insertion loss) mufflers will bephysically larger than lower quality units.

Although size is not the only factor, you cannotget good acoustical performance without it.

ASHRAE TC 2.6

Exhaust Muffler Grades

n Industrial/Commercial: IL = 15 to 25 dBA Body/Pipe = 2 to 2.5 Length/Pipe = 5 to 6.5

n Residential Grade: IL = 20 to 30 dBA Body/Pipe = 2 to 2.5 Length/Pipe = 6 to 10

n Critical Grade: IL = 25 to 35 dBA Body/Pipe = 3 Length/Pipe = 8 to 10

ASHRAE TC 2.6

The super-critical grade muffler generallyrepresents the “top of the line” for reactivemufflers. Some manufacturers include anabsorptive section to reduce high frequencysound transmission, but that is not the case forthe silencer design shown in the next slide.

This drawing shows a 3 chamber critical grademuffler. It achieves its “super-critical” statusprimarily from its length, as much as 16x pipediameter.

ASHRAE TC 2.6

Exhaust Muffler Grades

n Super Critical Grade: IL = 35 to 45 dBA Body/Pipe = 3 Length/Pipe = 10 to 16

ASHRAE TC 2.6

Absorptive silencers use fiberglass or otheracoustic fill material to absorb noise without anyreactive elements (tubes &chambers).Absorptive silencers provide very little noisereduction at low frequencies, so they shouldnever be used as the only silencer in an engineexhaust system. The straight-through designshown here is very useful for absorbing highfrequency self-generated noise created byreactive mufflers.

ASHRAE TC 2.6

Absorptive (secondary) Silencers

n Straight through designwith fiberglass shieldedfrom the exhauststream by perforatedsheet metal

n Provides mostly highfrequency IL with lowpressure drop

ASHRAE TC 2.6

Some manufacturers offer combinationreactive/absorptive silencers in a single packageunit. Although this sounds like a good idea, yougenerally will get better overall acousticalperformance by using a reactive muffler followedby a separate absorptive silencer.

Of course, a combination silencer may beappropriate for installations where there is notenough length in the exhaust system to fit twoseparate units.

ASHRAE TC 2.6

Reactive/Absorptive Silencers

n These devicescontain fiberglassshielded from theexhaust stream byperforated sheetmetal

n Provides broad-bandnoise control

ASHRAE TC 2.6

This graph shows the approximate insertion lossas a function of frequency for the various gradesof mufflers.

Note that all values are approximate since nomuffler has repeatable IL performance fromengine to engine. Also note how the ILperformance of the absorptive silencer is best inthe frequency region where reactive mufflersstart to deteriorate.

ASHRAE TC 2.6

Muffler Insertion Loss

n Reactive mufflerswork best at 125 Hzand 250 Hz(IL is reduced athigh frequencies byself-noise)

n Absorptive mufflerswork best at 1000Hz and 2000 Hz

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ASHRAE TC 2.6

The first step in generator exhaust noise controldesign is to determine if a single muffler (byitself) can meet the project requirements.

Step 1 in this process is to obtain the unsilencednoise level from the engine manufacturer. This istypically given as sound pressure level at 1 meter(or similar distance).

Step 2 is to determine the noise criteria at thereceiver. Provide a 5 dBA margin to allow forother sound transmission paths.

ASHRAE TC 2.6

How to Select a Muffler

Step 1: Unsilenced Noise Level(e.g. UNL = 116 dBA @ 1 m)

Step 2: Calculate Exhaust NoiseCriteria ENC = RNC - 5 (dBA)

(e.g. to meet a total noise level of60 dBA, design muffler for 55 dBA)

ASHRAE TC 2.6

Step 3 is to calculate the unsilenced exhaustnoise at the receiver location. The followingequation provides a correction for distanceassuming free-field spreading.

Reflections from large objects (e.g. buildings)can cause the actual noise level to be higher thanthat predicted by this equation.

Conversely, shielding provided by barriers(partial or total) can cause the received noiselevel to be lower.

ASHRAE TC 2.6

How to Select a Muffler

Step 3: Correct UNL to ReceiverDistance Lp(xr) = Lp(x0) - 20 log (xr/x0)

for example:

Lp(25 m) = Lp(1 m) - 20 log (25/1)

Lp(25 m) = 116 -28 = 88 dBA

ASHRAE TC 2.6

In Step 4 the required insertion loss of the muffleris determined by subtracting the receiver noisecriteria from the unsilenced receiver noise level.Note that a 5 dB safety factor is recommended toaccount for the fact that actual mufflerperformance often falls short of themanufacturer’s claims.

Once a muffler grade is determined, the last stepis to size the inlet/outlet pipe for pressure drop.

ASHRAE TC 2.6

How to Select a Muffler

Step 4: Required IL = UNL - ENC + 5

IL = 88 dBA - 55 dBA + 5 = 38 dBA (super critical grade required)

Step 5: Select inlet/pipe size forpressure drop no more than 50%of maximum

ASHRAE TC 2.6

Octave band IL data must be used to computethe expected noise level if barriers are involvedin the sound transmission path. If barriers arenot involved, it is much simpler to use theapproximate dBA attenuation figures availablefrom a reputable manufacturer.

Keep in mind that octave band values areapproximate because they vary from engine toengine and also with load.

ASHRAE TC 2.6

Using Octave Band IL Data

n Octave band IL data must be used toassess effects of barriers

n Accuracy is probably no better thanthe dBA method (without barriers)

n Manufacturer’s octave band data arelisted as “typical” because they areinconsistent from engine to engine

ASHRAE TC 2.6

This graph presents the results of an insertionloss field test, where the contractor wanted tosubstitute a less expensive muffler from an “off-brand” manufacturer. Even though themanufacturer “guaranteed” that his mufflerwould meet the IL of the specified unit, it failed atnearly every octave band. The overall noisereduction was only 18 dBA with the initial testshown here.

ASHRAE TC 2.6

Insertion Loss Field Test

n “Off brand” mufflerwas substituted, butwas “guaranteed”

n Muffler failed the fieldtest “repeatedly”

n After 2 years oftesting & re-testingthe specified mufflerwas finally installed

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DIL = 18 dBA

ASHRAE TC 2.6

One aspect of the engine exhaust systemacoustical design that is often overlooked is thetail pipe. The section of pipe downstream of thefinal silencer will have acoustic resonances thatcan amplify engine tones if they match.

Resonances can be avoided simply by keepingthe length of the tail pipe less than 1/2wavelength at the tone frequency. Even better,size the tail pipe to exactly l/4 to cancel the tone. `

ASHRAE TC 2.6

Tail Pipe Design

n Exhaust tail pipe willhave resonances thatcan amplify enginetones

n Avoid amplification oftones by using shorttail pipe or size L to1/4 wavelength (λ/4)

Muffler

Tail Pipe

L

ASHRAE TC 2.6

ASHRAE TC 2.6

The next equation will give you the variousresonance frequencies of any exhaust tailpipe. The number n is any positive integer,but usually it is only the low frequenciesthat are a concern. Note that resonanceoccurs when L = nl/2, so this tail pipelength should be avoided at all times.

Tail Pipe Resonances

fn = nc/(2L)where:

fn is resonancefrequency of pipen = 1, 2, 3, …c is speed of soundL is length of pipe (ft)resonance occurs if L = nλ/2

Muffler

Tail Pipe

L

ASHRAE TC 2.6

This example goes through the various stepsrequired to design a tail pipe for a 4-cycle engine.The frequencies to avoid are 15 Hz, 30 Hz, 45 Hz,60 Hz, 75 Hz, 90 Hz, 105 Hz, etc.

The most important frequency is the 90 Hz EFR.The wavelength at 90 Hz is 20 feet, so we want toavoid a tail pipe length of 10 feet, 20 feet, 30 feet,etc. The best length is exactly 5 feet becausethis will cancel the 90 Hz tone at the outlet.

ASHRAE TC 2.6

Tail Pipe Design Example

n 6 cylinders @ 1800 RPM (950 oF)n CFR = (1800/120) = 15 Hzn EFR = 6(CFR) = 90 Hzn c = 49.03(460+950)0.5 = 1841 ft/secn λCFR = 1841/15 = 122 ftn λEFR = 1841/90 = 20 ftn Tail Pipe Length = 20/4 = 5 ft (or 15 ft)

ASHRAE TC 2.6

Tuned resonators can be used to attenuatespecific frequencies in the exhaust spectrum.There are a lot of different resonator designs, butthe simplest and easiest to understand is theside branch resonator, sometimes called the 1/4wave resonator. A side branch resonator isnothing more than a dead-end section of pipeconnected to the main exhaust pipe.

ASHRAE TC 2.6

Tuned Resonators

n Resonators can beused to removetones from theexhaust spectrum

n Sketch shows aside branchresonator tuned to1/4 wavelength (λ)

Muffler

L

λ/4

ASHRAE TC 2.6

The diameter of the side branch should be equalto the diameter of the main pipe. As the acousticwave from the source reaches the branch, theenergy splits equally. The wave travelling downthe side branch reflects off the closed end and isreflected back toward the branch. When itarrives back at the branch it is exactly 180degrees out of phase from the source wavebecause the branch has a length of l/4.

ASHRAE TC 2.6

Side Branch Resonator

n Side branch is deadend pipe (no flow)

n Length of pipe istuned to λ/4 or 3λ/4

n Speed of soundincreases with temp.

c = 49.03(TR)0.5 (ft/sec)TR = 460 + Ta(

οF)

Wavelength, λ = c/f

λ/4

Speed of Sound, cFrequency, f (Hz)

ASHRAE TC 2.6

Side branch resonators are effective only at lowfrequencies and only in three narrow frequencyregions. Side branch resonators areinconsequential at other frequencies.

Problems inherent with the side branch resonatorinclude de-tuning caused by changes in thespeed of sound as the engine warms up. Inengine exhaust systems, soot buildup and watercollection in the side branch can also degradethe attenuation.

ASHRAE TC 2.6

Side Branch Resonator

n Only effective at low frequencies(when λ > 10 times pipe diameter

n Only effective in 3 narrow frequencyregions (L = λ/4, 3λ/4, and 5λ/4)

n Can achieve 20 to 30 dB insertion lossif tuned properly

n May become ineffective if end of pipedoes not remain reflective

ASHRAE TC 2.6

Another resonator design that is particularlysuited for engine exhaust systems is theHerschel-Quincke tube, which I like to call thetrombone resonator. Acoustically, it works verymuch like the side branch resonator - except thatit doesn’t require a reflection.

The only difference is that you have to considerthe effects of flow velocity on the speed of soundfor proper tuning.

ASHRAE TC 2.6

Trombone Resonator(Herschel-Quincke Tube)

ASHRAE TC 2.6

This drawing shows a floor plan of the generatorroom in a field application using a tromboneresonator in a 2 MW generator installation on theground floor of an office building. In order toobtain the necessary vertical space, the groundfloor slab was depressed 4 feet in the vicinity ofthe generator. A custom fabricated acoustictransition followed by 7 ft. long duct silencerscontrolled the radiator and engine noise.

ASHRAE TC 2.6

Resonator Field Application

ASHRAE TC 2.6

The trombone resonator was designed toeliminate the 240 Hz engine firing rate of the 16-cylinder diesel. The other two resonatorfrequencies were 80 Hz and 400 Hz.

ASHRAE TC 2.6

Trombone Resonator Design

n Tuned for EFR = 240 Hz (3λ/4)n Also effective at λ/4 = 80 Hz

and 5λ/4 = 400 Hzn Gas Flow Rate = 16,745 CFMn Gas Temperature = 946 oFn Speed of Sound = 1,838 ft/secn Additional Loop Length = 138 inches

ASHRAE TC 2.6

After completion of the acoustical design,the owner decided to incorporate a 1MWload bank between the radiator dischargeand the acoustical transition.This worked very well from an acousticalstandpoint because no additional noisecontrol was required for the load bank.

ASHRAE TC 2.6

Engine, Radiator & Load Bank

ASHRAE TC 2.6

ASHRAE TC 2.6

This next slide is a view of the tromboneresonator (note the super-critical primary muffleron the left).

The exhaust pipe and muffler are insulated andexternally wrapped for thermal purposes.

Acoustical panels with perforated metal facingare also visible on the slab above.

Trombone Resonator

ASHRAE TC 2.6

Dual Air Intake & MufflerASHRAE TC 2.6

View of the dual air intake filters with the primary mufflerbehind. This particular engine has dual 8” exhaust outlets.

Secondary Silencer & ElbowASHRAE TC 2.6

View of the secondary silencer as it passes through the generatorroom wall. The exhaust then makes an elbow up followed by twoadditional elbows before exiting the discharge plenum.

ASHRAE TC 2.6

These are views of building exterior showing the groundlevel radiator outlet with the exhaust outlet above.

This next graph shows the measuredexhaust noise levels 1 meter from theexhaust outlet with the generator runningat 50% load. Using the unsilenced engineexhaust noise data from themanufacturer, We show the calculatedtotal insertion loss of the exhaust system.Also shown is the manufacturer’sestimated octave band IL data for theprimary and secondary silencers.

ASHRAE TC 2.6

Field Test Results (50% Load)

n Exhaust systemachieved 48 dBAinsertion loss (basedon factory unsilenceddata)

n Performance waslimited by self-noiseat mid-frequencies

n 67 dBA at 1 meter

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ASHRAE TC 2.6

The next slide shows a narrow bandspectrum of the octave band datapresented in the previous slide andshows the effect of the tromboneresonator. Note the dip in the curve inthe vicinity of 80 Hz and 240 Hz.The fact that there is no EFR tone (240Hz) at all is very impressive.

ASHRAE TC 2.6

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30 Hz (2xCFR)

45 Hz (3xCFR)

60 Hz(4xCFR)

120 Hz(8xCFR)

180 Hz (12xCFR)

270 Hz (18xCFR)390 Hz(26xCFR)

90 Hz(6xCFR) 240 Hz (EFR)

Measured Spectrum at 1 meter

ASHRAE TC 2.6

Summary

n In most cases a reactive muffler isadequate to control exhaust

n In very sensitive installations add asecondary absorptive silencer (sized to3000 ft/min) with a short tail pipe

ASHRAE TC 2.6

Summary

n CAUTION: Do not use two reactivemufflers in series (too much self-noise)

n In extreme cases use super-criticalprimary muffler with secondaryabsorptive silencer plus a tunedresonator to eliminate tones

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ASHRAE TC 2.6