engine exhaust sound control barrier wall

8/20/2019 Engine Exhaust Sound Control Barrier Wall

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Engine Exhaust Noise Control

Jerry G. Lilly, P.E.

JGL Acoustics, Inc.Issaquah, WA

[email protected]

Return to www.enoisecontrol.com

http://www.enoisecontrol.com/http://www.enoisecontrol.com/


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ASHRAE TC 2.6

Engine Exhaust Noise Controln Reactive Mufflersn Absorptive Silencersn Reactive/Absorptive Mufflersn Tail Pipe Designn Tuned Resonatorsn Project 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.


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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:n Very High Noise (100 to 120 dBA @ 1 m)n High Temperatures (950 to 1050

οF)

n High Velocities (5,000 to 15,000 fpm)n Combustion By-Products (soot & corrosion)n Pipe Thermal Expansion


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ASHRAE TC 2.6

Performance Characteristicsn Insertion Loss (dB)

depends on design, size and frequency

n Pressure Drop (inches H 2O or Hg)

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

depends on velocity & designInsertion 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.


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Engine exhaust noise varies significantlywith loading. Typically the noise level atfull load is about 10 dB higher than theno-load condition. The next slide shows

typical engine exhaust sound levels atdifferent loads.The curves also show that the majority of

the engine exhaust noise is at lowfrequencies.

ASHRAE TC 2.6


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Unsilenced Exhaust Noise

n Unlike engine blocknoise, exhaust noise

increases significantlywith engine load

n Graph at right showsnoise level vs. load

for a 16-cyl. 2000 KWengine (1800 RPMdiesels)

100

105

110

115

120

125

130

6 3

1 2 5

2 5 0

5 0 0

1 0 0 0

2 0 0 0

4 0 0 0

8 0 0 0

Octave Band Frequency (Hz)

S o u n d

P r e s s u r e

@ 1

m ( d B )

25% 50% 75% 100%

ASHRAE TC 2.6


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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


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Unsilenced Exhaust Noise

n Unsilenced engineexhaust noise is

broad-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)

100

105

110

115

120

125

130

6 3

1 2 5

2 5 0

5 0 0

1 0 0 0

2 0 0 0

4 0 0 0

8 0 0 0


S o u n

d P r e s s u r e

@ 1

m ( d B )

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

ASHRAE TC 2.6


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The exhaust noise spectrum will always containstrong tones associated with the rate of cylinder firings. In 4-cycle engines each cylinder firesonce every oth er revo lut ion 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


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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 = # cylindersn Harmonics of CFR and EFR

ASHRAE TC 2.6


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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


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500 KW Engine Exhaust Tones

70

80

90

100

110

120

130

2 5

3 2

4 0

5 1

6 4

8 1

1 0 1

1 2 8

1 6 1

2 0 2

2 5 5

3 2 1

4 0 4

5 0 8

6 4 0

8 0 6

1 0 1 5

1 2 7 7

1 6 0 8

2 0 2 4

2 5 4 8

1/24th Octave Band Center Frequency (Hz)

S o u n

d P r e s s u r e

L e v e

l ( d B

No Silencer, 6" Pipe (113 dBA)

90 Hz(1xEFR)

60 Hz(4xCFR)

180 Hz(2xEFR)

270 Hz

(3xEFR)

30 Hz(2xCFR)

ASHRAE TC 2.6


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The most common element used to silencegenerator exhausts are reactive mufflers.Reactive mufflers are available in a wide range of cost 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 of these elements that constitutes muffler design.

ASHRAE TC 2.6


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Reactive Mufflers

n 2, 3 or 4-chamber designs

n All metal constructionwith no soundabsorptive materials

n Maximize ratio of bodydiameter to pipediameter & volume

ASHRAE TC 2.6


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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


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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


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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 for the 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


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Exhaust Muffler Grades

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

ASHRAE TC 2.6


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Absorptive silencers use fiberglass or other acoustic fill material to absorb noise without anyreactive elements (tubes &chambers).Absorptive silencers provide very little noisereduction at low frequencies, so they should

never 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


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Absorptive (secondary) Silencers

n Straight through designwith fiberglass shielded

from the exhauststream by perforatedsheet metal

n Provides mostly highfrequency IL with lowpressure drop

ASHRAE TC 2.6


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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 followed

by 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


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Reactive/Absorptive Silencers

n These devicescontain fiberglass

shielded from theexhaust stream byperforated sheetmetal

n Provides broad-bandnoise control

ASHRAE TC 2.6


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

Note that all values are approximate since no

muffler 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


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Muffler Insertion Loss

n Reactive mufflerswork best at 125 Hz

and 250 Hz(IL is reduced athigh frequencies byself-noise)

n Absorptive mufflerswork best at 1000Hz and 2000 Hz

05

10

15

20

25

30

35

40

45

50

6 3

1 2 5

2 5 0

5 0 0

1 0 0 0

2 0 0 0

4 0 0 0

8 0 0 0


I n s e r t

i o i n L o s s

( d B )

Industrial

Residential

Critical

Super Critical

Absorptive

ASHRAE TC 2.6


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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 unsilenced

noise 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 for other sound transmission paths.

ASHRAE TC 2.6


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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 of 60 dBA, design muffler for 55 dBA)

ASHRAE TC 2.6


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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


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Step 3: Correct UNL to Receiver

Distance Lp(xr ) = L p(x0) - 20 log (x r /x0)

for example:

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

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

ASHRAE TC 2.6


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In Step 4 the required insertion loss of the muffler is 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 muffler

performance 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


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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 for pressure drop no more than 50%of maximum

ASHRAE TC 2.6


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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 available

from 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


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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


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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 muffler

would 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


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Insertion Loss Field Test

n “Off brand” muffler was substituted, but

was “guaranteed”n Muffler failed the field

test “repeatedly”n After 2 years of

testing & re-testingthe specified muffler was finally installed

05

10152025

303540

6 3

1 2 5

2 5 0

5 0 0

1 0 0 0

2 0 0 0

4 0 0 0

8 0 0 0


I n s e r t

i o n

L o s s

( d B )

No Load Full Load Spec

DIL = 18 dBA

ASHRAE TC 2.6


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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


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Tail Pipe Design

n Exhaust tail pipe willhave resonances that

can amplify enginetones

n Avoid amplification of

tones by using shorttail pipe or size L to1/4 wavelength ( λ /4)

Muffler

Tail Pipe

L

ASHRAE TC 2.6


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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 frequencies

that are a concern. Note that resonanceoccurs when L = nl/2, so this tail pipelength should be avoided at all times .


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Tail Pipe Resonances

f n = nc/(2L)where:

f n is resonancefrequency of pipen = 1, 2, 3, …

c is speed of soundL is length of pipe (ft)resonance occurs if L = n λ /2

Muffler

Tail Pipe

L

ASHRAE TC 2.6


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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


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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 λ CFR = 1841/15 = 122 ftn λ EFR = 1841/90 = 20 ftn Tail Pipe Length = 20/4 = 5 ft (or 15 ft)

ASHRAE TC 2.6


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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 1/4

wave resonator. A side branch resonator isnothing more than a dead-end section of pipeconnected to the main exhaust pipe.

ASHRAE TC 2.6


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Tuned Resonators

n Resonators can beused to remove

tones from theexhaust spectrum

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

Muffler

L

λ /4

ASHRAE TC 2.6


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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 is

reflected 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


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Side Branch Resonator

n Side branch is deadend pipe (no flow)

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

n Speed of soundincreases with temp.

c = 49.03(T R)0.5 (ft/sec)TR = 460 + T a (

οF)

Wavelength, λ = c/f

λ /4

Speed of Sound, cFrequency, f (Hz)

ASHRAE TC 2.6


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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 resonator include de-tuning caused by changes in thespeed of sound as the engine warms up. Inengine exhaust systems, soot buildup and water collection in the side branch can also degrade

the attenuation.

ASHRAE TC 2.6


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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, 3 λ /4, and 5 λ /4)

n Can achieve 20 to 30 dB insertion loss

if tuned properlyn May become ineffective if end of pipe

does not remain reflective

ASHRAE TC 2.6


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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 that

it doesn’t require a reflection.The only difference is that you have to consider the effects of flow velocity on the speed of soundfor proper tuning.

ASHRAE TC 2.6


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Trombone Resonator (Herschel-Quincke Tube)

ASHRAE TC 2.6


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This drawing shows a floor plan of the generator room 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 ground

floor slab was depressed 4 feet in the vicinity of the generator. A custom fabricated acoustictransition followed by 7 ft. long duct silencerscontrolled the radiator and engine noise.

ASHRAE TC 2.6


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Resonator Field Application

ASHRAE TC 2.6


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

ASHRAE TC 2.6


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Trombone Resonator Design

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

and 5 λ /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


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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


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ASHRAE TC 2.6

This next slide is a view of the trombone

resonator (note the super-critical primary muffler on the left).

The exhaust pipe and muffler are insulated and

externally wrapped for thermal purposes.

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


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Trombone Resonator

ASHRAE TC 2.6


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Dual Air Intake & Muffler ASHRAE TC 2.6

View of the dual air intake filters with the primary muffler

behind. This particular engine has dual 8” exhaust outlets.


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Secondary Silencer & ElbowASHRAE TC 2.6

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


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ASHRAE TC 2.6

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


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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’s

estimated octave band IL data for theprimary and secondary silencers.

ASHRAE TC 2.6


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Field Test Results (50% Load)

n Exhaust systemachieved 48 dBA

insertion loss (basedon factory unsilenceddata)

n Performance was

limited by self-noiseat mid-frequencies

n 67 dBA at 1 meter

010203040

506070

6 3 2 5 0

1 0 0 0

4 0 0 0


I n s e r t

i o n L o

s s ( d B )

Measured PrimarySecondary Pri + Sec

DIL = 48 dBA

ASHRAE TC 2.6

ASHRAE TC 2 6


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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 (240

Hz) at all is very impressive.

ASHRAE TC 2.6

ASHRAE TC 2 6


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30

40

50

60

70

80

90

1 0

1 3

1 6

2 0

2 5

3 2

4 0

5 1

6 4

8 1

1 0 1

1 2 8

1 6 1

2 0 2

2 5 5

3 2 1

4 0 4

5 0 8

6 4 0

8 0 6

1 0 1 5

1/24th Octave Band Center Frequency (Hz)

S o u n

d P r e s s u r e

l L e v e

l ( d B )

15 Hz(CFR)

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

ASHRAE TC 2 6


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Summary

n In most cases a reactive muffler isadequate to control exhaust

n In very sensitive installations add asecondary absorptive silencer (sized to

3000 ft/min) with a short tail pipe

ASHRAE TC 2.6


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Summary

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

n In extreme cases use super-criticalprimary muffler with secondary

absorptive silencer plus a tunedresonator to eliminate to ne s

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engine exhaust sound control barrier wall

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