Engine Exhaust Noise Control

1 Engine Exhaust Noise ControlJerry G. Lilly, Acoustics, , to TC Exhaust Noise ControlnReactive MufflersnAbsorptive SilencersnReactive/Absorptive MufflersnTail Pipe DesignnTuned ResonatorsnProject ExamplesThe above are the subjects that we will data will also be presented from field tests:One an example of a project failure and the other abig TC 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 F)nHigh Velocities (5,000 to 15,000 fpm)nCombustion By-Products (soot & corrosion)nPipe Thermal ExpansionASHRAE TC Characteristicsn Insertion Loss (dB)depends on design , size and frequencyn Pressure Drop (inches H2O or Hg)depends on velocity & designn Self-Generated Noise (dB ref.)

2 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, data on self Noise is generally not 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 curves also show that the majority ofthe Engine Exhaust Noise is at TC Exhaust NoisenUnlike Engine blocknoise, Exhaust noiseincreases significantlywith Engine loadnGraph at right showsnoise level vs.

3 Loadfor a 16-cyl. 2000 KWengine (1800 RPMdiesels)10010511011512012513063125250 5001000200040008000 Octave Band Frequency (Hz)Sound Pressure @ 1 m (dB)25%50%75%100%ASHRAE TC overall Noise level from most unsilencedengine Exhaust systems varies from about 110dBA to 120 dBA, when measured 1 meter fromthe pipe Noise level does not always align with thepower rating of the generator as you can see bythis next Noise can also be affected by engineturbochargers and after-coolers. It is best toobtain Exhaust Noise data from the TC Exhaust NoisenUnsilenced engineexhaust Noise isbroad-band withhighest levels at lowfrequenciesnData compares KW and 500 KWengines with a KW Engine (all1800 RPM diesels)10010511011512012513063125250500 1000200040008000 Octave Band Frequency (Hz)Sound Pressure @ 1 m (dB)150 KW (114 dBA)350 KW (110 dBA)500 KW (113 dBA)2000 KW (120 dBA)ASHRAE TC Exhaust Noise spectrum will always containstrong tones associated with the rate of cylinderfirings.

4 In 4-cycle engines each cylinder firesonce every other revolution of the drive fire once every rev in 2-cycle lowest tone is always the CFR, which is thefiring rate for any one Engine firing rate is generally the strongesttone in the Exhaust TC Exhaust TonesnCylinder Firing Rate (CFR)CFR = RPM/60 for 2-cycle enginesCFR = RPM/120 for 4-cycle enginesnEngine Firing Rate (EFR)EFR = N(CFR) where N = # cylindersnHarmonics of CFR and EFRASHRAE TC 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 the strong tone at 90 Hz, which is the that the second and third harmonics arealso prominent in the TC KW Engine Exhaust Tones70809010011012013025324051648110112 8161202255321404508640806101512771608202 425481/24th Octave Band Center Frequency (Hz)Sound Pressure Level (dB) No Silencer, 6" Pipe (113 dBA)90 Hz (1xEFR)60 Hz(4xCFR)180 Hz (2xEFR)270 Hz(3xEFR)30 Hz(2xCFR)

5 ASHRAE TC most common element used to silencegenerator exhausts are 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 element in the muffler has sound reductionproperties that vary greatly with acousticfrequency, and it is the mixing and matching ofthese elements that constitutes muffler TC Mufflersn2, 3 or 4-chamberdesignsnAll metal constructionwith no soundabsorptive materialsnMaximize ratio of bodydiameter to pipediameter & volumeASHRAE TC the years a series of muffler grades haveevolved to describe the approximate insertionloss performance for Engine Exhaust words do not necessarily imply where themufflers should be used.

6 Note that better quality( higher insertion loss) mufflers will bephysically larger than lower quality size is not the only factor, you cannotget good acoustical performance without TC Exhaust Muffler GradesnIndustrial/Commercial: IL = 15 to 25 dBA Body/Pipe = 2 to Length/Pipe = 5 to Grade: IL = 20 to 30 dBA Body/Pipe = 2 to Length/Pipe = 6 to 10nCritical Grade: IL = 25 to 35 dBA Body/Pipe = 3 Length/Pipe = 8 to 10 ASHRAE TC 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 drawing shows a 3 chamber critical grademuffler.

7 It achieves its super-critical statusprimarily from its length, as much as 16x TC Exhaust Muffler GradesnSuper Critical Grade: IL = 35 to 45 dBA Body/Pipe = 3 Length/Pipe = 10 to 16 ASHRAE TC 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 TC (secondary) SilencersnStraight through designwith fiberglass shieldedfrom the exhauststream by perforatedsheet metalnProvides mostly highfrequency IL with lowpressure dropASHRAE TC manufacturers offer combinationreactive/absorptive silencers in a single packageunit.

8 Although this sounds like a good idea, yougenerally will get better overall acousticalperformance by using a reactive muffler followedby a separate absorptive course, a combination silencer may beappropriate for installations where there is notenough length in the Exhaust system to fit twoseparate TC SilencersnThese devicescontain fiberglassshielded from theexhaust stream byperforated sheetmetalnProvides broad-bandnoise controlASHRAE TC graph shows the approximate insertion lossas a function of frequency for the various gradesof that all values are approximate since nomuffler has repeatable IL performance fromengine to Engine .

9 Also note how the ILperformance of the absorptive silencer is best inthe frequency region where reactive mufflersstart to TC Insertion LossnReactive mufflerswork best at 125 Hzand 250 Hz(IL is reduced athigh frequencies byself- Noise )nAbsorptive mufflerswork best at 1000Hz and 2000 Hz05101520253035404550631252505001000200 040008000 Octave Band Frequency (Hz)Insertioin Loss (dB)IndustrialResidentialCriticalSuper CriticalAbsorptiveASHRAE TC first step in generator Exhaust Noise controldesign is to determine if a single muffler (byitself) can meet the project 1 in this process is to obtain the unsilencednoise level from the Engine manufacturer.

10 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 TC to Select a MufflerStep 1: Unsilenced Noise Level( UNL = 116 dBA @ 1 m)Step 2: Calculate Exhaust NoiseCriteria ENC = RNC - 5 (dBA)( to meet a total Noise level of60 dBA, design muffler for 55 dBA)ASHRAE TC 3 is to calculate the unsilenced exhaustnoise at the receiver location. The followingequation provides a correction for distanceassuming free-field from large objects ( buildings)can cause the actual Noise level to be higher thanthat predicted by this , shielding provided by barriers(partial or total) can cause the received noiselevel to be TC to Select a MufflerStep 3: Correct UNL to ReceiverDistance Lp(xr) = Lp(x0) - 20 log (xr/x0)for example.