Transmission Loss

[geeovana]

HI

I am designing an exhaust system for a 450cc engine. I have designed the new system out of lightweight materials with a similar internal configuration as the stock item with a range of end cap sizes. I have run the model in Solidworks Flow Simulation to calculate back pressure produced at a certain RPM.

Is there anyway to calculate the approximate dB produced by the exhaust if I know the theoretical pressure and velocity of the gas exiting the silencer??

Thanks for any help in advance

Regards
Graeme

 

[SomptingGuy]

The pressure is likely to be 1e5Pa at the tailpipe, but the instantaneous velocity will be quite lively. If you consider the orifice to be a monopole source (ok for the low frequencies you're producing), then it's quite simple. For a monopole source in the free field, pressure P(t) at a distance R from a source of strength S(t) (a.k.a. volumetric flow rate) is given by:

P(t)=rho/(4.pi.R).dS(t-R/c)/dt

Where c is the speed of sound in the air between source and receiver, -R/c is the transport delay. Then you just need to analyse the pressure signal at the receiver as you would with measured pressure.

A good book on this is: "Theoretical Acoustics", Philip McCord Morse.

- Steve

 

[Tmoose]

4 stroke, I presume

 

[GregLocock]

FSAE I presume

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[geeovana]

Thankyou for the replies

Yes it's a four stroke single cylinder

So if i had a velocity of 150 m/s and a maximum total pressure of 123000 Pa how would I calculate it?

Thankyou for for all the help

Regards
Graeme

 

[Tmoose]

Even if silencer's internal chambers, tube lengths etc are a " similar internal configuration as the stock item" I expect what comes out the end has large pressure variations with strong multi-frequency content.
Pressure variation similar to the red curve in Fig 2 here -

http://www.epi-eng.com/piston_engine_technology/exhaust_system_technology.htm

Or, the sounds heard around 0:40 here but with frequency based or rpm.

http://www.youtube.com/watch?v=_y1Nog5SuM4

I think for the variation in end cap size ( opening diameter(s) or area, or ??) to change the exhaust sound very much it might be a result along the lines of an acoustic "plenum chamber" as shown about 1/3 down the page here -

http://www.mom.gov.sg/Documents/saf...elines for Noise Control Vibration Part 7.pdf

If the end cap succeeds in creating "back pressure" of a few psi in operation the power may be suppressed a bit too.


Some general silencer effects here-

http://www.paraglidingteam.nl/PPGTechnics/sound and noise/Mufflers/1155795969.pdf

 

[SomptingGuy]

If you want a CAE prediction of exhaust noise, your CAE tool will need to be able to predict tailpipe velocity as a function of crank angle (or time). The mean value will not really help - it's the pulsations that make/are the noise. There are some CAE tools available for free that can simulate this - having just a single cylinder makes that choice wider.



- Steve