What Makes An Exhaust Sound Raspy

[kyepan 291]

if you notice e46's have this tinny rattle sound to them when getting nailed, like someone is blowing through a metal bassoon.

 

Can this be caused by harmonics and gas bouncing off the inner walls of the pipes / silencers, or is it just that stainless is more sonorous than mild steel at the raspy frequencys.

 

Reason i ask is I've just fitted a brand stainless exhaust to the mx5, its almost identical to the racing beat stainless one I removed (except for not being bent where I collected some armco) The new one has a pronounced rasp once gas speed reaches a certain point.

 

Is this due to my fitting, the design, or some inherent characteristic of the pipe diameter and bends.

 

J

[welshpug 1,627]

I think half the sound of the E46 M's is the 3.2 24v straight 6 howling down the ITB's that have a nice chamber over the top of them

 

Stainless do tend to be more raspy or tinny, something to do with the damping qualities of the material I think, stainless is a lot harder than mild steel.

 

 

I think someone recently commented on a similar thing on the PSOOC forum when they fitted a full maniflow stainless system to their 206 GTi180.

Edited July 1, 2009 by welshpug

[kyepan 291]

how can we explain the difference between the two nearly identical stainless exhausts.

[Batfink 201]

thickness of the steel will affect the resonance

[base-1 17]

Nothng to do with difference, but MX5s are inherently raspy, it was designed into them on purpose, same as the snickety gearchange!

[Cameron 16]

How do you know that they are identical? They could be completely different inside and therefore sound completely different.

I've been doing a lot of research into this at work, you'll find that an M3 will have a very large silencer as inside there are lots of chambers to sort out various dominant frequencies. There is no actual sound deadening material, which is why the mid to high frequencies are allowed to pass through, hence the rasp.

 

It could be that your old one was a straight through box, and had lots of sound absorbing material, whereas the new one is chambered and has less or no material. The exhaust noise has nothing to do with gas speed* instead it is a function of the firing frequency and it's harmonics, and therefore wholly dependant on engine speed. The material won't affect the tone, at least not noticeably, the only affect it will have is that it may have very slightly different sound insulating properties.

 

*Except that the gas speed will change with engine speed.

[welshpug 1,627]

I do know that the M3 has a massive rear silencer that sits across the back of the car, as does the 206 gti180!

[Cameron 16]

That's because you need large resonators to get rid of the low frequencies. If you put a straight through box on an M3 the booming would drive you mad!

[bales 1]

It could be that your old one was a straight through box, and had lots of sound absorbing material, whereas the new one is chambered and has less or no material. The exhaust noise has nothing to do with gas speed* instead it is a function of the firing frequency and it's harmonics, and therefore wholly dependant on engine speed. The material won't affect the tone, at least not noticeably, the only affect it will have is that it may have very slightly different sound insulating properties.

 

*Except that the gas speed will change with engine speed.

 

You can't really say it has nothing to do with gas speed when the the 2 are inherently linked. I personally disagree with you over that.

 

If you had 2 identical cars with identical exhaust systems i.e the same design of silencers, chambers/ material inside them but they were of a different bore, the exhaust note will change and the gas speed will increase in the smaller bore / and backpressure will increase.

 

I know this as me and my dad used to have a tuned MG midget and the exhaust note was very dependent on the bore of the system. We used the same straight through pipe with same type rear box and the noise was very raspy on the smaller bore and considerably deeper when we changed to a larger bore exhaust.

 

Now a larger bore isnt inherently changing any of the firing frequencies of the engine, but it does directly change gas speed throughout the engines rev range. So I persoanlly don't see how your theory is true, especially as with most things to do with engines everything is linked and dependant on lots of other factors.

[Cameron 16]

That wasn't what I said though, I said that the material had nothing to do with the sound, not the pipe diameter. And where the diameter is different it isn't the gas speed that changes the tone of the noise. I'll briefly try to explain why:

 

The noise is caused by the sudden rise in pressure when the exhaust valve opens, which creates a pressure wave which travels through the gas in the pipe until it reaches the tailpipe and disperses to the atmosphere / listener. The pressure wave moves through the gas at the speed of sound (which varies according to temp), so it isn't actually the gas pulse that makes the noise, the gas moves far slower. Hotter gas will therefore have a higher frequency at a certain rpm, as the sound waves move faster through it, likewise colder gas will have a lower frequency at the same rpm. This is why a large diameter pipe will have a lower tone, as the gas slows down and has more time to cool, lowering the frequency.

 

So yes, larger diameter systems will have a lower note, but not because the gas speed is slower, because the gas is cooler.

[bales 1]

That wasn't what I said though, I said that the material had nothing to do with the sound, not the pipe diameter. And where the diameter is different it isn't the gas speed that changes the tone of the noise. I'll briefly try to explain why:

 

The noise is caused by the sudden rise in pressure when the exhaust valve opens, which creates a pressure wave which travels through the gas in the pipe until it reaches the tailpipe and disperses to the atmosphere / listener. The pressure wave moves through the gas at the speed of sound (which varies according to temp), so it isn't actually the gas pulse that makes the noise, the gas moves far slower. Hotter gas will therefore have a higher frequency at a certain rpm, as the sound waves move faster through it, likewise colder gas will have a lower frequency at the same rpm. This is why a large diameter pipe will have a lower tone, as the gas slows down and has more time to cool, lowering the frequency.

 

So yes, larger diameter systems will have a lower note, but not because the gas speed is slower, because the gas is cooler.

 

Ha ha but you can turn that around both ways, the gas is cooler due to the gas speed being less with a larger bore. You cant seperate the two of them out as they are both linked!

 

So are we saying that the gas temperature is partly governed by the bore of the exhaust as obviously the main factor is the degree of completion of the combustion process the valve timing and compression ratio...

 

I'm not disagreeing with you over this but I am genuinely interested in the theory, so what your saying is the if we had a number of identical exhaust systems it is the gas temperature that dictates the frequency of the exhaust note...not the flow rate / gas speed coming through it....assuming the source is putting the gas out at the same temp.

 

If that is the case how come turbocharged engines have very high EGT's but low frequency exhaust notes, would you not say that the main factor in a turbocharged cars exhaust note is the very low backpressure hence slow gas speed? and would the slow gas speed not be a function of the minimal restriction posed by a large bore exhaust?

 

So the high egt's in a turbocharged engine (hence quicker pressure waves) are negated by the large bore which give's the gases time to cool very quickly and therefore slows the pressure waves down to give them the lower frequency...

 

But....my 205 Turbo exhaust system runs massively hotter than most n/a engines, i.e the floor pan is hot to the touch so this doesn't match with the fact that you have to have a cool gas to get the low frequency noise...so what we have is very hot exhaust gas travelling relatively slowly but the pressure waves are still travelling slowly enough to give the lower frequency note??

 

I do agree with pressure waves travelling at different speeds due to temperature, but I do think flow rate /gas speed has an effect aswell.

 

Afterall exhaust gas is a fluid like anything else, what if I use the analogy of a hosepipe. One hosepipe runs at a massively high pressure and squirts the water out a mile and one hosepipe (with the same id) runs at a much lower flow rate and flows out normally. You would be able to hear the difference in them and yet there is no temperature differential worth talking about....

[Tom Fenton 1,526]

My thoughts on turbo exhaust notes is that the turbo takes a lot of energy out of the exhaust gases before they reach any silencers.

[bales 1]

My thoughts on turbo exhaust notes is that the turbo takes a lot of energy out of the exhaust gases before they reach any silencers.

 

And thats exactly my point!! So we are saying the energy behind the gas is having an effect on the frequency, not the fact that the pressure wave is dictated purely by the temperature of the fluid it is moving through...

 

Noise at the end of the day is just a wasted byproduct from the combustion process, with a turbo'd engine there is less energy in it as it has been used already to spin a turbine so there is less energy to move the gas and therefore the gas moves slower.....which gives a low frequency noise....so the pressure wave is moving slowly even though it is in very hot egt's.

[Cameron 16]

Not quite right, but nearly. Its because a lot of the high frequencies are dissipated by the blades of the turbine. The stronger low frequency pulse still passes through so you get that low bassy drone.

 

Gas speed has nothing to do with the speed of sound, it appears nowhere in the formula! I'd write it down but its a bit of a faff.

Edited July 2, 2009 by Cameron