Silly question about engine exhaust noise

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Silly question about engine exhaust noise

Postby Graeme M on June 22nd, 2017, 3:49 am 

Ok, this is just an idle question (cool pun yeah?) but a little Googling didn't help and this has bothered me for years!! I hope it's sciency enough for this forum.

If I have a single cylinder four stroke engine (say my motorcycle) idling at 1200rpm, then there are 10 power strokes per second. Now, I can actually hear what sounds like very slow beats of the engine - it's why people say that singles are "thumpers". At 1200rpm, a single makes an audible thump-thump-thump sound that to my ear sounds like a power stroke every quarter/half a second or so. Not at every .1 of a second as it really is!

At 5000rpm, that same engine still makes quite a slow sound, yet there are something like 40 power strokes per second. It should be making a high pitched buzzing I'd have thought. And as for my single cylinder racebike at 12000rpm, that is one power stroke every .01 of a second!!

Why does the exhaust sound not match the mechanical reality?

PS I am also deeply sceptical that at 12000rpm with my engine's tiny little valves that it can physically flow enough mixture to burn properly and produce power in some tiny fraction of a second... :)
Graeme M
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Re: Silly question about engine exhaust noise

Postby bangstrom on June 22nd, 2017, 6:11 am 

This example of lower frequency ranges might be of some help. ... ecklow.php
Ten Hz (cps) is below what is considerer the “frequency hearing range” and forty Hz is a low hum but you can still distinguish the individual beats or thump-thump-thumps.

Human hearing ranges from about 20 Hz to 20 thousand Hz ( kHz).
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Re: Silly question about engine exhaust noise

Postby Graeme M on June 22nd, 2017, 6:45 am 

Hmmm... OK, I know little about sound and so on, so please excuse me if my response is really ignorant! I'm just trying to understand what you are saying.

Doesn't "frequency" in your answer simply refer to the speed of the sound vibration in a medium, rather than the frequency (in the sense of timely repetition) of the sound source? The frequency of the waveform generated by an explosion would determine the pitch of its sound, but isn't that a different thing from the frequency of the sound itself? I guess that's clumsily stated, what I mean is that each explosion when heard from the muffler would be at a particular pitch which would be a function of the frequency of its waveform?
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Re: Silly question about engine exhaust noise

Postby Dave_Oblad on June 22nd, 2017, 8:09 am 

Hi Graeme,

When I've removed the muffler off an engine.. I find I can easily hear each separate explosion as the combusted gas is released to the exhaust outlet. And of course it goes up in frequency when you increase the flow of gas and the engine increases its RPM.

But add a muffler and it changes. A muffler is supposed to absorb the sound but still pass the exhaust without creating too great a restriction. Example: Breath in through your nose and exhale through a straw.

Anyway, there are a lot of techniques to quell the noise and absorb the noise. One design uses echo cancellation. The bang enters a chamber and bounces back causing two waves to null each other when out of phase. But this produces an alias frequency.. like +10Hz added to -10.2 Hz leaves a bit (0.2hz) that is not canceled. It's a bit more complex than that, but my best guess is you are hearing an alias Frequency in a muffler system that is using some echo cancellation technique.

Like take two musical notes where one is just slightly higher (or lower) than the other. When combined, you will hear a low frequency (warble) note as the two main notes fall in and out of phase with each other.

There might be more useful information here:

Hope this has helped..

Dave :^)
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