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Engines make noise out of their exhaust pipes because it is the fundamental nature of piston engines to produce regular pulses of high-pressure air, aka sound waves.
Sound waves can be canceled out, or at least reduced in volume, by combining them with an equal but opposite-phase sound wave.
Proposal:
Split the exhaust pipe into two separate pipes. One pipe is just a simple straight pipe. The other pipe goes through an electronically controlled expandable loop, like a robotic trombone. Both pipes rejoin, recombining the split exhaust, and leave the vehicle.
By adjusting the length of the trombone tube relative to the engine RPM, one can achieve several useful effects. One is that, by making the exhaust pulses rejoin each other exactly out of phase, the exhaust sound level can be greatly reduced. The other is that, if performance is desired over sound reduction, the trombone section can be treated as a dynamically adjustable tuned exhaust system, generating greater or lesser exhaust backpressure depending upon desired performance at any given engine speed/throttle level.
The dynamic tuned exhaust is especially notable--tuned exhausts (which improve engine power) are common features on high-performance vehicles, but because of their static design, they only function well at certain specific engine speeds when the resonant lengths are correct. By being able to adjust the length of the exhaust system, correct resonant properties can be maintained at any engine speed.
Satan's Calliope
http://flame-effect...liope-probably.html
"musical" instrument made from tuned pulse jets [rmutt, Dec 12 2006]
Large Hot Pipe Organ
http://www.testdept.org.uk/PPP-3.html
Search for "LHPO" in the link. I can't seem to find a recent reference for this thing, so maybe it's nonfunctional now. [Vernon, Dec 12 2006]
[link]
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Could there be an override function that
handed control of the trombone to a
small steering-wheel mounted
keyboard? |
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And then we could drive around in groups of seventy-six, with all of our systems linked. |
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The problem here is that you're not dealing with a single sine wave, but a whole bunch of them with different frequencies, requiring different mixing lengths. Still, you might be get rid of a predominate bass note this way. |
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I've been wondering about this one for a while, [5th]. You'd need a long, straight pipe of adjustable length, closed at one end, and an opening roughly half way down its length whose position could be adjusted. This would give you independent control over the two variables of noise and power. |
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The difficulty is making this reliable within the aggressive environment of an automotive exhaust. You've got to produce a good seal on moving parts that are subject to heating and corrosion. |
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How about a flexible chamber (?some
super-super silicone?) which could be
squashed by an [external] actuator to
change its volume? Would that work? |
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It might... maybe something like a long sausage-shaped balloon, so that squashing it changed its length rather than any other dimensions? |
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Also, for good resonant properties, does the chamber not need to be stiff? |
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I'm not sure about the stiffness (long
story there....) - you're probably right.
But if the balloon were just the liner of
an otherwise rigid box, it might help.
(NB - the human vocal tract is lined with
squishy stuff but seems to be tuneable,
though not in my own case.) |
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The "trumpet" concept might work better for this application. As noted before, because its nearly impossible to maintain an air tight seal with the heat and vibration in the trombone scenario, the trumpet tends to have more of an air tight system. I dont know how many scenarios that could exist with the length of the exhaust, however, if you had 6 valves to control the various lengths, essentially you could have 36 different combinations of exhaust lengths. The computer could control the tubing length dynamically through the shifting of the valves. |
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Well if you need two equal sounds, why not just split the exhaust, have each pipe equal in length, and have them curve in to meet each other, thus removing the whole adjusting process. |
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I just want smoke pouring out of my trombone, after a fiery solo performance. |
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// why not just split the exhaust, have each pipe equal in length, and have them curve in to meet each other // |
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They actually did that on the mid 80's camaro. |
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This might make an interesting science project. Generate a tone with a stereo speaker, then try to neutralize it using your apparatus. |
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if only you could make it painful for the other driver... |
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(+)...for 'Out of the Box Thinking' about 'Exhaust Pipe Internals'. |
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MikeOxbig, the problem with merely splitting the exhaust and bringing it back together is that it does nothing about the phase of the sound. |
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If you simply add together two identical sounds, they produce a louder sound. If you have two sounds with the same volume, and one sound is the inverse of the other (high pressure/low pressure swapped), they cancel each other out. |
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Engine exhaust sounds are close enough to sine curves, that a 180 degree phase-shifted version of the sound will resemble an inverted version. |
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Thus, if you split the exhaust into two unequal length pipes, making one pipe longer than the other by one half of the sound's wavelength, then join them back together, the resultant exhaust noise should be much quieter than the original. |
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The sound's wavelength of course depends on the engine's rpm, though, so the lengths need to change with the engine's speed... thus 5th's idea. |
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This is an interesting HB classic. |
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// This is an interesting HB classic. // |
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Thanks for bringing it up; I hadn't seen it before. |
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// You'd need a long, straight pipe of adjustable length, closed at one end, and an opening roughly half way down its length whose position could be adjusted. This would give you independent control over the two variables of noise and power. // |
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You could implement that as a Y arrangement too, right? And then just use a trombone on each arm of the Y to control its length. |
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// As noted before, because its nearly impossible to maintain an air tight seal with the heat and vibration in the trombone scenario, the trumpet tends to have more of an air tight system. // |
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Instead of sliding joints, maybe it could use some kind of flexible bellows? Stainless steel bellows for making flexible exhaust joints are widely available and cheap, IIRC. But the uneven internal surface might scatter the sound somewhat, if its undulation is not much smaller than the sound wavelength. |
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// Well if you need two equal sounds, why not just split the exhaust, have each pipe equal in length, and have them curve in to meet each other, thus removing the whole adjusting process. // |
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I fail to see how that is not exactly equivalent (acoustically at least) to a single pipe. |
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