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How to reduce exhaust noise.

04/27/2007 9:27 AM

I've recently been looking at reducing noise emissions from an exhaust from a single seat race car. The idea is to reduce the noise without effecting performance (as in a silencer). To cut a long story short, after an extended design meeting in the pub one Saturday night, I came up with the idea of putting a propeller blade in the pipe to spin the gases. I have no idea where the original idea came from, other then Mr Daniels finest. Anyway, we tried it and got a reduction of 13 decibels, and went up the grid 10 places! Has anyone any info on any research on this topic? My (alcohol fuelled) theory was that introducing spin would create an interference with the sound pulses. I'd love to hear your theories.

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#1

Re: Exhaust noise.

04/27/2007 9:39 AM

This "sounds" like a brilliant idea. My half-baked theory is that anything you do, to interfere with or disrupt the wave-pattern of sound as vibrating-air, is welcome, and thus you may somewhat reduce their audible level, by means of dispersing it's initial potential energy-level.

A gun silencer, for instance, is only designed to interfere with the micro-molecular swirls (micro-cyclones created by supersonic movement, stretching the molecules) which are created by the shot, thus reducing audible level by more than 60 db SPL, sometimes.

Interfere with the given pattern, I say.

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#2

Re: Exhaust noise.

04/27/2007 11:20 AM

When I was a teenager I had an off road bike with a really loud exhaust. It was a straight through end can with mufflers. The father of a friend of mine told me to open it up and insert a bold in the mesh across the flow of exhaust gasses. This would split the sound pulse and reduce the noise.

I never tried it cos as teenager I rather liked the noise. Loud exhausts make every boy feel like a man! Wish I had tried it now though cos if it works it is a very simple method to reduce sound.

If you don't want to restrict the flow through then you could perhaps use an oval cross section or perhaps multiple thin strands to break the sound pulses.

Just another thought though. If the gasses were restricted could you place tubes of narrowing diameter over the end can towards the exhaust outlet. Kind of like air scoops to blow faster moving air past the outlet. This would reduce the pressure and help suck the gasses out. Feel free to comment on this and tell me I'm talking rubbish if thats the case.

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#3

Re: Exhaust noise.

04/27/2007 11:28 AM

Kewl. Am I correct in assuming the the propeller was fixed, so the the airflow spiraled? Or did the propeller spin, so there would be a slight spin imparted to the airflow, but a more significant spin imparted to the propeller itself?

Years ago, I built exhaust systems for four stroke and two stroke motorcycles, in which acoustic tuning can have a profound effect (especially, and dramatically so, in two strokes). In both, the negative reflected wave (from the outer end, or diverging part of the pipe) can travel back up the pipe to help scavenge the cylinder just before the valve closes (or port closes, in the case of a two stroke).

In a four stroke, the overall effect of a divergent cone and reflected wave or waves is fairly simple. In two strokes, it is not quite so simple, because there are two effects working. In the first, the divergent cone of the expansion chamber reflects a negative pressure wave back to the exhaust port, to help scavenge the cylinder. Later in the cycle, the convergent cone sends a positive pulse back down the pipe to shove unburned mixture (copious amounts of which have been pulled up through the transfer ports, and right out the exhaust port) back into the cylinder, just before the exhaust port closes. As you'd guess, the more dramatic the cone angles, the more pronounced the effects, but over a shorter rpm band. (In the extreme case, there were 50cc racers producing 21 hp, with a useful power band extending from about 20,000 to 20,500. At 10,000 rpm these bikes had all the power of an electric shaver. OK, I m exaggerating the peakiness a bit -- but the 7 hp/cu in (440 hp per liter) is accurate.)

On some of the two stroke pipes, I would invert the stinger (the small pipe at the end) and put it inside the chamber, so that it's end was in about the center of the fat section, where positive and negative pulses would collide, cancelling at some rpms. The resulting pipe was about as quite as one fitted with a silencer, but lighter. Performance was the same.

I wonder if your pipe performs better than a straight pipe, or is your comparison with a silenced one. If it is better than a straight pipe, then my guess would be that for the rpm at which the engine spends most of it time on the track, the straight pipe was disadvantageously tuned. The propeller would serve to reduce the acoustic effects, and spread the power band better, making the car faster in the real world.

I love this stuff -- real science in action, as opposed to running the numbers.

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#12
In reply to #3

Re: Exhaust noise.

04/28/2007 4:48 AM

Yes, the 'propeller was basically made from a piece of thin stainless sheet I had laying around cut into 3 identical vanes and welded into the pipe so as to cause minimum obstruction. I kept them connected in the centre with a very thin section bent at 90 degrees to the flow. I didn't think tho take a picture, when we take it apart again I'll try and remember, or post a sketch. I know the sound waves are travelling faster then the gases, but I was thinking along the lines of the gases are a carrier for the waves, so that an imparted spin introduced into the flow has to have an effect on the wave - however minimal! (Not my area of expertise, obviously!)

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#4

Re: Exhaust noise.

04/27/2007 12:03 PM

The noise reduction is consistent with the theories of propagation of sound, however it has nothing to do with the spinning of gasses.

The sound is travveling alot faster than the gasses.

As a sound wave which normally travells in all directions is confined in a pipe, the sound wavefront builds up power by adding on itself while travelling along the walls of the pipe. Any obstruction, or in this case deflection will interrupt that accumulating effect and reduce the sound comming out of the pipe.

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#5
In reply to #4

Re: Exhaust noise.

04/27/2007 12:10 PM

...wavefront builds up power by adding on itself while travelling along the walls...

Additive (as opposed to subtractive) interference.

And vice versa:

...Any obstruction, or in this case deflection will interrupt that accumulating effect and reduce the sound...

This concurs with what I thought I knew.

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#6
In reply to #5

Re: Exhaust noise.

04/27/2007 11:17 PM

analyze the sound frequency spectrum. Then make an inline plenum that splits the exhaust stream into, say 8 flows. Make sure the combined area of the 8 pipes is about 15% greater than the area of the inlet. These are then combined in another inline plenum about 2-3 feet along. Here is what you do. You need to try to make some cancellation of the major frequencies by delaying one by 1/2 wave. With 4 pipes, pick 4 frequencies in the major sound energy band. Use 1000 feet/second and if the major band is 500 hertz 1 wave = 2 feet. 1/2 wave = 1 foot.

All this will reduce the power out of the engine, but it will be quieter.

In race cars they do the opposite to try to create cancellations at certain points for more power.

http://www.google.com/search?hl=en&q=%22tuned+exhaust+systems%22&btnG=Google+Search

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#8
In reply to #4

Re: Exhaust noise.

04/28/2007 12:20 AM

Boilers (and water heaters) are usually equipped with turbulators to induce spiral mixing of the gases, but they would also break the wall continuity. Possibly an impressed spiral for the length of the pipe. Small water heaters use a spiral plate nearly full length, loose in the flue. Maybe a spiral-weld pipe with a lip turned in? Or a filler inserted into the weld of the spiral weld? I could use a pair for my Pantera. Thanks

Rich

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#9
In reply to #8

Re: Exhaust noise.

04/28/2007 1:46 AM

Pantera -- there's a name I haven't heard for a while. Drove and worked on one or two. Great cars. I'm envious.

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#10
In reply to #9

Re: Exhaust noise.

04/28/2007 1:49 AM

Needn't be. a summers rework and a winter paint job before it can get the use it deserves.

RichH

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#7

Re: How to reduce exhaust noise.

04/27/2007 11:43 PM

The talk of a fan did give me an idea, though it seems a bit impractical. Place a supercharger in the exhaust to boost the pressure and then restrict the outlet.

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#11

Re: How to reduce exhaust noise.

04/28/2007 4:41 AM

Quit eating beans!

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#13

Re: How to reduce exhaust noise.

04/28/2007 6:27 AM

This is of at a tangent slightly , but I was just reading how Harley reduce noise with a variable exhaust on the silver wing 400

Even more off tangent , this has a clip of engine noise reduction for boats ;

http://www.freshairexhaust.com/

I also wondered about active anti-noise devices (?) . I'll leave y'all alone to address the specific question . Many a good idea has come from a bar mat doodle . You may need further Board meetings. Personally I would see that as critical.

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#14
In reply to #13

Re: How to reduce exhaust noise.

04/28/2007 7:02 AM

This idea has already been suggested, and we are in fact having a board meeting tonight to discuss the proposal. Right before the next design meeting!

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#15
In reply to #14

Re: How to reduce exhaust noise.

04/28/2007 7:32 AM

The wonderful thing about board meetings is that they can be re-convened ad infinitum . Every cloud has a silver lining ! On completion of the project I suggest continued review meetings.

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#35
In reply to #14

Re: How to reduce exhaust noise.

04/30/2007 6:28 AM

This thread seems to be drifting a bit however, it sounds like what you have done is to tune the exhaust back pressure & improved the engine power. I have seen tunable race exhausts that let you add or remove baffles from the end of the pipe to achieve the same result, see http://www.supertrapp.com/product_sections/motorcycle/index.asp

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#36
In reply to #35

Re: How to reduce exhaust noise.

04/30/2007 6:55 AM

As it happens, I have a two into one system on my Moto Guzzi with a Supertrapp can on it!

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#16
In reply to #13

Re: How to reduce exhaust noise.

04/28/2007 10:20 AM

Your link look like an amazing solution.

Does it work (These days I have to ask...)?

...wondered about active anti-noise devices...

In Public Address amplification systems, they use what's called "Main Frequency Shifter", to cancel ambient noise feed-backing through the microphone. They shift the frequency by some 5 to 8 percent up or down, and feed it back into the signal, to cancel the feedback screech, and speech pops.

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#18
In reply to #16

Re: How to reduce exhaust noise.

04/28/2007 10:48 AM

No idea capn Yuval me matey - wos lookin' fer a shipmate to tell ee. Arr

see thar she blows -'talk like a pirate day' .aye ladde

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#22
In reply to #13

Re: How to reduce exhaust noise.

04/28/2007 11:49 AM

This is of at a tangent slightly , but I was just reading how Harley...

I'm flying over today to break your legs! Honda makes the Silver Wing.

Along the lines of active noise reduction: Perhaps cars should always drive in pairs (same models) at precisely the same speed. Via a radio link, the engines would be kept perfectly in phase. However, one car would be just behind the other, in the correct spacing to put the sound waves from one exactly 180 degrees out-of-phase with the other.

Of course, the Bose solution would be to sell everyone their noise-cancelling headphones -- then we could all run straight pipes, reducing car weight, and thus improving economy and reducing emissions.

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#24
In reply to #22

Re: How to reduce exhaust noise.

04/28/2007 12:18 PM

Typo Ken , typo thing I does ,Arrgh.

splice me mainbrace I did ,near shivered me timbers aye

<nudge ,nudge,> ,, know what i mean.

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#25
In reply to #24

Re: How to reduce exhaust noise.

04/28/2007 1:38 PM

Aye, matey, we'll let yer slide, this once. Next time, we run ya through!

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#23
In reply to #13

Re: How to reduce exhaust noise.

04/28/2007 12:06 PM

Cool link. The underwater video alone is worth seeing. The prop, as it approaches the frame rate, appears to stop and reverse direction.

I wonder what the solubility of CO in water is? The unburned hydrocarbons which would otherwise go straight into the atmosphere would end up in the water for a while, as they do with outboard motors (which exhaust below water). So, do we like fishes better, or birds?

I'm sure the figures re CO near the transom are correct, because the unburned HC levels (which you can smell, unlike CO) are extremely high near the backs of boats. After a day of sailing, firing up the engine is enough to make me gag.

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#17

Re: How to reduce exhaust noise.

04/28/2007 10:39 AM

I wish someone would sell an exhaust-noise reduction system to Harley Davidson!

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#19
In reply to #17

Re: How to reduce exhaust noise.

04/28/2007 10:51 AM

http://world.honda.com/environment/2004report/24_zisseki_01_2r03.html

.We aims ta pleeze. Dont go west young man -go eest

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#21
In reply to #17

Re: How to reduce exhaust noise.

04/28/2007 11:34 AM

If only they'd buy it!!

I'm getting this wrong, I suspect, and don't have time to look it up... but Harley considers its sound part of its trademark, or some such. So they've tried to sue manufacturers who have come too close. (Again this is foggy -- but it's something equally ludicrous.)*

As an old roadracer, who, in 24 hr endurances races, would race against highly modified Harley's of almost 3 times the displacement of my Honda, I can say the Harley sound truly means "slug" to me. We could out handle anything (and beat even larger Japanese bikes on that basis) but the Harleys were about the only thing out there that we could pass on the straightaways! We might just as well have been racing against lawn tractors.

* Although as a Porsche fan, I'd have to say the the fact that a new Boxter has a lot of the sound characteristics of an old 911 still appeals to me.

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#26
In reply to #21

Re: How to reduce exhaust noise.

04/29/2007 6:22 AM

You are 100% correct, Harley copyrighted their exhaust sound, the description is 'PotatoPotatoPotato' The average Harley owner would never consider buying one that they couldn't hear. The best use we came up with for a Harley was as a ships anchor! (Ooops - cue lots of abusive posts!) As an aside, why do animal rights activists never protest against the use of animal fur/leather outside biker rallies? Is this a case of the sense of self preservation overcoming deeply held personal beliefs? Huh, don't sound that commited to the cause to me!

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#27
In reply to #26

Re: How to reduce exhaust noise.

04/29/2007 9:49 AM

Come to think of it , you don't see animal rights outside fetish clubs . Somebody told me that.. Excuse me while I take my sustainable-timber clog out of my mouth.(I got rid of artificial and animal products ages ago)

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#28
In reply to #26

Re: How to reduce exhaust noise.

04/29/2007 2:01 PM

the description is 'PotatoPotatoPotato'

This is PERFECT! It certainly conjures up 1. the very agricultural (tractor like) sound, 2. the couch potato nature of most Harley riders, 3. even the facial characteristics of most riders -- who here, at least, tend to be middle aged, with that kinda puffy potato-ey look, and 4. the sort of heavy, inert, lifeless quality of a potato as an analogy for the heavy, inert lifeless quality of a "vehicle" best suited for, as you correctly state, anchoring boats.

On the other hand, you have to admire anyone who would (like a Hummer buyer) be so willing to advertise the insecurities that inevitably come with aging. Harley and Hummer buyers verily shout: "I'm insecure about my sexual prowess, and therefore have purchased this loud, flashy thing to compensate." I have to admire the boldness. I drive pretty nondescript cars to keep my own insecurities under wraps.

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#29
In reply to #28

Re: How to reduce exhaust noise.

04/29/2007 2:58 PM

& don't forget the "recliner" rider position, or the "ape Hangers" or the raked out forks

I was following a guy w/apehangers that put his arms almost fully extended [ straight up ], every minute or so the bike would do a little shake

I kept waiting for him to crash & burn

On the other hand I'm too old & my backs too weak, to have all the weight on my wrists [ crotch rocket ].

Nor do I feel the need for 100+hp [ on the street anyway ]

I've had way more fun on little lite bikes, they just need to be able to keep up w/traffic + 10-20mph

I'd love to have say a moutain bike w/10-15hp & a total weight of around 1/2 my load, 10hp & 120#s wet, nearly silent, 6"-8" of travel

Not exactly the potato-potato expeirence

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#33
In reply to #29

Re: How to reduce exhaust noise.

04/30/2007 12:49 AM

I'd love to have say a moutain bike w/10-15hp & a total weight of around 1/2 my load, 10hp & 120#s wet, nearly silent, 6"-8" of travel

Sounds great! I had an observed trials bike for a while that was not quite that light, but otherwise was a good fit. I rode some trials with it, and was never particularly good at it , but had a great fun -- and it made a really nice bike for poking around in the woods.

Light, small, fast road bikes (250cc or even less) are great fun too. Although I've had a real crotch rocket, and have campaigned some powerful bikes on tracks, the small ones are more fun, I think.

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#30
In reply to #28

Re: How to reduce exhaust noise.

04/29/2007 5:56 PM

Oops! I seem to have touched a nerve here. Hehehe. I have a Kawasaki sports tourer for those comfy, and fun, long journey's (I have the weight on wrist problem also) and a Moto Guzzi twin for JUST fun! Now that is agricultural, being originally an Italian tractor design!

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#34
In reply to #30

Re: How to reduce exhaust noise.

04/30/2007 1:15 AM

Oops! I seem to have touched a nerve here.

Actually I'm really not all that offended by Harley riders -- but I do like poking fun at them! Hummers actually strike me as a bit more obscene -- it's as if the buyers are saying "I don't give a hoot about the environment, and I'm proud of that!"

My back and wrists are not all that well-suited to crotch rockets either. I do some pedalling too, and have decided to trade my bike with dropped bars for one with straight bars so my neck doesn't ache so much. I'm lusting for a mountain bike with hydraulic disc brakes, half because I just like the fact that the brakes work so well. The new shifters are great too.

I'd occasionally race against moto guzzi's -- and liked the sound of them. Quite a contrast with the whine of our Honda, which we shifted at 14,500 rpm all day and night long, and it just ticked off the miles. The Guzzis would pull us on the straights, and it was a little like a roaring elephant passing a bumble bee.

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#20

Re: How to reduce exhaust noise.

04/28/2007 11:14 AM

Another dirt bike trick w/no power loss

Hose clamp a 4"-6" long piece of inner tube to the outlet, point the curve down.

used this trick to knock a couple of db's of & pass sound check

the propeller trick sounds like a kritzman spark arrestor, which causes the hot particles to spin just long enough to reduce the temp of the bits of carbon that can start a fire

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#31

Re: How to reduce exhaust noise.

04/29/2007 6:40 PM

Turn off the engine it cuts out all the noise. It never fails.

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#32

Re: How to reduce exhaust noise.

04/29/2007 7:45 PM

Coming back to the original question, after reading all the entries to date, while I was working at Joseph Lucas Australia - subsidiary of the English automotive electric company - one of their subsidiaries "CAV" which worked mainly on products associated with diesel engines had developed a system which reduced exhaust noise.

It was a system which was programmed to reduce the noise by adding a noise in anti-phase using a set of speakers near the exhaust system.

Reports at the time said it was very effective and could be turned on and of at will, like when entering a town.

I don't lnow what became of it, but perhaps someone in the present Lucas empire could look through some archives.

Nowadays it should not be difficult to program a microprocessor to do that,

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#37

Re: How to reduce exhaust noise.

04/30/2007 9:34 AM

About spinning propellers, or vanes, in an exhaust system...

If any one here has ever run a bus, truck, grader, scraper, skidder, etc. with a natural aspirated 53 series Detroit with open exhaust you have a special understanding of loud exhaust from a normal operating engine (excluding dragsters and other exotic performance racers.) I understand that the diesel engine is out of the scope of your race car application. Where they do connect is when someone upgrades one of those screaming diesel engines with a turbo charger they become much quieter even without mufflers. This effect might be very similar to your propeller in exhaust as far as disrupting pulses. Sometimes aggressive turbos will add noise as a blades spine fast and start sounding like a tiny jet engine. My point (arguable that I even have one here) is if you like toying around try rigging up a little turbo in the exhaust and see what it sounds like. I assume regulations require natural asp engines, so leave the compressor side unhooked for the little experiment. Might need an oil can handy to keep the turbo bearings in decent rig.

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#38
In reply to #37

Re: How to reduce exhaust noise.

04/30/2007 10:18 AM

Well done sir! you have kicked the old grey matter into some semblance of order and made me remember why I got the idea in first place. I remember spectating at Le Mans a few years ago (it was early in the weekend, that's why I remember it!) and noting the marked difference between the turbo cars (Bentley, Audi et el) and the normally aspirated cars. Particularly the CR5 Corvettes. The turbo cars made a soft whooshing noise braking down into the corners, the engine on overrun making hardly a sound. The Corvettes braking into the same corner made a...well...get an empty 40 gallon drum, chuck a load of spanners into it and shake vigorously! It made your ears bleed. QED!

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#39

Re: How to reduce exhaust noise.

05/02/2007 2:12 AM

Excellent approach,

From one side you can use gun silencers, as model, because engine, is like a quick repetetive shots??

From other side i remember one old Russian invention, where silencing was done by whirling exhaust gases by fixed blades inside silencer in form of turbine blades. Most problematic was implementation, eg manufacture such shapes and fixing. As final goal was make it inexpensive, and like homemade, lol.

andrewzol@mail.ru

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#40

Re: How to reduce exhaust noise.

07/14/2009 7:23 AM

I'm going to try it... Alway's building or modifying little bikes or go-karts for my son's and myself. Flow master exhaust for cars crosses the two streams so it actually helps one another, sounds better and increases horse power... not sure if actually quiter though. My friend is helping me learn about pc's and on the subject of noise cancellation said they take a microphone and speakers and play 180 of the sound back - guess it's supposed to cancel the other out w/in the area of the speakers, it would be cool to see a motorcycle going buy but here no noise... probably not real practical for the application your looking for... thought the subject was interesting, just a tinkering DAD. GOOD LUCK!!!

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Join Date: Jan 2011
Location: Phoenix, AZ
Posts: 2
#41

Re: How to reduce exhaust noise.

01/26/2011 10:12 AM

Hi guys!

Old topic, but relevant to what I am working on.

I build motorized bicycles, the little Chinese motor kind, and my showpiece bike has a 66cc with an expansion chamber and automatic transmission.

It has a silencer, bit still was to loud and high pitched for my taste, so I set out to quite it down and hopefully lower the pitch some. This is what I have come up with thus far but I always love to hear other views and the 'propeller in the pipe' sounds like it could help.

(hmm, no embedded pics..)

OK, this is the bike with the stock Expansion Chamber and silver silencer on the end.

http://kcsbikes.com/pics/KCnv1-21-11-Left.jpg

This is what the inside of the silencer looks like, just a 4" long 1 1/8th" perforated tube in a 2" can stuffed with fiberglass.

http://kcsbikes.com/pics/XchamSilencer2.jpg

This is the end of Expansion Chamber where the silencer hooks on. It is a 2" flange.

http://kcsbikes.com/pics/XchamSilencer1.jpg

I want to replace the the silencer with a 3" diameter 12" long automotive Glass Pak muffler and mount it like this with 2" exhaust pipe bent to curve it in and parallel to the ground to a muffler.

http://kcsbikes.com/pics/muffler1a.jpg

A sound wave travels at, duh, the speed of sound, and has a frequency relitive to the frequency of the sounds creation, again, duh.
Keeping a sound wave in a tube the frequency stays the same and actually gains in amplitude (strength) as it bounce back on itself.

I can see how holes in the pipe to insulation quiets the motor now, the holes trap some of the sound wave in the insulation and disrupts it some.

My pipe [i]should[/i] do two things...
1. It is 3x longer, so it should lower the amplitude (loudness) by a factor of 3, or 3 times quieter, and

2. It is has an internal perforated pipe almost twice the diameter, so it should lower the frequency (pitch) of the sound wave by close to a factor of 2.

The result should be a much quieter and deeper sounding exhaust don't you think?

Now before I weld it up, what do you think about a little butterfly valve or propeller in the 2" pipe just before the 3" muffler? Think it would help any? I am not to qorring about back-pressure, the pipe is already much bigger than stock.

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#42
In reply to #41

Re: How to reduce exhaust noise.

02/02/2011 7:35 AM

That sounds intresting! Let us know how you get on...

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#43
In reply to #42

Re: How to reduce exhaust noise.

02/02/2011 1:05 PM

I'll let you know how it turned out when I get back home.

It sounded a deeper and much quiter in the brief time I tried it, but I wrecked and typing from a laptop in a hospital bed at the moment.

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Location: Phoenix, AZ
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#44
In reply to #42

Re: How to reduce exhaust noise.

02/04/2011 5:02 PM

I got the auto exhaust mounted.

It is just temp mounted. I need to pull it off and paint it black, and then it is getting a chrome reducing end piece that points down but it does as intended. It dropped the noise level and deepened the tone without effecting the Expansion Chamber performance at all.

The process was interrupted by my taking a spill during the Before Video of the sound. It is rather gruesome and I just got home from the hospital last night, but you can read all about that and see the before video here:

http://kcsbikes.com/topic.asp?TOPIC_ID=163

Once I can use my right leg again I'll get an After video with sound so you can hear the difference.

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