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[Brad Weber]

How far away are the residences and how do they compare in elevation?  A common misconception is to assume that the noise reduction noted a few feet from a berm or barrier will also apply at geater distances away but that is not true with any significant change in distance or elevation for either the source or receiver.  When I worked at an amphitheater the complaints came not from adjacent properties but from those some distance away where the berms and barriers had limited effect while thermal, wind and other factors diffracted the sound back down.
 
So trucks, berms or whatever may help where the cars and listeners are both at ground level and physically close to the barrier but they may not be as effective where the distances to the cars or listeners are larger and where the listeners may be at a higher elevation.

[John Roberts {JR}]

For today's outside the box suggestion we need an anti-horn on the end of each tailpipe. Instead of the typical megaphone-like flare at the end of the header/tailpipe, we need a reverse taper or an inverted cone a few inches from the pipe outlet so the sound goes into the cone and bounces around to interfere with itself, rather than couple to the outside world.

The speed of sound is faster(?) than the speed of the exhaust gas traveling in the headers so it seems the two can be somewhat treated separately. If a larger cone is located a few inches from the pipe outlet it shouldn't introduce that much back pressure. 

How many dB attenuation is needed to keep the neighbors happy?

JR

[Patrick Tracy]

For today's outside the box suggestion we need an anti-horn on the end of each tailpipe. Instead of the typical megaphone-like flare at the end of the header/tailpipe, we need a reverse taper or an inverted cone a few inches from the pipe outlet so the sound goes into the cone and bounces around to interfere with itself, rather than couple to the outside world.

The speed of sound is faster(?) than the speed of the exhaust gas traveling in the headers so it seems the two can be somewhat treated separately. If a larger cone is located a few inches from the pipe outlet it shouldn't introduce that much back pressure. 

How many dB attenuation is needed to keep the neighbors happy?

JR

If you look at 2-stroke motors you'll see something just like that, but it's designed to produce precisely timed reflections of the exhaust pulse that return to the exhaust port just in time to assist with compression.

[John Roberts {JR}]

If you look at 2-stroke motors you'll see something just like that, but it's designed to produce precisely timed reflections of the exhaust pulse that return to the exhaust port just in time to assist with compression.

I think you're talking about expansion chambers(?) and they are only tuned for a specific rpm range. I always thought they were returning a negative pressure wave to scavage gas out of the cylinder and pull more charge through it. That may be a 4 stoke thing, but it seems any motor would want more suck than blow at the exhaust port. A reflected pressure wave at the exhaust port seems like it would reduce charge flowing in a 2 stroke motor, and reduce BHP.



JR

[Tommy Peel]

If you look at 2-stroke motors you'll see something just like that, but it's designed to produce precisely timed reflections of the exhaust pulse that return to the exhaust port just in time to assist with compression.

I'm guessing that you're talking about something like the muffler below from a Stihl FS 45/46/55 trimmer? They have an inverse cone on the exhaust exit. I'm not sure that they improve the engine performance(they seem to run just as good w/out the plug). They do significantly reduce the volume of the exhaust as well as change the pitch to a less offensive one. The plug on the Stihl does have a spark arrestor screen behind it that frequently gets stopped up(I worked for around a year in a Stihl dealer's shop), but that's a topic for a different forum.  When those things get stopped up they do get real quiet(and weak) though assuming they run at all.

Edit: added picture.

[Patrick Tracy]

I think you're talking about expansion chambers(?) and they are only tuned for a specific rpm range. I always thought they were returning a negative pressure wave to scavage gas out of the cylinder and pull more charge through it. That may be a 4 stoke thing, but it seems any motor would want more suck than blow at the exhaust port. A reflected pressure wave at the exhaust port seems like it would reduce charge flowing in a 2 stroke motor, and reduce BHP.



JR

Yes, expansion chambers. In addition to helping draw the fuel-air mixture with negative pressure they also improve compression with positive pressure. They work over a wide RPM range since they're acoustically tuned and the speed of sound is pretty consistent.

[John Roberts {JR}]

Perhaps also an analysis of what frequencies in the noise, carries the furthest and is the source of annoyance. Tuning exhaust to reduce noise could be pretty different depending on the frequency range of that noise being addressed.

JR

[duane massey]

Might be cheaper to hire a 100 or so "gangsters" with competition-level sound systems to cruise thru the neighborhoods playing recordings that mimic the cars when there is an event......

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