Helmholtz resonators in exhaust

Reposting this in here for more traffic..

Has anyone tried it?

In case you're wondering, helmholtz resonance is a phenomenon where a cavity resonates at certain frequencies of sound.

http://docs.lib.purdue.edu/cgi/viewcontent.cgi?article=2200&context=icec

http://www.diracdelta.co.uk/science/source/q/u/quarter wave tube/source.html#.VpbfN7x-jdk

By designing a helmholtz resonator in to an exhaust system, you can attenuate particular frequencies of sound depending on the shape, volume, and temperature of the gas.

I've researched and found that lots of car guys do it to kill drone on their 911/vette/g35 or whatever. The simplest idea is to create a cavity that is a quarter wavelength in length of the peak drone frequency. That way, the sound waves travel a total distance of a half wavelength (putting the wave out of phase with the target frequency) and cause destructive interference to reduce noise.

I measured the drone on my truck at various speeds with my laptop and plotted amplitude vs log frequency. At 70mph the drone peaks at 89 Hz. Real annoying on the long drives.



Here's the basic calculation

L = 1/4*lambda

L = 1/4 * v_sound / f_peak

v_sound depends on temperature. Assuming 200*F, v_sound = 1275 ft/s

L = 1/4 * 1275 ft/s / (89 1/s) * 12 in/ft = 43 in

43 inches is LONG, but there is enough space under the truck.

One concern is that the drone appears to be pretty wide band, which means that if the resonator is to "peaky" it's not going to do a whole lot, since the peak drone frequency changes with RPM and temperature. Looking for a "low" Q factor resonator, but this is not easily calculated. It seems like it will be just trial and error with all the variables involved.


I guess what I'm looking for is thoughts/advice/first hand knowledge on the subject. Surely someone has tried this on their tacoma.

 

Idk that's a lot of techno babble for me. And the resonator I got wasn't expensive, and is like a foot long.. Just sayin. Seems like a lot of hassle to me. But interesting nonetheless. If that's the kind of thing you can do. But doubt many here would be willing to mess with a project like this. I'm happy with the ultra quiet res I got put in. they all seem to do the trick. Haven't heard of any bad things regarding slap on resonators.

 

Op did you ever follow through with this project? I just installed TRD exhaust and it has a much louder drone than I expected. I inquired about a J pipe today at local custom exhaust. They want $225 to fabricate and install.

 

Throw a glass pack inline

low frequency sound has long wavelengths, as OP mentioned. J-pipe is good for the factory intake, not your exhuast. You're better off deadening it than effing around with some clever resonance tricks

Edit: you know what? engineer time. The same effect can be achieved by welding a v shape in your exhaust tube 1/4 wavelength from the muffler to achieve reflection without an added cavity. (use the cavity you already have) Good luck and you can't sue me for your own experiments