Helmholtz resonator in exhaust

Who's tried this?

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 eliminate 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 eliminate the 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. 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.

 

I don't have much to contribute on Helmholtz resonators, but the guys that I bought my blower kit from seem to have some experience with it on Toyotas.

http://dirtydeedsindustries.com/helmholtz-resonator/
http://www.bamufflers.com/

 

Are you shit'in me? The most technical TW gets with exhaust sound, is after asking for a "which one" opinion, is listening to exhaust sounds on you tube videos.