Quote:
Originally Posted by ryank382
Do you know if you need a specific length to cancel out the right frequency?
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Yes. A J-pipe works by
phase-cancellation. Part of the sound from the engine enters the J-pipe and is reflected back with a time delay proportional to the length of the J-pipe. When the reflected sound is exactly half a wavelength delayed compared to the origin sound wave, the peaks and valleys cancel each other out.
In order for the reflected sound wave to be 1/2 a wavelength delayed, the J-pipe must be 1/4 of a wavelength long. The sound will travel to the end of the J-pipe and back, 1/4 + 1/4 = 1/2. To calculate the optimal length of the J-pipe:
1. Identify the frequency you want to cancel out, using a sound meter app.
2. Calculate the wavelength of that frequency. Wavelength = How far the sound wave propagates in one wave cycle = speed of sound divided by frequency. Note that the speed of sound is temperature dependent.
3. Build a J-Pipe 1/4 the wavelength.
For example, to cancel out 90hz, assuming you want to cancel it when the exhaust is hot (I don't know, 500 F?), the speed of sound is 460 m/s (
calculator), the wavelength is 460/90 = 5.1 meters, a quarter wave is 1.25 meters! however, the air in the J-pipe may actually be cooler that the exhaust gas since exhaust gas doesn't flow through it. If we use 100 F, the speed of sound is 353.4 m/s, a quarter-wave is 1 meter, which is still a very long J-pipe.
I suspect the J-pipe in the muffler above goes deep into the muffler and maybe does a bend. It's also possible that the muffler itself does a better job of cancelling low-frequencies than the stromung due to a different internal design. It's also possible that you don't need to get the optimal length in order to reduce the drone sufficiently.
Ultimately trial and error is a simple and guaranteed way to get to the results you want. Depending on how much time and money you want to spend, you might be better off modding the OEM muffler because you'd get the good low-frequency cancellation it provides.