[WARP TEN]

Originally Posted by mike100

.... Loud muffler with the stock resonator drones loudly at 1800 rpm. I forget what the resonator is tuned for specifically......

Mike is correct--the Helmholtz resonators in the stock mufflers are tuned to quiet the 120 Hz resonance at about 1,800 rpm and the stock resonator handles the 230 Hz resonance at about 3,400 rpm. Our engines are notorious for those resonance peaks.

I wrote an article many years ago for the then-equivalent of the HOB after I had an acoustics engineer wire the car and write a report for me. Here is the section on resonance. Remember it was written quite a few years ago so technology has improved in many systems--Bob

Controlling Resonance. If resonance is a concern to you, the only way to really tame it is to utilize a devices called Helmholtz resonators (or quarter wave tuners). To my knowledge, only the stock system and the Corsa use such devices; Corsa’s trade name for its patented approach is Reflective Sound Cancellation (RSC).
What is it a Helmholtz resonator? Imagine a closed coffee can with an opening into it. Exhaust doesn’t pass through, but sound waves can enter and, depending on the size and shape for a specific frequency, cancel each other out. If you cut open a stock muffler, you will discover that the rear half of the muffler is in fact a closed chamber with a pipe opening into it for sound waves. While you can notice the resonance points on both the stock and Corsa systems when you go through them under load, they are muted and much less objectionable than others. While Flowmaster doesn’t turn to the Helmholtz principle for resonance control, it does use a patented sound canceling design that utilizes V-shaped deflectors to lessen unwanted sounds.