The "J Pipe" talked about in the above posts is more correctly called a "Resonant Acoustic Stub" and has been discussed here on Moparts several times in the past. It does not have to be shaped like a J. It does have to be a specific length to cancel a specific tone. If the temperature of the exhaust gas changes, so does the frequency of the sound tone cancelled.

This same scientific principle is used to quickly measure the static compression ratio of NASCAR engine cylinders during inspections.

A "slide whistle" works this way too.

https://www.amazon.com/American-Slide-Wh...s=slide+whistle