a sound test on a moving vs a non-moving vehicle will give you different results. Also the sound transmission thru a thicker-walled pipe will be less than thru a thin pipe. There are a lot of variables to play with, including turbo size, downpipe, headwork, etc.
Actually, the test that most States recognize and use for vehicle sound control is the SAE J1169.
The exhaust test is measured at about 4500 rpm @ 0.5m from the tip. The specifics of the test can be found at the bottom of the page on the APS website here:
I have the 3.5" APS - it's gargantuan and LOUD. But it's going bye bye in favor of the MadDad Whisper Dual Tip - I just got it delivered last week and I the quality is top notch. Rich - thanks for the prompt delivery!
I also love the fact that it looks rather "stock" (no huge tips) so appearance helps to go undetected.
A Review in the above format would be helpful to get a good idea of exhaust - for sound level measurements you do have to specify the distance the microphone is placed from the tip. Since noise level decrease with distance is a significant factor, as well as the RPM of the engine and the sound scale used. "C" scale will give you higher levels than the "A" scale. I believe you need to use the "A" scale as that one is weighted for human hearing.