When we made our CD of muffler sounds back in 2001, we used very nice recording and editing equipment. We had one microphone for each pipe and everything was measured and recorded (for our benefit and analysis) including temperature, humidity, time of day, etc. We did it all one Sunday and when you think about making 54 runs, that was a lot of muffler and pipe changing and engine revving.
Even so, we had to watch out for the system reducing the sound levels to protect itself. In other words, when done, the loud mufflers are louder but not as much as they sound in real life. Using cell phones and stock computer speakers, as Big Block says, is often not as accurate, even with today's very good phones and computer speakers.
I used a calibrated dB meter always placed and locked into the same spot and we were very careful to observe loud background sounds (like wind) when doing the actual recording. I followed SAE testing guidelines and when we were all done, it work out very well. For example, with the 20" long Dynomax Super Turbo mufflers, which are quiet for sure in most cases, you can listen closely to the CD with good speakers and/or headphones and hear relatively quiet background sounds like birds chirping and the electric fan we used on our test engine which was a long ways from the exhaust outlets. When listing to the sounds from the loudest chambered mufflers, you hear no background noise, only exhaust sounds. The reduction in levels the microphones or input to the recorder did to the sound level is very apparent to me and so much so, I put a caveat with the paperwork that went with the CD that explained that the CD sounds were accurate but the sound level was to be compared from the data collected with the dB meter and not from the CD itself.
Another interesting thing was the mufflers that drone in cars, tended to drone on the CD and with good playback equipment, you can actually hear the systems going into resonation. I intentionally selected the rpm to use that is very close to the point where V8 cars resonated. With the engine we used, around 2200 rpm was that point. With my 340, that is also about where my loud mufflers would resonate and roar.
A dB meter used consistently and according to guidelines is a must when making true comparisons. Even a few feet and directional orientation can make a big difference when dealing with sound levels. Just like Yellow Rose mentioned above.
Tom Hand
