So we have an autocross site that is having sound issues. The club started taking sound levels this past event at a different site so competitors know how loud they are and gives them a chance to change it before the next event at the troubled site.
There are large buildings on 3 of the 4 sides of the track, and sound was measured on the side without buildings and per the SCCA rules book (50ft off track, and the sound meter on the correct settings).
There has been a heated argument over the sound results. Many folks are saying that the echoes off the nearby buildings are louder than the cars themselves, causing louder than normal readings. Is that possible?
My thinking on it is that sound loses strength as it passes through the air, so traveling a few hundred feet to a building, bouncing off of it and traveling a few hundred feet back to the meter, it should be at a lower Db. But I'm not the smartest guy by a large margin, so I could be way off base.
I'm just curious which side is correct.
And for a baseline, my car measured the same at this event as it did at the Wilmington champ tour (huge open airport) and at a neighboring regions event with one large building close by.
Acoustics play funning things. Neighbor had a typical outdoor shooting range about 1/4 mile away. Depending on exactly where they were, you could feel the concussion on your face and chest standing on my place.
SCCA standards are nifty, but don't hold any legal water with regards to noise and nuisance complaints and a county/state zoning agency.
Your site is probably experiencing reverberant field conditions. In the reverberant field, sound radiated directly from the source exists in conjunction with sound reflected from surfaces in the area (walls ect.). The reflections add significantly to the sound level produced by the noise source. The reflections will not be louder, but they will add to the total sound level. You may want to take this additive condition into account if you want to keep you neighbors happy.
In reply to Entropyman:
Good point, but the peak Db shouldn't be higher, correct?
The site we measured at isn't close to anything, so we don't have to worry about sound levels there. We were just trying to be proactive by giving drivers their sound levels before going to the troubled site.
If you have one noise at 80db, and one at 100db both occurring at the same time, your total will be higher than 100db.
At least, i think. I know it won't be 180db, though.
Peak Db will be higher than that experienced in an area with fewer reflective surfaces.
if the reflected sound is more or less in tact, you can have 2 different conditions occuring. If the reflection is inverted in phase to the origin of the sound, then the reflected wave can partially cancel the original wave, to our ears it sounds unnaturally softer for some reason.
if its IN phase with what is reflected, it can sum and partly spike amplitude.
here's a good visualization of both processes. the red line is the final sound wave you would hear. notice how the hills double in height when in faze, but the red line approaches flat (silence) when they're out of phase.
https://en.wikipedia.org/wiki/File:Waventerference.gif
