Benspeed,
If you are talking about reducing the sound pressure from 103 to 89 when measured from the same distance...good luck!! If I remember correctly a 6db reduction is half as loud. 103-97 is half as loud. 97-91 is half as loud again. You are talking about less than 1/4 as loud.
Maybe an acoustic engineer can put his/her $.02 in. But I also believe that certain frequencies travel better than others. Weather is also a factor.
I ran with a club that had absolutely no noise limits at all the tracks we raced at EXCEPT Laguna, which for us, used to be 92. It was lowered to 90 this year and I don't know of any Rx7's who are going.
Laguna measures it on driver's right approx, 100' from the center of the track, halfway up the hill between T5 and T6. It was very difficult to get the Rx7's down to 92.
Non Laguna set-up was the same as yours.
We then added a 3" in - 5" disc only series supertrap at the end of a 45 degree turn out (with the mud shield***). We went out with 8 discs and got black flagged for 92 db and change. 8 discs had worked in years prior. Rather than remove a couple of discs and get black flagged again (ending our weekend) we put 4 discs and asked for a courtesy check. This put us between 90-91db and absolutely killed our top end. We decided to try 6 discs and got our last courtesy check...we were in the 91's and gained about 400 rpm over the 4 disc setup. Still about 1000rpm shy of our normal setup.
According to the folks at Supertrap 14 discs is approximately equal to no supertrap.
Our testing found that each disc removed cost us between 100-200 rpms of top end and seemed to move our bottom end down about the same amount. Adjust your shift points accordingly and maybe it won't cost you much time.
We experimented with a RadioShack meter on a tripod at home and found that a 45* out and 45* down was the best set up.
A 90 out meant that the sound deflected off the ground then the hill and straight back to the meter. We guessed that the 45 would mean that by the time we were at the angle where the sound would deflect off the hill and to the meter we would be farther up the hill and therefore, farther from the meter.
We witnessed many other contraptions that seemed to work in other cars. Coffee cans packed with steel wool and holes punched in the side away from the meter.
At 90 db you need to start thinking about intake noise as well. We had some "quiet" turbo cars have their blow off valve thing exceed the limit.
*** Supertrap exhaust discs divert the sound (and heat
) outwards and back towards the exhaust pipe. The mud shield deflects the sound and heat back out the rear of the car.
Good luck. I hope wherever you are racing they measure it farther than 100' away OR at less than max rpm under zero load.
