Originally Posted by stephentonks
Wow !! Thanks for all the advice. I shall definitely have a go with the design submitted by Curtis. I would also like to respond to some of the other comments. There is a fence at the side of the car but there is nothing behind the car. There way be some echo, so I will try the test again where hopefully nothing will interfere. My sound meter does have a weighting switch but unfortunately it is already set to "C". I am conducting the test based on information I got off one of the circuits' web sites. They say the reading should be taken at 50 cm away from the exhaust at 45 degrees.
I have seen some silencers sold by Merlin Motorsport. An additional silencer fitted to the end of an exhaust tail pipe. Used on track days where the existing system fails the noise test. The Merlin Muffler can take from 0 to 9 Db off the noise reading. They do a try before you buy service, so it might be worth a look.
50 CM is Awfully close... (like 20 inches?) Racetrack DBA meter readings here in the states is generally 100 feet from the track. Are you certain that their spec isn't 50 m (as in Meters?) - which would be more like it...
One thing I've learned over the years is to NOT point the exhaust down (at least not in close proximity) to the ground. It "Cracks" much louder and the sound pressure reading is much higher.
I've used the Vortex cones that Chuck has shown here. On one of my cars (310 Cubic Inch) - the engine would no longer make it to over 6200 RPM - shift points were normally at 6800 RPM. While it did knock the pressure reading down by 5 dB, it was quite literally like stuffing a potato in the tailpipe as the engine was very lazy afterward..