Im not sure on the exact methods but i believe the dvice is to be held at roughly half a meter from the exhaust tail pipe at a 45 degree angle and the noise sampled

Various tracks have different limits but as long as you aim for a 98db MAX you can get on any track in the UK..

Taken from Castle Combe web site

On all our Action Days, the sound limit is 100 dB(A) - measured at the exhaust pipe at 4500rpm on a static test. Your car may be completely road legal, but if it doesn’t meet our check - then sorry, it can’t go out.