It's an SPL meter you want, Maplins do them for about £50
http://www.maplin.co.uk/products/mod...1&P roducts=1 The standard cans are stamped as complying to the 80dB noise requirement, so you should be fine with these.
http://www.chathamclose.com/pics/endcan.jpg The standard test for road use is a top gear drive by at around 60 mph, whereas circuits tend to measure at 3/4 redline 1m from the exhaust at an angle of 45degrees. I'd suspect MIRA would use the driveby test becuase of who they are and the low limit they've set.
This is why most standard cans on most roadbikes score well into the 90's at circuits, even though the noise limit for road use has been 82dB since '91 and most manufacturers (inc Ducati) work to the tighter US EPA regs with a limit of 80dB.
I got sick of my standard piped R1 failing noise tests, so did a bit of digging into why the figures don't match up, even bought my own SPL meter to dispute the uncalibrated units at Goodwood & Castle Combe.