All readings were taken at 5500rpm with calibrated meter 45' to exhaust outlet and 0.5meters away.
Manifold, B-pipe, Backbox, level (db)
Std, Std, Std, 93
Std, Std, Spoon, 102
Spoon 421, Spoon, Std 94
Toda, Spoon, Spoon, 106
Std, Spoon, Spoon, 104
Jr , TG, TG, 109
Std, TG, TG-Ti. 96
Std, Cobra us, Cobra, 109
DC, Spoon, Spoon, 108
JR, Eurosport, Eurosport, 105, Spercharger & Cams
Std, Mugen, Mugen, 89
Toda, Toda, Toda, 94
Couple of points from personal experience:
1. Spoon bung doesnt actually work very well. Yes it does reduce level slightly but when you lift off it bangs and pops going even higher than a box without the bung.
2. Its worth carrying your standard backbox with you as even with a full race manifold and Spoon B-Pipe it will still see you under 100db. 8)