[skidlids]

Castle Combe trackdays
The Noise limit is 105dB(A), measured at 0.5m from the exhaust outlet at 3/4 max revs

I haven't got this years ACU handbook yet and I can't find my 2004 copy so this is from the 2003 Roadrace rules

ACU
The limit is 105dB(A), measured at 0.5m from the exhaust outlet at 45 degrees @ 5500 rpm for a 600cc twin

A few hundred revs and a angle of 45 degrees makes quite a bit of difference.