How exactly ...

Note is controlled by altering the resonant frequency of the exhaust, volume by the amount / size of silencers. Do a search on google for resonant frequencies of tubes or something similar for the basic principles.

They dont really, They guess, if its too loud or to quiet they add or remove silencer boxes. They would have fitted loads if they are any good so will have an idea what will be loud or not..

Ed

Just wondering. Had one fitted to the ole Civic as the original was hanging off. The chappy asked how loud I wanted it, then ran off to a backroom and returned about 10 mins later (could have gone for a cuppa for all I know!). Never figured out what he actually DID to the exhaust that's all...

-- Ali.