Distinctive sound of a Bristol 2 Litre Engine
 

There is a quite distinctive sound from the exhaust of a Bristol six cylinder engine.

This distinctive sound can be heard from a Bristol car - miles away.
Nothing else sounds just like a Bristol 2 litre engine in good tune.

So what is it mechanically about the motor that gives it such distinctive resonance?

I'm going to guess the unique sounds is partially created by having single choke triple cars. If you work out the timing the pulses thru carbs 1 and 3, is different to the middle carb. Carbs 1 and three have a 240/480 degree firing whereas the middle carb has a 360/360 pattern.

Also I think the smaller size of the block possibly encourages higher resonant frequencyies.