Uncovering the Engine Sound Mystery: How Can Different Cylinders Sound Alike?

[bmwguy400394]

Ok, not sure if this is the right place to post this but here it goes. Many people have stated and I agree that 3, 6, and 12 cylinder engines all sound nearly identical at the same RPM. This is also true for 5 and 10 cylinder (4 and 8 being the exception due to most V8s being crossplane, but a flatplane V8 sounds just like a 4-cylinder as well).

My question is, how is this possible?? It would maybe make sense for, say, a V12 to sound like a V6 at half the RPM, but the V12 is firing twice as often and yet still sounding just like a 6-cylinder. This just doesn't add up.

It also begs the question, if you had a two-stroke 6-cylinder, would it have the sound characteristics of a 4-stroke V12 given the fact that it would have the same number of combustions, or would it have the sound characteristics of a 4-stroke 6-cylinder at twice the RPM?

Anyone skeptical visit these links:
(BMW 750i V12, redlining about 6000RPM)
http://www.youtube.com/watch?v=d9kVQVt7xPY&feature=related (BMW 325i Inline-6, redlining a little over 6500RPM)

 

[brewnog]

The key here is that the V12 is not necessarily firing twice as often as the straight six. Some vee engines have the same firing frequency as their equivalent inline engines, but with two cylinders on opposite banks firing together. This could be the case with the BMW engines you cited. If not, perhaps the vee engine sound was recorded at a position where the exhaust outlet from one bank 'drowned out' that of the other bank.

2 stroke engines have very different sound characteristics as an inherent result of their design. Mathematically (just considering exhaust pulses) then your prediction may be true, but 2 strokes sound so different anyway that it just becomes a subjective matter.

The other thing to remember is that while the above all holds true when listening to exhaust noise (because of the pulses), noise created by rotating components, imbalance, ancilliaries etc all play a big part in the sound profile of an engine, and can dramatically affect the way the engine sounds.

Having said all that, I wouldn't agree that a 3 cyl, 6 cyl and 12 cyl all sound the same! Probably too much time spent standing next to all of them.