RPM and perceived pitch

[tommymck]

There was a thread here recently having to do with "buzziness" and the CB1100. I don't actually find the CB to be "buzzy" in any case, but I put about 300 miles on my Triumph Speedmaster this weekend and it got me to wondering.... The Triumph has a much lower note than the CB.

What is it that we're hearing relative to the pitch of the sound a bike makes? The triumph rev's more or less the same rpm at the same speed. But it's noticeably lower in pitch.

I'm not sure I can see why. Let's say you're at 3600 RPM... Which is relatively high for both bikes when cruising around. That's 60 Hz, which is the low organ bass note at the intro to Also Sprach Zarasthrustra. Hardly audible it's so low. Sure, there are things going on that are 2 or 4 times the rate of the RPM, but still relatively low pitch. What's clacking around in the engine that's 10X RPM?

[Retsel_imp]

Remember in the CB you have double the number of cylinders cycling for every engine revelation. Double the ignitions, double the opening and closing of valves, double the exhaust outputs. When you double the cycles you double the pitch! At the same time you are reducing the cylinder size by half, creating a smaller resonance chamber for each ignition. Higher pitch is inevitable.

Sent from my SM-T710 using Tapatalk

[PowerDubs_imp]

Engine layout first and foremost- then Bore / Stroke / and to much lesser extent- # of cyl / exhaust diameter and design.

My 6.0 W12 engine exhaust sounds an awful lot like my 3.2 VR6 and it to my 2.8 VR6... cyl head design, cyl angles, etc all extremely similar.