[EvaH (OP)]

Michael asks:

Why does the pitch of an engine go higher as RPM is increased?

What do you think?

[mrsmith2211]

I was wondering if it might be a Doppler effect as the volume and speed of the air increases as rpms increase.

[Halc]

The sound produced by an engine is typically due to a repeating cycle, one frequency based on perhaps one revolution of the crank of the engine, but more likely based on say the number of gear teeth going by somewhere, or the frequency at which the cylinders are firing.  So as the engine speed increases, so does the pitch of all these sounds, and thus the overall pitch is a direct function of the RPM of the engine.

[evan_au]

Why does the pitch of an engine go higher as RPM is increased?

Pitch (or Frequency) is measured in cycles per second (or Hertz, abbreviated Hz).

If you have a tachometer on your car, you will see it calibrated in Revolutions Per Minute (RPM).
So if your tachometer is showing 3000 RPM, that is 50 Revolutions Per Second, or 50 Hz.
- If your car has (say) a 6 cylinder/4-stroke motor, there will be a lot of sound energy at multiples of 150 Hz.
- Rev it to 6000 RPM, and there will be more energy at multiples of 300 Hz, a higher pitch.