Naked Science Forum

Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: EvaH on 08/09/2020 12:05:04

Title: Why does the pitch of an engine go higher as RPM is increased?
Post by: EvaH on 08/09/2020 12:05:04

Michael asks:

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

What do you think?

Title: Re: Why does the pitch of an engine go higher as RPM is increased?
Post by: mrsmith2211 on 08/09/2020 16:05:47

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

Title: Re: Why does the pitch of an engine go higher as RPM is increased?
Post by: Halc on 08/09/2020 16:15:10

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.

Title: Re: Why does the pitch of an engine go higher as RPM is increased?
Post by: evan_au on 08/09/2020 23:01:11

Quote from: OP

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.