mjkall
10-17-2002, 09:40 AM
I was thinking of measuring the speed and RPM of a passing nitro boat by recording the sound as the boat passes me. All you would need to do is to bring a taperecorder with a microphone and record a couple of passes at full throttle. Once you get home you can sample the data to your computer usnig a sound card and by processing the audio data one could determine the speed and RPM of the boat.
When looking at the spectrum of the sound when the boat is approaching you and when it is moving away from you, you will get two peaks. The first peak is at a higher frequency and the second peak is at a lower frequency. By averaging these two frequencies and multiplying by 60, you get the engine RPM (since frequency is 1/sec, you convert to 1/min by multiplying by 60). By calculating the difference between the two frequencies, you get the speed by using the following formula (Doppler):
V= (df/(2*f))*340 m/s
where df if the difference between the two frequencies and f is the average of the two frequencies. 340 m/s is the speed of sound in the air. By changing the unit of the speed of sound (kph, mph etc.) you get the speed in different units.
Example:
When a boat is moving towards you, you hear it at a frequency of approx 515 Hz. When the boat moves away from you, you hear it at approx. 485 Hz. Taking the average of that you get 500 Hz which in turn gives you an RPM of 60 * 500 = 30.000. The difference between the two frequencies is 515 - 485 Hz = 30 Hz. By using the above formula you get:
V = (30 Hz / (2 * 500 Hz)) * 340 m/s = (30 / 1000) * 340 m/s = ~10 m/s
So the boat is travelling at 10 m/s.
When looking at the spectrum of the sound when the boat is approaching you and when it is moving away from you, you will get two peaks. The first peak is at a higher frequency and the second peak is at a lower frequency. By averaging these two frequencies and multiplying by 60, you get the engine RPM (since frequency is 1/sec, you convert to 1/min by multiplying by 60). By calculating the difference between the two frequencies, you get the speed by using the following formula (Doppler):
V= (df/(2*f))*340 m/s
where df if the difference between the two frequencies and f is the average of the two frequencies. 340 m/s is the speed of sound in the air. By changing the unit of the speed of sound (kph, mph etc.) you get the speed in different units.
Example:
When a boat is moving towards you, you hear it at a frequency of approx 515 Hz. When the boat moves away from you, you hear it at approx. 485 Hz. Taking the average of that you get 500 Hz which in turn gives you an RPM of 60 * 500 = 30.000. The difference between the two frequencies is 515 - 485 Hz = 30 Hz. By using the above formula you get:
V = (30 Hz / (2 * 500 Hz)) * 340 m/s = (30 / 1000) * 340 m/s = ~10 m/s
So the boat is travelling at 10 m/s.