6.10 What is the doppler effect ?
Description
This article is from the Acoustics FAQ, by Andrew Silverman with numerous contributions by others.
6.10 What is the doppler effect ?
When a sound source is moving, a stationary observer will detect a different frequency to that which is produced by the source. The speed of sound in air is approximately 340 m/s (see 2.11). The wavelength of the sound emitted will be foreshortened in the direction of motion by an amount proportional to the velocity of the source. Conversely the wavelength of a receding sound source will increase. The doppler effect may be noticed as a marked drop in pitch when a vehicle passes at high speed.
Example 1: A sound source, S, emits 1000 waves per second (1 kHz) and is moving directly towards an observer, O, at a speed of 100 metres per second (equivalent to approx 225 miles per hour).
After 1 second the wave front, which is travelling at the speed of sound, will have travelled 340 metres from the original source position. Also after that second the sound source will have moved 100 metres towards the observer.
0 m 340 m S | | | | | | | | | O <-------------- 1000 waves ------------------>
100 m 340 m
S | | | | | | | | | O
<------- 1000 waves --------->
Therefore the same number of waves will occupy a space of 340-100 = 240 metres and the wavelength will be 240/1000 = 0.24 metres. To the observer the frequency heard will be the speed of sound divided by its wavelength = 340/0.24 = 1416.7 Hz.
Example 2: An observer moving at 100 metres per second directly approaches a stationary sound source, S, which is emitting 1000 waves per second (1 kHz). In this example there is no change in wavelength. In one second, the observer will hear the number of waves emitted per second plus the number of waves which s/he has passed in the time (1000+100/0.34) = 1294.1 Hz.
Note the interesting result - a stationary observer with moving source will not hear the same frequency as a would a moving observer with stationary source.
Continue to:
My Books
