# Doppler effect and multiple independant sound sources.

## Main Question or Discussion Point

If I am stationary and a sound source moves towards me at a constant velocity, I will hear it (the sound) increase in frequency until it is adjacent to me and then as it moves away from me, I will hear a decreasing frequency.

Fundamental stuff right? It works with radio waves and light which is how we can look for red shifted and blue shifted galaxies and make assumptions about their heading.

But, and this is my question... lets say in my example above, the sound source was a car travelling at 50km/h with the driver leaning on his horn for the whole time of observation and that being so, the source of sound is constant in frequency and although moving, the source remains the same as it travels linearly (ie the horn as the car moves towards, then away from me).

What if, I set up, say, 100 speakers with a sound frequency generator attached to each (at the same frequency of the car horn as a rather arbitrary decision) and laid them out in a line, say 1 metre apart.

Then, using a timer, I have them emit their sound (all emitting the exact same frequency), starting at one end and at such intervals as to simulate a vehicle moving at 50km/h.

If I were to then stand in the middle of this line of speakers, would I hear the Doppler effect as the sound "approaches" me, then "moves away" from me?

I had a reasonable argument in class about this today and would like some outside input.

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## Answers and Replies

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The Doppler effect is not related to sound moving towards and then away from you.
The source of the sound (or the observer) should move.

You may have some interference effects between the sounds emitted by your sources.
It is not clear if they emit a short pulse or they start at a given time and then emit continuously.

The Doppler effect is not related to sound moving towards and then away from you.
The source of the sound (or the observer) should move.

You may have some interference effects between the sounds emitted by your sources.
It is not clear if they emit a short pulse or they start at a given time and then emit continuously.
Yes, I understand about how the movement of the source of the sound compresses the sound (or energy) waves, and that was part of the animated discussion in class we had. I'm glad to hear that repeated. Do you have any reference to this fact?

In my example, each sound emitter emits a constant amplitude and frequency, but just for a moment. They don't turn on and stay on, they just emit for a moment.

Yes, I understand about how the movement of the source of the sound compresses the sound (or energy) waves, and that was part of the animated discussion in class we had. I'm glad to hear that repeated. Do you have any reference to this fact?
What fact? You mean Doppler effect being produced by the motion of the source or receiver?
Any textbook with a chapter on acoustics is a "reference". Or you would like to see some research paper?

In my example, each sound emitter emits a constant amplitude and frequency, but just for a moment. They don't turn on and stay on, they just emit for a moment.
Then you will receive pulses with the same frequency, separated by some time interval.

Then you will receive pulses with the same frequency, separated by some time interval.
That's what I thought.

That's pretty much answered my question.

If there is no relative movement between the source of the the sound (or energy, we were actually discussing radio waves from a DVOR air nav installation) and the receiver, then no Doppler effect is experienced by the receiver.