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This is a question I got for my class.

We have the following situation. We take two tuning forks, we hit them, then we take one of the tuning forks and walk away with it. The sound changes as we walk away and it isn't consistent anymore.

Why?

EDIT: I see I was a bit unclear, tho the question itself is unclear since i don't know anything except that. Silly teacher.

So, to clarify, two forks(my guess is that they have the same frequency). We hit each fork at the same time, one student is holding the fork #1 next to the blackboard, other student is walking away with fork #2.

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  • $\begingroup$ What do you think? $\endgroup$
    – Wouter
    Commented Nov 20, 2013 at 16:31
  • $\begingroup$ When you say "isn't consistent anymore" does the tone sound modulated at about the same frequency as the steps you take? $\endgroup$
    – Brandon Enright
    Commented Nov 20, 2013 at 16:36
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    $\begingroup$ If the second student is walking fast enough there will be a Doppler shift, but I doubt this would be audible at walking speeds. Running might do it. $\endgroup$
    – John Rennie
    Commented Nov 20, 2013 at 17:25
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    $\begingroup$ Tuning forks love people. If you walk away from one, it gets sad and the pitch drops. If you walk towards it, it perks up and the pitch rises in joy. <--- ok, so I'm having a bad day; gimme a break. PS yes, the teacher was thinking of a Doppler shift. $\endgroup$
    – Carl Witthoft
    Commented Nov 20, 2013 at 17:52
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    $\begingroup$ @John: The doppler shift won't be insignificant at all, even at a slow walk, if you're listening to the beat between the fixed and moving tuning forks. In fact, there will be one beat cycle for each wavelength to additional distance traveled. At middle A of 440 Hz, that's once every 750 mm or 2 1/2 feet. $\endgroup$
    – Olin Lathrop
    Commented Nov 20, 2013 at 19:39

3 Answers 3

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What you hear in this experiment is the combination of the Doppler effect and the beat. As John Rennie points out, the frequency change due to the Doppler effect would be hardly audible. However, the frequency between the two tuning forks will now be slightly different, which results in a intensity modulation, called the "beat".

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Imagine instead of pressure fluctuations, that the tuning fork was throwing balls at you at a rate of 1 balls per second, and that the balls were moving at 1 m/s. Now imagine that the tuning fork is moved away from you at 1 m/s, but the balls are still moving at the same speed. What do you think would happen to the rate at which the balls hit you in this situation? (How much farther/shorter does each ball have to travel, at the same speed, compared to the one before it?)

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Let me add one more possibility that I've only just thought of:

The two tuning forks behave as point sources for the sound, so they will generate an interference pattern just like a Young's slits experiment. This means that as you change the separation of the tuning forces the intensity of the sound at some fixed point in space will oscillate up and down, though the frequency of the sound will not change.

Calculating the oscillation of the volume is relatively straightforward: the volume will be a maximum when the difference in the distances from the observer to the two tuning forks is an integral number of wavelengths, and a minimum when the difference in the distances is a half integral number of wavelengths.

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  • $\begingroup$ Any idea how clean the signal from a tuning fork is? I could easily imagine the two tines producing the same frequency but at different phases. Then you're into analyzing dipole moments :-( $\endgroup$
    – Carl Witthoft
    Commented Nov 21, 2013 at 14:33
  • $\begingroup$ @CarlWitthoft: The phase difference between two sources does not destroy the interference pattern, it just alters it. $\endgroup$
    – Aecturus
    Commented Nov 21, 2013 at 15:42
  • $\begingroup$ @AlphaCentauri yes, but the more separate sources (phases) there are, the less likely you are to find a significant peak and valley in the overall interference pattern. $\endgroup$
    – Carl Witthoft
    Commented Nov 21, 2013 at 16:38

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