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In my book, it is written that transverse waves are produced during physical motion of tuning fork. But vibration of tuning fork produces sound which are compressional in nature . Please tell me the correct answer and reason behind it.

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  • $\begingroup$ "in this case." Which case, the waves in the air or the physical motion of the the tuning fork itself? There are probably both compression/longitudinal and transverse waves in the tuning fork, and depending on how you strike the fork, torsional waves. $\endgroup$ – Bill N Apr 30 '19 at 17:40
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The acoustic wave in air is a longitudinal wave. The mechanical movement of arms of the tuning fork could be interpreted as a transversal wave, but it's actually more of vibration. There is no real wave propagation.

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  • $\begingroup$ There is no real wave propagation. Huh? How do you think a tuning fork emits sound then? $\endgroup$ – Gert Apr 30 '19 at 19:05
  • $\begingroup$ @Hilmar (but also to Bill N). There is no real wave propagation. Vibration of an extended body always involves wave propagation. Vibration modes are nothing but stationary waves. And a tuning fork is a good frequency standard because it's relatively insensitive to the exact excitation way. Probably because there is a mode much less damped than the others and after a short time it survives alone. $\endgroup$ – Elio Fabri Apr 30 '19 at 19:26
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    $\begingroup$ As to OP question, the standard mode of vibration of a tuning fork is transverse: displacement orthogonal wrt prongs' length. That vibration mode, together with prongs' shape, is well suited to excite a compression wave in air. $\endgroup$ – Elio Fabri Apr 30 '19 at 19:27
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    $\begingroup$ @ElioFabri Actually the transverse vibration of the prongs is very badly suited to excite air vibrations, because the sound radiated from the equal opposite motion of the prongs cancels out almost exactly. If you hold a tuning fork close to your ear and rotate it, you will discover that the sound is highly directional. A tuning fork generates sound efficiently because the base of the fork vibrates axially, and when you press it against a large flexible surface (e.g. a table top) the complete table top vibrates and creates the sound in the air. $\endgroup$ – alephzero Apr 30 '19 at 20:54
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    $\begingroup$ @alephzero You're right. After writing my comment I recalled just the experiment you are quoting of a tuning fork whose handle is pressed against a table. I also add that in science museums tuning forks can be seen mounted over a wooden box with an open side. It makes clear that sound radiation doesn't come directly from fork but from the air in the box, put into vibration by the box' wall touching the fork's base. $\endgroup$ – Elio Fabri May 1 '19 at 7:58
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A tuning fork vibrates like this (in its fundamental mode):

Tuning fork

(image from here)

As the tines move sideways they alternately compress and expand the air next to them:

Tuning fork

So the sideways motion of the times produces a compression wave in the air.

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  • $\begingroup$ Add to that that the base is also moving ("up" and "down" in those drawings). Putting it on a sounding board makes it much loader. Or on a resonance box with a length of about one quarter of the wavelength in air. $\endgroup$ – Pieter May 1 '19 at 8:59

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