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In the Sound function section of the Turbo C Version 2.0 Reference Guide 1988 I found the following comment.

/* Emits a 7-Hz tone for 10 seconds.
   True story: 7 Hz is the resonant frequency of a chicken's 
   skull cavity. This was determined empirically in Australia, 
   where a new factory generating 7-Hz tones was located too 
   close to a chicken ranch: When the factory started up, 
   all the chickens died.

   Your PC may not be able to emit a 7-Hz tone. */

Is this story true?

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    How loud? A 7-Hz tone that bangs you against a wall could kill a human. – Daniel R Hicks Jan 30 '18 at 3:31
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    Since the comment did not mention any human casualty, I think it is safe to assume that the sound is not that loud. – tsukumogami Jan 30 '18 at 5:38
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    @tsukumogami The question brings up a good point though. The frequency is only a part of the situation here, and the amplitude will be a necessary consideration. Below a certain threshold, the resonance wouldn't really work quickly enough. – JMac Jan 30 '18 at 15:28
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    So it seems that it breaks down into two questions of fact. First, is the resonant frequency of the chicken skull 7 Hz (or close to it), and second, can that be exploited to kill chickens with 7 Hz tones at intensities that would not be meaningfully threatening at other frequencies. – Ben Barden Jan 30 '18 at 16:47
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    It's probably also safe to assume when researching that the claimed real-world incident was not precisely 7.00 Hz, that some rounding is involved in the retelling. – Riking Jan 31 '18 at 9:51

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