7 hertz - the resonant frequency of a chicken's skull (thing)

Indeed it is in documents of the legendary Borland Turbo C++. For nostalgy, I digged out the disks. Here's the aforementioned help entry in its whole (don't worry about copyrights, isn't TC under GPL now? I think saw a story about it on slashdot.):

---

  ________________
  |sound, nosound|                     
  ________________

• sound turns the PC speaker on at the specified frequency
• nosound turns the PC speaker off

Declaration:

• void sound(unsigned frequency);
• void nosound(void);

Remarks:

• sound turns on the PC's speaker at a given frequency.
• nosound turns the speaker off after it has been turned on by a call to sound.

frequency specifies the frequency of the sound in hertz (cycles per second).

Return Value: None

Portability:

 + DOS + UNIX + ANSI C + C++ Only +
 | Yes |      |        |          |
 +-----+------+--------+----------+

See Also:

delay

Example (for both functions):

/* 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. */

 #include 

 int main(void)
 {
    sound(7);
    delay(10000);
    nosound();
    return 0;
 }