https://everything2.com/title/7+hertz+-+the+resonant+frequen...

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;
   }

from the comments over there (2002)