title: Recognition of a short high frequency sound in low frequency noise (objc/c)
body: <p>I am currently creating an application which signals readiness to other devices using a high frequency sound.</p>
  
  <p><strong>(transmitter):</strong> A device will produce a short burst of sound of around 20khz.</p>
  
  <p><strong>(receiver):</strong> Another device will be listening for a sound at this frequency at a small distance from the transmitter(10m approx) The device recieves audio data from a microphone</p>
  
  <p>The background noise will be fairly loud, varying from around 0 - 10khz(about human speech range), and would be produced by a small crowd of people.</p>
  
  <p>I need the receiving device to be able to detect the 20khz sound, separated from the noise, </p>
  
  <p>and know the time at which it was received.</p>
  
  <p>Any help with an appropriate algorithm, a library, or even better, code in C or</p>
  
  <p>Objc to detect this high frequency sound would be greatly appreciated.</p>