title: Ultrasound iphone (Shopkick signal technology)
body: <p>I think shopkick is detecting very high frequency signal which is not audible to human ear.But the real question is how they can detect signal of more than 22khz in iphone. I have checked frequency response of iphone mic,it seems to be from 20 hz to 22 khz within the human audible range.</p>
  
  <p><a href="http://blog.faberacoustical.com/2009/iphone/iphone-microphone-frequency-response-comparison/" rel="nofollow">http://blog.faberacoustical.com/2009/iphone/iphone-microphone-frequency-response-comparison/</a> http://www.businessinsider.com/shopkick-crate-barrel-2010-12?op=1</p>
  
  <p>Can you guide me on this. If it is possible with iphone mic,then we can able do some signal processing specifically FFT in order to get frequency.</p>