Thanks for the note, Paul. I am thankfully not speeding up the sound. I need to take a 24khz - 26khz sound and pitch shift it down to an audible range, maybe 12-13khz, or 6-7.5 khz if there aren't too many artifacts in the time stretch
I am trying to test using 27khz frequencies but I either am having a hard time generating a frequency or I am having hard time sensing it with the teensy. I have to test a sound generation device that I don't have in my hands just yet. I will be ordering a few of these ultrasonic transducers to see how they fare with the teensy mic in.
http://www.mouser.in/ProductDetail/Kobitone/255-250SR16P-ROX/?qs=sGAEpiMZZMvtrnhC60i%2bOsZHfFLa1IkJ
There is a corollary to Paul's comment.
If you use a sampling frequency of 44 kHz (say PJRC audio card with Audio software) you cannot have frequencies above 22 kHz. so your 27 kHz does not exist after the ADC.
If you use 96 kHz sampling rate with your own software, then you can do it.