August 2012
Is it possible to take the sound heard in the frequency range of 0 to 40 kHz and space it evenly into the 10 Hz to 20 kHz range? I want to hear what a dog hears but in the human range. I know this will make people’s voices sound funny, but how would this be done and is there such a device? Is there even a mic for that upper range of up to 40 kHz?
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He might refer to the June 2011 issue of Nuts & Volts and the article on the "Bat Detector" by Jonathan Berber. Using this concept, a hetrodyne circuit, utilizing a piezo transducer (microphone), should result in an audible, to human ears, signal.
One easy approach is the digital divide circuits used in Bat detectors. For example: http://pw1.netcom.com/~t-rex/BatDetector.html
Basically it just divides the input frequency by some power of two (2,4,8,19, ...). In this case divide 0 to 40KHz by 4 and get 0 to 10KHz. The digital divide circuit is pretty simple but you will lose all amplitude information. Given how simple the circuit is that might be acceptable.
A suitable microphone will be a bit of a problem. Many mics will give a signal at frequency up into the 40KHz range but the sensitivity will drop off a fair amount. Piezo transducers are often used for Bat calls and they'll give some signal up to, perhaps as high as, 100KHz. But piezo have resonant frequencies and they'll be super sensitive near that frequency. A typical 1" diameter thin piezo resonates at about 3KHz.
There are ultrasonic transducers designed for 40KHz that work well as a mic at that frequency, but their sensitivity at lower frequencies drops off.
