Infrasound, Pitch-shifters and Pigeons

I remember being told a while back about the exceptional hearing range of pigeons and it wasn't until an especially random daydream that I decided to investigate further. I was listening to some electronic music at the time, which subsequently made me wonder about what a pigeon may keep on its MP3 player?

My line of thought does have some academic merit before you judge, so bear with...

Having a personal interest in the signal synthesis involved in making electronic music, made me think about the LFOs or Low Frequency Oscillators artists' use in their instruments to modulate their musical tones. These frequencies are deliberately in the infrasonic range of human hearing so our ears cannot pick them up i.e. 0 - 20 Hz, however what if it is not heard by a human but rather another animal such as a pigeon?

Based on an article by M.Kreithen and D.Quine (1979) pigeons can hear as low as 0.05 Hz in isolated environments allowing them to easily hear the LFO tones, and with their high pitch-sensitivity would make unwanted tones easily perceivable, leaving me to conclude the common bird would probably not be a fan.

Further reading shows that this increased acoustic range is to allow for the detection of weather systems and geological activity from a long range, as the lower frequencies of the landscape's movements attenuate over a larger distance. This made me think that having this physiology could be quite handy, even if implemented artificially.

The primary focus of interest here is how do I pitch shift my own hearing?

Well I'm sure with a cheap mic and a pair of headphones it should be possible, electronically this can be done by using a simple full wave rectifier circuit which will increase the input signal frequency by two.

This overly-simplified circuit shows the basic concept and could be implemented totally passively through the use of a sensitive mic and germanium diodes that have a low voltage drop similar to the ones used in cheap AM receivers.



Mounted on a pair of headphones you could listen out for the weather and hear the world go by from miles away as if you were one of the unsung birds yourself.