Eco Sound

 

Sounds spectrographs (e.g. Merlin sound ID) usually show frequencies up to 8 or 10 kHz, but some humans have the ability to hear up to 20 kHz. Most bird song, and indeed most sounds humans care about are less than 10 kHz - usually much less. For example, middle C on a piano is 440 Hz and the highest note on a piano is just above 4 kHz. 

 So what do humans hear above 10 kHz? Some crickets make sound in the 10-20 kHz range, and some rodent squeak distress calls are audible there too. The hiss of water or air from a pinprick hole can be in this range, but it’s not clear humans would need to evolve to hear that. 

So far, I think the most likely sound in this range is the whine of mosquito wings - maybe we evolved to swat mosquitoes? 

 Also, slowed bird song sounds reeeeaaalllly cool! The 2nd to last audio file on this web page is a good example: https://donaldkroodsma.com/?page_id=49

lang eliot also has a recording that is pitch-lower the recording about one octave, to bring the notes into a frequency range that is, at least generally speaking, more appealing to our human musical preferences (note that the vast majority of human-created melodic instruments produce dominant tones that are lower in pitch than the majority of bird songs). https://musicofnature.com/celestial-music/ 

 lang Eliot current project: app that employs advanced algorithms to lower the pitch of bird songs in real time, thereby making them audible during walks in nature. https://hearbirdsagain.org/