Why we can only hear between 20hz and 20,000hz
Below 20 Hz the waves are huge like 17 meters long, you’re no longer passing them through the ear drum to the three bones and then to the hairs in the inner ear, you’re just detecting the vibration of your chest. You are significantly less sensitive to a 100 Hz tone than a 1000 Hz tone (~25dB different) and a 20 Hz tone has to be at 70 dB for you to detect it at all. That’s a large amount of air moving for you to be able to perceive the tone unlike a 1 kHz sound that you can detect at 0 dB or 3 kHz that’s almost -10 in a lot of people
For the higher end its back to structural limitations. Your ear drum and the attached bones have weigh and friction and some springiness associated with them. A high frequency tone is going to try to move them back and forth really really reallyyyyy quickly but the drum and bones simply cannot respond that fast, and a lot of the energy starts getting lost. The cochlea is what the bones connect to at the other end and it is a fluid channel filled with tiny hairs, when the frequency is just right(or a harmonic) the little hairs bend over to let some chemical signals through to trigger the nerve. For extremely high frequency sounds the hairs are itty bitty and you only have hairs that get so small so there had to be a cutoff somewhere. The itty bitty hairs are also more prone to breakage which is why kids could hear the high pitched whine of a CRT but teachers and parents often couldn’t, the hairs responsible for the 15 kHz and up are some of the first to go
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