Pitch and place.

Step 1 of 6

Tiny pushes

Sound is the air squeezing and stretching. It travels along the ear canal and pushes on the eardrum, and three tiny bones pass the movement on to the cochlea, a coiled tube full of fluid.

Loudness is measured in decibels, a scale of ratios: $0$⁠ dB is the quietest sound a young ear hears, when the air’s pressure swings by only 20 µPa. Every 10 dB is $10$⁠ times the sound energy, and every 20 dB is $10$⁠ times the pressure:

$p = 20\ \mu\text{Pa} \times 10^{L/20}$$p =$$20\ \mu\text{Pa} \times 10^{L/20}$⁠

Step 2 of 6

A wave that sorts

Uncoil the cochlea is at Cochlea uncoiled: the cochlea is straightened into a tube, with a scale of pitch along it. Along its length runs the basilar membrane: narrow and stiff at the base, by the oval window, and wide and floppy towards the tip, about 35 mm away. Every tone sends a wave from the base towards the tip, and the wave peaks at one place. The tone is 1 kHz for now.

Step 3 of 6

One place, one pitch

About $3500$⁠ inner hair cells sit along the membrane. Each sits at one place, so each nerve fibre that listens to it stands for one pitch. The brain knows which pitch it hears from which fibres are active. Pitch (powers of ten) uses powers of ten because the membrane is built roughly that way too: equal ratios of pitch take up roughly equal lengths.

Step 4 of 6

Quiet and loud

The quietest sound you can hear, the threshold of hearing, depends on its pitch. The ear is most sensitive between about 2 kHz and 5 kHz, partly because the ear canal resonates near 3 kHz. The tone now is Low hum 50 Hz at Conversation.

Step 5 of 6

High notes go first

Every tone, whatever its pitch, passes the base of the cochlea on its way. There, over a lifetime, the hair cells wear out more than anywhere else. A Mosquito tone 17 kHz is playing now, and Listener’s age is 20 years.

Step 6 of 6

Noise

Rock concert, 105 dB. Above about 85 dB, hours of noise a day kill hair cells.