How people hear and understand the frequency of sounds?

Answer 1

(Not sure if this means 'How do people hear and how do they understand the frequency of sound' or 'How do people hear the frequency of sound and understand it' so I'm answering both.)

The ear can be split into three basic parts: outer, middle and inner.

At the outer ear, sound waves pass through the ear canal and hit the ear drum, causing the eardrum to vibrate at the frequency of the sound waves hitting it.

In the middle ear, the ossicles (the 'hammer, anvil and stirrup' bones) transmit the vibrations from the eardrum to the oval window (a membrane linking the middle ear to the inner ear).

The middle ear is filled with air, whereas the inner ear is filled with fluid. When sound passes between media of different densities, some of the sound is reflected and lost rather than transmitted. The ossicles help to transmit sound energy from air to fluid with as little reflection as possible. This is called impedance matching, fyi.

Additionally, the way the ossicles are arranged allows for a mechanical advantage of about 1.5. This means the forces of the vibrations at the oval window are increased by half.

Furthermore, the area of the oval window is around 15 times smaller than the area of the ear drum. Since pressure = force/area, this fact as well as the mechanical advantage of 1.5 means that there is a greater pressure on the oval window due to the vibrations.

Essentially, the amplitude of the vibration is increased between the outer ear and middle ear.

In the inner ear or cochlea, there is an organ called the Organ of Corti, in which these vibrations are turned into electrical signals and sent to the brain.

Within the Organ of Corti there is the basilar membrane, which is what distinguishes the frequencies of sound.

The basilar membrane is a stiff structure that is tapered on one end and is lined with hair cells. It is stiffer on one end (nearer to the oval window) and floppier on the far end.

When vibrations pass through the fluid in the inner ear, the differing levels of stiffness along the basilar membrane cause different areas of the membrane to resonate at different frequencies.

Higher frequency sounds cause maximum vibration (due to resonance) at the narrower, stiffer end of the basilar membrane. Lower frequency sounds resonate at the wider, floppier end of the membrane.

There are more than 2000 hair cells along the basilar membrane and they are very sensitive to movement. The vibration of the membrane causes the hair cells to be displaced, and this causes pulses in nerve fibers which translates them into electrical signals that are sent to the brain.