From sound to hearing

1. A vibrating source pushes and pulls nearby particles in a medium.
2. The particles oscillate parallel to the direction of energy transfer, producing alternating compressions and rarefactions.
3. The particles transfer the disturbance to neighbouring particles, so energy travels through the medium without the medium moving overall.
4. Sound cannot travel through a vacuum because there are no particles to pass on the vibration.

Sound and solid vibrations

1. When sound reaches a solid surface, changing pressure exerts an alternating force on it and can make it vibrate.
2. A vibrating solid can also push and pull the surrounding medium, converting its vibration into a sound wave.
3. The frequency of the sound wave matches the frequency of the source vibration.
4. A larger vibration amplitude usually produces a sound wave with a larger amplitude and a greater intensity.

The human ear

1. The pinna collects sound and channels it through the ear canal.
2. Pressure variations make the eardrum vibrate at the sound frequency.
3. The three ossicles transfer the vibration through the middle ear and increase the pressure applied to the oval window.
4. Movement at the oval window produces pressure waves in the fluid of the cochlea.
5. Different regions of the cochlea respond most strongly to different frequencies.
6. Sensory hair cells bend and convert the mechanical disturbance into electrical impulses.
7. The auditory nerve carries these impulses to the brain, where they are interpreted as sound.

A limited frequency range

1. A structure responds effectively only over a limited frequency range because its mass, stiffness and damping affect how easily it can vibrate.
2. At some frequencies the response amplitude is large, while very low or very high frequencies produce a weaker response.
3. The combined response of the eardrum, ossicles, cochlea and hair cells limits normal human hearing to roughly $20Hz$ to $20000Hz$.
4. The upper limit usually falls with age and after exposure to intense sound.