48. Sound Waves and Hearing

Wave Type: Sound waves are longitudinal waves.
Mechanism: They travel as a series of vibrations through the molecules of a medium.
Structure: They consist of:
- Compressions: Regions where particles are bunched closest together.
- Rarefactions: Regions where particles are spread furthest apart.
Vacuum: Sound cannot travel through a vacuum because there are no particles to vibrate.

The speed of sound depends on how densely packed the particles are:

Solids: Particles are very close; sound travels fastest.
Liquids: Mid-range speed.
Gases: Particles are far apart; sound travels slowest.
Transition Between Mediums:
- When sound moves from a less dense medium (air) to a more dense medium (solid), it speeds up.
- Frequency stays the same, but the wavelength increases to compensate for the higher speed (v = f x \lambda ).

Reflection: Hard, flat surfaces reflect sound waves, creating echoes.
Absorption: Soft materials tend to absorb sound.
Refraction: Sound can change direction (refract) as its speed changes when moving between different mediums.

The ear translates mechanical vibrations into electrical signals for the brain.

Ear Canal: Sound waves travel down the canal to the eardrum.
Eardrum: The waves cause the eardrum to vibrate.
Ossicles: These three tiny bones transmit and amplify the vibrations.
Cochlea: A snail-shaped structure that converts mechanical vibrations into electrical signals.
Auditory Nerve: Carries the electrical signals to the brain.
Brain: Interprets the signals as sound.
- Frequency is interpreted as pitch (higher frequency = higher pitch).
- Intensity is interpreted as volume (higher intensity = louder).

Standard Range: Humans can typically hear frequencies between 20 Hz and 20,000 Hz.
Ageing: As people get older, their hearing range often decreases due to wear and tear of the cochlea and auditory nerve.