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Nature of sound and the human ear

Short answer

Sound is a longitudinal pressure wave; speed in air ≈343 m/s at 20°C, c=fλ. Audible range 20 Hz–20 kHz. Outer ear directs waves to eardrum; middle ear ossicles (hammer, anvil, stirrup) amplify and transmit to oval window; inner ear cochlea fluid bends hair cells converting vibration to nerve signals. Hair cell damage from loud noise is permanent.

Explain the nature of sound (wave properties, frequency, wavelength and speed). With a neat sketch, describe the structure and working of the human ear and state the audible range.

Nature of sound

Sound is a longitudinal pressure wave that travels through an elastic medium (air, liquid or solid) by alternate compression and rarefaction of particles. It cannot travel in vacuum.

  • Frequency ff (Hz): number of cycles per second. Time period T=1/fT = 1/f.
  • Wavelength λ\lambda (m): distance between consecutive compressions.
  • Speed cc: c=fλc = f\lambda. In air c=331.4+0.6 Tc = 331.4 + 0.6\,T (T in ∘^\circC) ≈343\approx 343 m/s at 20 ∘^\circC; about 1480 m/s in water and 5000 m/s in steel.
  • Amplitude (sound pressure) decides loudness; frequency decides pitch.
  • Audible range for a healthy young person: 20 Hz to 20 000 Hz; below is infrasound, above is ultrasound.

Human ear

  Outer ear      Middle ear           Inner ear
 +--------+   +-----------------+   +--------------+
 | pinna  |   |  ossicles       |   |  cochlea     |
 | canal  |-->|  hammer-anvil-  |-->|  (fluid +    |--> auditory
 |   |    |   |  stirrup        |   |  hair cells) |    nerve
 |eardrum |   |  (Eustachian    |   |  semicircular|    to brain
 +--------+   |   tube)         |   |  canals      |
              +-----------------+   +--------------+
  1. Outer ear: pinna collects sound and the ear canal guides it to the eardrum (tympanic membrane), which vibrates.
  2. Middle ear: three small bones (malleus, incus, stapes) amplify and transmit the vibration to the oval window. The Eustachian tube equalises pressure.
  3. Inner ear: the fluid-filled cochlea contains the basilar membrane with thousands of hair cells. Vibration makes the fluid move, bending the hair cells that convert it into nerve signals, carried by the auditory nerve to the brain. High frequencies are sensed near the base and low frequencies near the apex of the cochlea. The semicircular canals do not hear but control balance.

Damage to hair cells by loud noise is permanent.

✓ Answer checked by Claude Opus 5.5 (high effort)

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