Sound Waves & Hearing (OCR GCSE Physics A (Gateway)): Revision Note

Sound Waves in Solids

• Sound waves are vibrations of air molecules

• When a sound wave comes into contact with a solid those vibrations can be transferred to the solid

  • For example, sound waves can cause a drinking glass to vibrate

  • If the glass vibrates too much the movement causes the glass to shatter

• Sound is an example of a longitudinal wave, hence it consists of:

  • Compressions - regions of higher density

  • Rarefactions - regions of lower density

Sound is a longitudinal wave consisting of compressions and rarefactions - these are areas where the pressure of the air varies with the wave

• These compressions and rarefactions cause changes in pressure, which vary in time with the wave

  • Therefore, sound is a type of pressure wave

• This is the process which converts wave disturbances between sound waves and vibrations in solids (or liquids)

• When the waves hit a solid, the variations in pressure cause the surface of the solid to vibrate in sync with the sound wave

When sound waves hit a solid, the fluctuating pressure causes the solid to vibrate

Sound Waves in the Ear

Higher Tier Only

• Hearing occurs when sound wave disturbances in the air outside the ear are converted into vibrations of the solid components of the ear

• In the case of the human ear, the sound waves are transferred to the small hairs (cilia) inside the cochlea by two main solid components:

  • the eardrum, which is made of tissue and skin

  • three small bones, also called ossicles, which consist of the hammer, anvil, and stirrup

• The sound wave travels down the auditory canal towards the eardrum

  • The pressure variations created by the sound wave exert a varying force on the eardrum, causing it to vibrate

  • The vibration pattern of the sound waves creates the same pattern of vibration in the eardrum

• The eardrum vibration is transferred to the three small bones (ossicles)

• The small bones (ossicles) then amplify the vibrations as they are transferred to the inner ear

  • In the inner ear, nerve cells detect the sound and send a message to the brain, giving the sensation of sound

  • This is primarily the cochlea in the inner ear, which contains small hairs (cilia) attached to nerve cells

  • The nerves produce electrical signals that pass through the auditory nerve in the brain, which is then interpreted as sound

Structure of the human ear

The human ear is made up of several components which turn sound waves into signals that the brain can interpret

Hearing changes due to ageing

• Since the transmission of the vibrations is dependent on the hairs in the cochlea, the transmission of sound to the human ear only works over a limited range of frequencies

  • This limits the range of sound frequencies a human can hear

The range of human hearing

• The range of frequencies a human can hear is 20 Hz to 20 000 Hz

• Human hearing of high frequencies becomes worse with age

  • This is due to changes in the structure of the inner area and the auditory canal nerves

• The cilia (small hairs) in the cochlea have different lengths and vibrate at different frequencies of sound

  • The range of frequencies a human can hear depends on the range of lengths of these hairs

  • As a person ages, the shorter hairs that respond to higher frequencies stop working

  • Therefore, ageing reduces the ability to hear high frequencies