Explain why a person with a damaged ossicle chain may struggle to hear quiet sounds, even though their eardrum and cochlea are functioning normally.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The ossicles are three small bones in the middle ear: the malleus, incus, and stapes. They transmit vibrations from the eardrum to the oval window of the cochlea.
Model answer (5 marks)
The ossicles amplify the vibrations from the eardrum and increase the force that reaches the oval window. Because the oval window has a much smaller area than the eardrum, the pressure on it is increased (pressure = force ÷ area). If a bone in the chain is damaged, this amplification and pressure increase do not occur. Quiet sounds produce only small vibrations and therefore only small pressure changes; without the ossicular amplification the vibrations that reach the cochlea are too small to be detected. Consequently the hair cells in the cochlea do not generate the electrical signals that would be sent to the brain, so the quiet sound is not perceived.
Examiner tips
- Use the term ‘amplify’ and ‘pressure increase’ explicitly; link to the area difference of the oval window.
- Explain that quiet sounds give small vibrations, so loss of amplification means they fall below the detection threshold.
- Mention the consequence for hair cells and signal generation.
Mark scheme (5 marks)
- The ossicles amplify vibrations / increase the force of vibrations transmitted from the eardrum to the oval window
- The oval window has a smaller area than the eardrum, so pressure is increased (pressure = force / area)
- If the ossicle chain is damaged, this amplification / pressure increase does not occur
- Quiet sounds produce only small vibrations / small pressure changes, so without amplification the vibrations reaching the cochlea are too small to detect
- The cochlea / hair cells therefore do not generate electrical signals to send to the brain, so the quiet sound is not perceived
Key terms in this question
Related
- All AQA A-Level Physics (7408) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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