Explain why sounds above 20 000 Hz are not heard by most adults, and why older adults often lose the ability to hear high-frequency sounds before they lose the ability to hear low-frequency sounds.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The human ear can detect sounds across a range of frequencies. This range changes with age.
Model answer (5 marks)
The audible range for humans is about 20 Hz to 20 000 Hz.
Sounds above 20 000 Hz do not stimulate the hair cells in the cochlea, so no electrical signal is sent to the brain.
The cochlea is tonotopically organised: different parts of the basilar membrane respond to different frequencies. Hair cells that respond to high frequencies are located near the base (entrance) of the cochlea and are subjected to greater mechanical stress and vibration.
With age the high‑frequency hair cells are damaged or destroyed first. Because hair cells are not regenerated, the loss of these cells results in permanent loss of high‑frequency hearing before low‑frequency hearing is affected.
Sounds above 20 000 Hz do not stimulate the hair cells in the cochlea, so no electrical signal is sent to the brain.
The cochlea is tonotopically organised: different parts of the basilar membrane respond to different frequencies. Hair cells that respond to high frequencies are located near the base (entrance) of the cochlea and are subjected to greater mechanical stress and vibration.
With age the high‑frequency hair cells are damaged or destroyed first. Because hair cells are not regenerated, the loss of these cells results in permanent loss of high‑frequency hearing before low‑frequency hearing is affected.
Examiner tips
- Mention the 20 Hz–20 000 Hz range first. Explain the lack of response above 20 kHz. Describe the tonotopic arrangement and location of high‑frequency cells. State that hair cells are not replaced, giving permanent loss.
Common mistakes
- Claiming that all high‑frequency cells are near the apex instead of the base. Saying that low‑frequency cells are damaged first. Omitting the reason why high‑frequency cells are more vulnerable (greater mechanical stress).
Mark scheme (5 marks)
- The audible range for humans is approximately 20 Hz to 20 000 Hz
- Sounds above 20 000 Hz do not cause the hair cells / receptor cells in the cochlea to respond / send electrical signals to the brain
- Different parts of the cochlea / basilar membrane respond to different frequencies
- Hair cells that respond to high frequencies are found near the base / entrance of the cochlea and are exposed to more mechanical stress / vibration
- Once hair cells are damaged / destroyed they are not replaced, so hearing at those frequencies is permanently lost
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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