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Sound is a mechanical pressure wave that needs a medium. We hear its frequency as pitch and its amplitude as loudness, measured in decibels. Humans hear up to about 20,000 Hz; higher frequencies are called ultrasound.

The eardrum drives the three ossicles (malleus, incus, stapes), which collect force and amplify the vibration. The stapes presses on the oval window, setting up pressure waves in the fluid of the cochlea.

The basilar membrane is stiffer and narrower near the base of the cochlea, where it responds to high frequencies, and floppier towards the apex, where it responds to low frequencies. So different frequencies make different places vibrate: a frequency map along the cochlea.

Experiment: School noise survey

1. Choose five places: e.g. a quiet library, the classroom, the corridor between lessons, the canteen, the playground.
2. Agree a method: hold the device at arm's length, 1 m from the nearest wall, at head height, for 30 seconds, and note the average (Leq or 'avg') reading.
3. Take three readings at each place at about the same time of day.
4. Calculate the mean and the range (highest − lowest) for each place.
5. Compare with the CDC guide values: about 60 dB conversation, above 70 dB for long periods may start to cause damage, above 85 dB is a warning level.
6. Evaluate: which places, if any, would be a concern for someone there all day? How reliable is a phone app?

Place	Reading 1 (dB)	Reading 2 (dB)	Reading 3 (dB)	Mean (dB)	Range (dB)
1
2
3
4
5

Questions

1. Explain how the cochlea separates a 200 Hz tone from a 10,000 Hz tone.

2. Calculate the interaural time difference for a head width of 0.20 m (v = 340 m/s). Give your answer in ms.

3. Your survey: which place had the highest mean level? Is it a concern? Use the CDC values and your range.

4. Give two reasons why phone-app readings are uncertain and one way you reduced the uncertainty.

Sources for every fact on this sheet: echoesof.science/school/ears/sources/

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