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School β€Ί Ears β€Ί Ages 15–16

Hearing: transduction, timing and noise dose

How the cochlea sorts frequencies, how hair cells turn movement into nerve signals, how the brain uses microsecond timing to locate sounds, and what noise exposure does. With a school noise survey.

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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.[2]

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.[2][1]

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.[2][1]

Transduction: when hair cells move, their stereocilia bend, ion channels open and the cell depolarises, creating an electrical signal. About 3,500 inner hair cells are the main receptors and about 12,000 outer hair cells fine-tune the response. Loudness is coded partly by how many hair cells are stimulated. Signals travel via the brainstem to the auditory cortex in the temporal lobe.[2][1]

Locating sounds: for a sound directly to one side the far ear is about 0.23 m further away, so with v = 340 m/s the interaural time difference is Ξ”t = 0.23 / 340 β‰ˆ 0.68 ms; real maximum values are a little over half a millisecond. The head also weakens the sound at the far ear (interaural level difference), and the shape of the pinna helps with up-down and front-back.[4][3]

Noise dose: risk depends on intensity and time. Lawn mowers and leaf blowers (80–85 dB) can damage hearing after about 2 hours, a motorcycle (95 dB) after about 50 minutes, and firecrackers (140–150 dB) cause pain and injury. Loud sound bends stereocilia too far and can kill hair cells, and cochlear hair cells cannot grow back.[5][6]

Types of hearing loss: conductive loss is a problem getting sound to the cochlea (a blocked canal, a hole in the eardrum, ossicle problems, fluid behind the eardrum) and is often helped by hearing aids. Sensorineural loss, the most common type, is a failure to send signals from the cochlea to the brain; causes include ageing and noise. A cochlear implant stimulates the auditory nerve directly. WHO projects that nearly 2.5 billion people will have some hearing loss by 2050, and more than 1 billion young adults risk avoidable loss from unsafe listening.[3][7]

Try it: School noise survey

⏱ 45 minutes

You need

  • A phone or tablet with a free sound-level-meter app (one per group)
  • A map of the school
  • A metre stick or tape measure

Safety

Do not go near loud machinery or traffic to take readings. Never put a device or earphones close to anyone's ear. If a place is uncomfortably loud, take the reading and leave.

What to do

  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 βˆ’ 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?
What happens, and why?

Risk to hearing rises with sound intensity and with time, so both the level and how long people stay matter. Phone apps give useful comparisons between places, but their microphones are not calibrated, so absolute values are uncertain; repeats and a fixed method reduce random error.[5]

Play: Conductive or sensorineural?

A cause of hearing loss appears. Is it a problem getting sound to the cochlea (conductive) or a problem sending signals from the cochlea to the brain (sensorineural)?

Your best score is kept only on this device. Nothing is sent anywhere.

Quiz

Other ages: Ages 6–7 Β· Ages 10–11