The human ear and the range of hearing

How do we hear?

Sound waves enter the ear and make a thin skin vibrate. Tiny bones and a coiled tube of liquid turn that vibration into signals that the brain understands.

The parts of the ear

1. The outer ear collects the sound and sends it down the ear canal.
2. The eardrum, a thin skin, vibrates with the compressions and rarefactions.
3. Three tiny bones (the hammer, the anvil and the stirrup) make the vibration stronger.
4. The cochlea, filled with liquid, changes the vibration into electrical signals.
5. The auditory nerve carries the signals to the brain.

The range of hearing

A healthy young person hears from about $20\ \text{Hz}$ to $20\,000\ \text{Hz}$. The upper limit drops with age.

• Below $20\ \text{Hz}$ is infrasound: elephants and whales call with it, and earthquakes make it.
• Above $20\,000\ \text{Hz}$ is ultrasound: dogs, bats and dolphins hear it.

The simulation is by PhET Interactive Simulations, University of Colorado Boulder (CC BY 4.0).

Protecting the ears

Very loud sounds, such as fireworks close by or loud music in earphones for hours, damage the cochlea for good. Keep the volume moderate and take breaks.

Watch out. "Ultrasound" does not mean "very loud". It means a frequency too high for us to hear.

A worked example

Find the wavelengths in air of a deep hum at $25\ \text{Hz}$, a high whistle at $16\,000\ \text{Hz}$, and a bat's call at $85\,000\ \text{Hz}$.

1. The hum. $\lambda = \dfrac{340\ \text{m/s}}{25\ \text{Hz}} = 13.6\ \text{m}$, longer than a bus.
2. The whistle. $\lambda = \dfrac{340\ \text{m/s}}{16\,000\ \text{Hz}} = 0.021\ \text{m}$, about $2\ \text{cm}$.
3. The bat. $\lambda = \dfrac{340\ \text{m/s}}{85\,000\ \text{Hz}} = 0.004\ \text{m}$, about the size of a small insect.

Practise with new numbers

Try it in Python

The program sorts sounds into infrasound, audible sound and ultrasound.

Remember these

• Outer ear, ear canal, eardrum, three bones, cochlea, nerve, brain.
• The cochlea changes vibrations into signals.
• We hear about $20\ \text{Hz}$ to $20\,000\ \text{Hz}$; below is infrasound, above is ultrasound.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. What does the eardrum do when a sound reaches it?

2. A sound has a frequency of $45\,000\ \text{Hz}$. Can a person hear it? What is it called?

3. What is the job of the cochlea?

4. Name an animal that uses infrasound.