Echoes, Hearing and Ultrasound: Quick Revision

A brief note on Echoes, Hearing and Ultrasound. Read it before a test. Try each question at the end before you open its answer.

The big ideas

• Sound reflects from hard surfaces; an echo is a reflected sound.
• Echo: $d = \dfrac{vt}{2}$, because the sound goes there and back.
• A separate echo needs at least $0.1\ \text{s}$: a wall at least $17\ \text{m}$ away in air.
• Many reflections make reverberation; soft materials absorb sound.
• Ear: outer ear, ear canal, eardrum, three bones, cochlea, nerve.
• We hear about $20\ \text{Hz}$ to $20\,000\ \text{Hz}$; below is infrasound, above is ultrasound.
• SONAR: $d = \dfrac{vt}{2}$ with about $1500\ \text{m/s}$ in sea water.

The facts at a glance

A worked example

A fishing boat's SONAR gets echoes after $0.48\ \text{s}$ and $1.96\ \text{s}$. How deep are the fish and the floor?

1. Fish. $\dfrac{1500\ \text{m/s} \times 0.48\ \text{s}}{2} = 360\ \text{m}$.
2. Floor. $\dfrac{1500\ \text{m/s} \times 1.96\ \text{s}}{2} = 1470\ \text{m}$.
3. So the fish are $1110\ \text{m}$ above the floor.

Practice with new numbers

A program to try

The program finds echo distances in air and in sea water.

Mistakes to avoid

• Forgetting to halve. The echo time is for the round trip.
• Using $340\ \text{m/s}$ under the sea. SONAR needs about $1500\ \text{m/s}$.
• 'Ultrasound means very loud.' It means too high to hear.
• Mixing up the eardrum and the cochlea. The eardrum vibrates; the cochlea makes the signals.

Check yourself

1. A shout's echo returns after $1.2\ \text{s}$. How far is the wall?

2. Why do we not hear echoes in a small room?

3. Is $25\ \text{Hz}$ infrasound, audible or ultrasound?

4. Which part of the ear changes vibrations into signals?