Ultrasound and its uses

Sound that we cannot hear

Sound with a frequency above $20\,000\ \text{Hz}$ is ultrasound. Its short waves travel in a narrow beam and reflect even from small objects. That makes it very useful.

Uses of ultrasound

• Cleaning: small parts such as watch pieces are put in a liquid, and ultrasound shakes the dirt off.
• Finding cracks: ultrasound sent through a metal block reflects from any hidden crack.
• Looking inside the body: in an ultrasound scan, echoes from organs are turned into a picture, for example of a baby before birth. Echocardiography looks at the heart.
• Breaking stones: ultrasound can break small kidney stones into fine grains.

SONAR

SONAR (sound navigation and ranging) finds the depth of the sea. A ship sends a pulse of ultrasound down and times its echo from the sea floor. Since the pulse goes down and back,

with $v \approx 1500\ \text{m/s}$ in sea water. SONAR also finds shoals of fish, wrecks and submarines.

You should find that the fish echo always comes first, because the fish are nearer. Doubling the depth doubles the echo time.

Animals that use echoes

Bats send out ultrasonic squeaks and listen for the echoes to find insects in the dark. Dolphins and porpoises do the same under water.

Watch out. Use $1500\ \text{m/s}$ in sea water, not $340\ \text{m/s}$, and halve the round trip.

A worked example

A SONAR pulse returns from a wreck after $0.9\ \text{s}$ and from the sea floor after $2.6\ \text{s}$. How deep is the wreck, and how far is it above the floor?

1. The wreck. $d = \dfrac{1500\ \text{m/s} \times 0.9\ \text{s}}{2} = 675\ \text{m}$.
2. The floor. $d = \dfrac{1500\ \text{m/s} \times 2.6\ \text{s}}{2} = 1950\ \text{m}$.
3. The gap. $1950\ \text{m} - 675\ \text{m} = 1275\ \text{m}$ above the floor.

Practise with new numbers

Try it in Python

The program turns SONAR echo times along a ship's path into depths.

Remember these

• Ultrasound: above $20\,000\ \text{Hz}$; a narrow beam that reflects from small objects.
• Uses: cleaning, finding cracks, body scans, breaking stones.
• SONAR: $d = \dfrac{vt}{2}$, with about $1500\ \text{m/s}$ in sea water.

Check yourself

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

1. A SONAR echo returns after $4\ \text{s}$. How deep is the sea?

2. Why is the time halved when we find the depth?

3. Name one medical use of ultrasound.

4. Why can ultrasound find a small crack inside a steel block?