Sound and ultrasound

What this chapter teaches

Sound is a longitudinal wave that needs a medium and cannot cross a vacuum. An echo covers the distance to a wall twice, so v = 2d ÷ t. Ultrasound is sound above 20 000 Hz. Sonar and body scans time its echoes, and the depth is half of v × t.

Three questions to ask

There is no one right wording. Listen for the idea.

1. A clap echoes from a wall 85 m away. Why does the echo take twice as long as the sound needs to reach the wall?
2. How does a ship find the depth of the sea with sound?
3. Why is ultrasound used to scan unborn babies, and why does the doctor use gel?

After the chapter, the learner can

• Measure the speed of sound using an echo from a wall
• Explain how ultrasound is used to scan unborn babies and find sea depths