# Uses of Sound Waves

## Uses of Sound Waves

#### Echoes

Echoes are sound waves bouncing off surfaces. Sound waves obey the same first rule of reflection. (Remember: the angle of incidence is the same as the angle of reflection.)

The echo is usually quieter than the original noise as energy is lost as the wave travels along.

You can work out how far away something is using echo-sounds.

If it takes 20 seconds for the echo to be detected it must have taken 20 seconds for the sound to travel to the object and back. Using:

Distance = Speed x Time

The distance can be calculated. The speed of sound is 330 m/s so the calculation becomes:

Distance = 330 m/s x 20 s = 6600 m

This is the distance there and back, so the object is half that distance away, 3300 m.

Watch out. Many students forget to halve the distance.

Shiny hard surfaces reflect sound better than soft, surfaces. Bathrooms are good rooms to sing in as the sound bounces well off tiled walls. If you sing in the living room most of the sound energy is lost, because the energy is absorbed by the carpet, furniture and curtains.

#### Ultrasound

Sound waves that have a very high frequency are called ultrasound or ultrasonic waves. These sounds are so high that humans can't hear them. Dogs and bats have a higher hearing range than humans and can hear some ultrasonic waves.

Ultrasonic sound waves are made by electrical devices (like a loud speaker), which change electrical signals into sound waves.

There are many uses for ultrasound in medicine and industry. Here are some of them:

Looking at babies in the womb (pre-natal scanning):

A receiver compares the length of time it takes for the ultrasound waves to be detected. The longer the time it takes for the wave to reach the receiver the deeper into the body the wave has gone. This information is then used to build up a picture of the baby in the womb, which is then shown on a visual display, like a computer screen.

Cleaning instruments: Ultrasonic waves can be used to clean delicate instruments without having to take the equipment apart. The instrument is held in a liquid. The ultrasonic waves make the liquid particles vibrate at a high frequency, which cleans the surfaces of the equipment.

Detecting flaws and cracks in metal: This works in the same way as scanning babies in the womb. The ultrasonic waves bounce off different surfaces in the metal. The time it takes for the waves to bounce back to the receiver allows us to work out the depth the wave has travelled into the metal.