Key Stage 3 science · Physics – Waves

KS3 hearing, microphones and ultrasound: a guide for parents

Key Stage 3 pupils learn what sound really is: a series of vibrations that travel as longitudinal pressure waves and carry energy. They find out how loudspeakers make sound, how microphones and the ear drum detect it, what the limits of human hearing are compared with other animals, and how ultrasound, sound too high for us to hear, is put to work in cleaning and physiotherapy.

Hearing, microphones and ultrasound, explained

Every sound starts with a vibration: a guitar string, the cone of a loudspeaker, your vocal cords. The vibrating object pushes on the air next to it, squashing the air particles together into a region of higher pressure called a compression, and then moves back, leaving a region of lower pressure called a rarefaction. These squashed and stretched regions travel outwards one after another. The air particles themselves only shuffle back and forth a tiny distance, in the same direction the wave is travelling. That is what makes sound a longitudinal wave, unlike a water ripple. It is also why the wavy line in a textbook is a graph of pressure and not a picture of what the air is doing.

Sound is detected when its pressure changes make something else vibrate. The ear drum is a thin, stretched membrane that is pushed in and out by the pressure variations, and tiny bones behind it pass the vibration on to the inner ear, where it becomes nerve signals. A microphone works on the same principle. Sound waves make a thin, flexible sheet called a diaphragm vibrate, and the microphone converts that movement into a varying electrical signal that copies the pattern of the sound. A loudspeaker does the reverse: a varying electrical current makes a coil act as an electromagnet that pushes and pulls against a fixed magnet, moving a paper or plastic cone back and forth to create new sound waves in the air.

Hearing has limits. The auditory range of a young, healthy person is roughly 20 Hz to 20,000 Hz, and the top of that range falls with age and after exposure to loud noise. Sound below 20 Hz is called infrasound, and sound above 20,000 Hz is called ultrasound. Other animals hear differently. Dogs and cats hear well above 20,000 Hz, bats and dolphins use ultrasound to find their way and catch prey, and elephants communicate using very low frequencies. Because loud sound carries a lot of energy, headphones turned up too high can cause permanent hearing damage, which is a point teachers often make in lessons.

The key idea behind ultrasound is that sound waves are pressure waves that transfer energy. Ultrasound carries energy in the same way as the sounds we hear, even though its frequency is too high for our ears. In an ultrasonic cleaner, jewellery, spectacles or surgical instruments are placed in a bath of liquid that is made to vibrate extremely quickly, shaking dirt off the surface without any scrubbing. Physiotherapists can direct ultrasound at an injured muscle or joint to deliver energy to the tissue beneath the skin. Many schools also mention medical scans, where echoes of ultrasound are used to build a picture, and the way bats and dolphins use echoes. These are common examples, not statutory requirements.

The thread running through the topic is that waves can transfer information as well as energy. A microphone turns the pattern of a sound into an electrical signal, and that signal can be amplified, recorded or sent along a wire and then turned back into sound. A bigger vibration gives a bigger signal and a louder sound, and faster vibrations give a higher frequency and a higher pitch. Pupils often draw or see these patterns on a screen, and practise explaining what changes when the sound gets louder or higher.

Words your child will hear

Longitudinal wave: A wave where the vibrations are in the same direction as the wave travels. Sound waves are longitudinal.
Compression and rarefaction: The squashed (higher pressure) and stretched (lower pressure) regions that make up a sound wave.
Diaphragm: A thin, flexible sheet in a microphone that vibrates with the sound. The ear drum does a similar job in the ear.
Auditory range: The range of frequencies an animal can hear. For humans it is roughly 20 Hz to 20,000 Hz.
Ultrasound: Sound with a frequency above 20,000 Hz, too high for humans to hear.
Infrasound: Sound with a frequency below 20 Hz, too low for humans to hear.
Pressure wave: A wave made of regions of high and low pressure that moves energy through a material.

What the National Curriculum says

The programme of study for Key Stage 3 says:

Pupils should be taught about sound produced by vibrations of objects, in loudspeakers, detected by their effects on a microphone diaphragm and the ear drum; sound waves are longitudinal
Pupils should be taught about the auditory range of humans and animals
Pupils should be taught about pressure waves transferring energy; use for cleaning and physiotherapy by ultrasound; waves transferring information for conversion to electrical signals by microphone

Objectives from the National Curriculum in England, © Crown copyright, used under the Open Government Licence v3.0.

Try it: explaining how a microphone and loudspeaker work together

A singer’s voice is picked up by a microphone and heard through a loudspeaker. Describe what happens to the sound at each stage, and how energy and information are passed on.

The singer’s vocal cords vibrate. They push on nearby air to create compressions and rarefactions, a longitudinal pressure wave that carries energy outwards from the singer.
The pressure wave reaches the diaphragm in the microphone and makes it vibrate in the same pattern. Energy has been transferred from the sound wave to the movement of the diaphragm.
The microphone converts the diaphragm’s movement into a varying electrical signal. A bigger vibration (a louder sound) makes a bigger signal, and faster vibrations (a higher pitch) make the signal change faster. The signal now carries the information about the sound.
The signal is amplified and carried along a wire. No air travels through the wire. Only the electrical pattern does, so it can be recorded or sent a long way.
In the loudspeaker the varying current flows through a coil, which becomes an electromagnet. It is pushed and pulled by a fixed magnet, and the cone attached to it moves back and forth in the same pattern as the original vibration.
The cone pushes on the air and creates new compressions and rarefactions. These reach a listener’s ear drum, which vibrates in the same pattern, so the listener hears the same pitch. Some energy is dissipated along the way, warming the wires and components slightly, and the amplifier is needed because the microphone’s signal is far too weak to drive the cone by itself.

Vocal cords vibrate, then a pressure wave in the air, then the microphone diaphragm vibrates, then an electrical signal, then the loudspeaker cone vibrates, then a new pressure wave in the air reaches the listener’s ear drum. The pattern of the vibration is copied at each stage, so the pitch stays the same, and the amplifier sets how loud the final sound is.

Common mistakes and how to help

Thinking sound waves are up-and-down waves like those on water

Almost every diagram of sound is a wavy line, which looks just like a water wave, so children assume the air moves up and down.

What helps: Explain that the line is a graph of pressure against time. In the real wave, the air moves back and forth along the direction the sound travels. Pushing and pulling the end of a long spring shows the same thing.

Believing ultrasound means a very loud sound

The prefix “ultra” suggests “extra strong”, and ultrasound is often described as powerful.

What helps: Ultrasound is about frequency, not loudness. It is above 20,000 Hz, so we cannot hear it, however much energy it carries. A loud sound is one with a big amplitude and may well be at a frequency we can hear.

Thinking air travels from a loudspeaker to your ear

You can feel air move in front of a big bass speaker, and “the sound came to me” sounds like something is moving across the room.

What helps: The air particles only vibrate to and fro about their positions. What travels is the pattern of compressions and rarefactions. If the air itself moved, a speaker would blow like a fan.

Believing sound travels down the wire between a microphone and a speaker

Sound goes into one end and comes out of the other, so it seems natural to think it passes along the cable.

What helps: The wire carries an electrical signal that copies the pattern of the sound. No sound exists in the wire. The loudspeaker makes completely new sound waves.

How to help at home

Feel the vibrations. Put your fingertips lightly on your throat and hum a high note, then a low one, then a quiet note and a loud one. You can feel the vocal cords vibrating, and discuss which changes are frequency and which are amplitude.
Make a simple diaphragm. Stretch a balloon or cling film tightly over the open end of a cardboard tube or cup and hold it with a rubber band. Sprinkle a few grains of rice on top and speak loudly close to the other end of the tube, or tap a tray nearby. The grains jump as the diaphragm vibrates.
Pitch on a rubber band guitar. Stretch rubber bands of different thickness across an open box and pluck each one. Tighter and thinner bands vibrate faster, and the note is higher. Ask your child to predict before each pluck.
Talk about safe listening. Many teenagers listen through headphones for hours. A sensible rule of thumb is that if someone next to you can hear your music clearly, or you cannot hear them talking from an arm’s length away, it is too loud. Explain that hearing damage from loud sound is permanent.
Wonder about animals. Ask how a bat can catch a moth in the dark, or why a dog responds to a whistle you cannot hear. Encourage your child to explain it using echoes, frequencies and the auditory range.

Leads on to

Key Stage 4: Properties of waves
Key Stage 4: The electromagnetic spectrum