Sound

A sound wave is produced by something vibrating — a loudspeaker cone, a guitar string, your vocal cords. The vibration pushes on the air next to it, and that disturbance travels outwards.

As the cone moves forward it pushes the air particles closer together, creating a region of higher pressure called a compression. As it moves back, the particles spread apart, leaving a region of lower pressure called a rarefaction. A continuous vibration sends out a train of compressions and rarefactions.

The particles vibrate back and forth along the direction the wave travels, which is what makes sound longitudinal. They do not travel with the wave — each one oscillates about its own position while the disturbance moves on.

Because sound needs particles to pass the disturbance along, it cannot travel through a vacuum. Ring an electric bell inside a jar and pump the air out: the hammer can still be seen striking, but the sound fades to nothing. This is the standard demonstration and a standard exam question.

Sound travels at about 330 to 340 m s⁻¹ in air, roughly 1500 m s⁻¹ in water and around 5000 m s⁻¹ in steel. The pattern is consistent: the closer the particles, the faster the sound.

The reason is straightforward. The wave travels by particles colliding with their neighbours and passing the disturbance on. In a solid the particles are packed tightly and strongly bonded, so a collision happens almost immediately and the disturbance is handed along very quickly. In a gas the particles are far apart and must travel some distance before colliding, so the wave is slow.

Sound in air also travels slightly faster when the air is warmer, because the particles are moving faster and collide more often. It does not depend on the loudness or the pitch — all sounds travel at the same speed in the same conditions, which is fortunate, or an orchestra would arrive at the back of a hall in the wrong order.

v

speed of soundm s⁻¹

d

distance to the reflectorm

t

time for the echos

Sound waves add exactly like this. Two loudspeakers producing the same note create places where crests meet crests and the sound is louder, and places where a crest meets a trough and it is quieter — interference you can walk through in a room.

Two properties of a sound wave map onto two things you hear. Frequency determines pitch: a high-frequency wave is heard as a high note. Amplitude determines loudness: a larger amplitude carries more energy and is heard as a louder sound.

Keeping these separate matters, because a question will often change one and ask about the other. Turning up the volume increases the amplitude and leaves the frequency alone — the note is louder but not higher.

The human ear responds to frequencies between roughly 20 Hz and 20 000 Hz. This range narrows with age, mostly at the top end. Sound above 20 kHz is called ultrasound, and although we cannot hear it, it is extremely useful.

Sound reflects from hard surfaces, and the reflection heard afterwards is an echo. The delay exists because sound travels at a finite speed — a fact worth stating explicitly, since "it bounces back" alone does not explain the pause.

That delay can be measured and turned into a distance. Send a pulse, time its return, multiply by the speed, and halve — because the pulse made the journey twice. This is echo sounding, used to map the sea bed and to find shoals of fish.

The same principle with ultrasound is used for medical imaging. Ultrasound is sent into the body, reflects from the boundaries between different tissues, and the returning pulses are timed to build a picture. Because it is not ionising, it is safe enough to use on an unborn baby, which X-rays are not.

Ultrasound is also used industrially to detect cracks inside metal castings without cutting them open, and to clean delicate objects such as jewellery and surgical instruments by shaking dirt loose in a liquid bath.