Sound waves: what type of wave is sound?

What is a sound wave?

A sound wave is a travelling pattern of vibrations that carries energy through a material. It starts with something vibrating: a guitar string, a loudspeaker cone, your vocal cords. As a speaker cone pushes forwards, it squeezes the air molecules in front of it together. As it moves back, it leaves a region where they are spread out. Each molecule bumps into the next, passing the pattern along, until it reaches your ear and makes your eardrum vibrate too.

The air itself doesn't travel from the speaker to you. Each molecule only jiggles back and forth around the same spot. What travels is the energy and the pattern of squeezes and stretches.

Why sound is a longitudinal wave

Waves come in two kinds, depending on how the particles move compared with the direction the wave goes (our guide to transverse and longitudinal waves covers both in depth):

Sound fits the longitudinal pattern: the air molecules vibrate along the line the sound travels. The AQA GCSE specification states it directly: "Sound waves travelling through air are longitudinal." A good model is a slinky spring on a table. Push one end in and out and you see bunched up sections travel along it, with stretched sections between.

• Compression: a region where the particles are pushed close together, so the pressure is a little higher than normal.
• Rarefaction: a region where the particles are spread further apart, so the pressure is a little lower than normal.
• Wavelength: the distance from one compression to the next (or one rarefaction to the next).

Why do diagrams show sound as a wavy line?

Oscilloscopes and textbooks often draw sound as a transverse looking wave. That line is a graph, not a picture: it plots the air pressure (or the displacement of the particles) at each point. The peaks are the compressions and the troughs are the rarefactions. It is easier to read amplitude and wavelength from a graph like this, but the sound itself is still longitudinal.

Sound is a mechanical wave: it needs a medium

Because sound is passed on by particles bumping into each other, it needs particles. Sound travels through gases, liquids and solids, but not through a vacuum. In the classic experiment, a ringing bell inside a glass jar gets quieter as the air is pumped out, even though you can still see the hammer hitting it. Light, which is an electromagnetic wave, still gets through. That is why space is silent.

The speed of sound

Sound travels at about 343 m/s in air at 20 °C, and faster in liquids and solids, where the particles are closer together and more strongly linked. These values are from OpenStax University Physics:

Sound is about 4 times faster in water than in air, and about 17 times faster in steel. Warmer air carries sound slightly faster, because the molecules move faster.

Worked example: how far away is the storm?

You see lightning, then hear thunder 6 seconds later. Light arrives almost instantly, so the delay is the time the sound took. Distance = speed × time = 343 × 6 = 2,058 m, about 2 km. That gives a handy rule: sound takes about 3 seconds to travel 1 km in air (1,000 ÷ 343 = 2.9 s).

Worked example: an echo

You clap in front of a cliff and hear the echo 1.2 s later. The sound went to the cliff and back, so the distance to the cliff is half of the total: 343 × 1.2 ÷ 2 = 205.8 m, about 206 m. Ships use the same idea with ultrasound to find the depth of the sea, called sonar.

Frequency, wavelength and the wave equation

Like all waves, sound obeys the wave equation, which links speed (v), frequency (f) and wavelength (λ):

A tuning fork vibrating at 512 Hz in air at 20 °C makes sound with a wavelength of λ = v ÷ f = 343 ÷ 512 = 0.670 m. The same note under water, at 1,480 m/s, has a wavelength of 1,480 ÷ 512 = 2.89 m. The frequency stays the same when sound moves into a new material; the speed and wavelength change. Check these in our Wave Speed Calculator.

Try one: this question asks you to use the wave equation.

Pitch, loudness and what we can hear

• Pitch depends on frequency. More vibrations per second sound higher. A small bird's song has a high frequency; a bass drum has a low one.
• Loudness depends on amplitude. Bigger pressure changes carry more energy and sound louder. Loudness is usually measured in decibels (dB).
• The range of human hearing is about 20 Hz to 20,000 Hz (20 kHz), as AQA puts it. In air, that means wavelengths from about 17 m (343 ÷ 20) down to about 1.7 cm (343 ÷ 20,000). The upper limit falls as people get older.

Sound above 20 kHz is ultrasound, used for scanning unborn babies and finding cracks in metal. Sound below 20 Hz is infrasound, made by elephants, earthquakes and large machines.

Common mistakes

• Calling sound transverse. In air it is longitudinal. The wavy graph is a pressure plot, not the shape of the wave.
• Saying the air travels to your ear. The particles only vibrate in place; the energy travels.
• Saying sound is faster in air than in solids. It is slower in air. It is light that slows down in denser materials.
• Saying sound can travel through space. It needs a medium; space is a near vacuum.
• Mixing up pitch and loudness. Pitch is frequency; loudness is amplitude.
• Forgetting to halve the distance for an echo. The sound travels there and back.

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