Sound, ultrasound and infrasound

Sound waves can travel through solids causing vibrations in the solid.

Within the ear, sound waves cause the ear drum and other parts to vibrate, which causes the sensation of sound. The conversion of sound waves to vibrations of solids works over a limited frequency range. This restricts the limits of human hearing.

You should know that the range of normal human hearing is from 20 Hz to 20 kHz.

You should be able to describe, with examples, processes which convert wave disturbances between sound waves and vibrations in solids — the effect of sound waves on the ear drum being the obvious example. A microphone works the same way, its diaphragm vibrating in response to the sound; a loudspeaker runs the process backwards, its cone vibrating to produce sound waves in the air.

You should also be able to explain why such processes only work over a limited frequency range, and the relevance of this to human hearing.

The explanation: the solid parts that have to vibrate — the ear drum and the small bones behind it — have a size and a stiffness, and they respond well only to a limited range of frequencies. Below about 20 Hz and above about 20 kHz they do not vibrate enough to pass the wave on, so no sensation of sound is produced. That is what sets the limits of human hearing, and it is why the upper limit falls with age as the ear becomes less flexible.

You should be able to explain in qualitative terms how the differences in velocity, absorption and reflection between different types of wave in solids and liquids can be used both for detection and exploration of structures which are hidden from direct observation.

That is the idea behind all three examples below: a wave that reaches somewhere we cannot go, and comes back changed, carries information about what it passed through.

Ultrasound waves have a frequency higher than the upper limit of hearing for humans — so above 20 kHz. Ultrasound waves are partially reflected when they meet a boundary between two different media. The time taken for the reflections to reach a detector can be used to determine how far away such a boundary is. This allows ultrasound waves to be used for both medical and industrial imaging.

Echo sounding, using high frequency sound waves, is used to detect objects in deep water and measure water depth.

The calculation in both cases uses s = v t, with one correction that is easy to forget: the wave travels to the boundary and back, so the distance found is twice the depth.

Seismic waves are produced by earthquakes.

• P-waves are longitudinal seismic waves. P-waves travel at different speeds through solids and liquids. • S-waves are transverse seismic waves. S-waves cannot travel through a liquid.

P-waves and S-waves provide evidence for the structure and size of the Earth's core.

How that becomes evidence: after an earthquake, detectors around the world record which waves arrive where. S-waves are not detected on the far side of the Earth at all, and since S-waves cannot travel through a liquid, part of the core must be liquid. P-waves are detected, but they arrive later than expected and are refracted, because they change speed on entering a different medium — and the size of that shadow zone gives the size of the core.

You should be aware that the study of seismic waves provided new evidence that led to discoveries about parts of the Earth which are not directly observable. Nobody has ever been near the core; everything known about it was inferred from waves.