6.1.4 Sound waves (HT)

Sound waves and vibrations in solids

1. Sound waves can travel through solids and make parts of the solid vibrate.
2. A sound wave in air can be converted into vibrations in a solid; for example, loud sound can make a window vibrate.
3. The conversion also works the other way: a vibrating solid produces sound waves in air, as when the metal prongs of a tuning fork vibrate and disturb the surrounding air.

Sound waves and the ear

1. A sound wave travels through the air into the ear.
2. The wave reaches the ear drum and makes it vibrate.
3. These vibrations are passed to other parts of the ear.
4. The vibrations are then processed by the body, producing the sensation of sound.
5. This works because the ear drum is a thin solid membrane that vibrates when a sound wave reaches it, so it is an example of a sound wave being converted into vibrations in a solid.

The limited frequency range of hearing

1. The conversion between sound waves and vibrations in a solid only works over a limited frequency range.
2. The parts that vibrate, such as the ear drum and the tiny structures behind it, have a fixed size, mass and stiffness, which means they can only be made to vibrate efficiently by a limited range of frequencies.
3. If a sound wave's frequency lies outside this range, these parts cannot vibrate in step with it, so the wave is not converted into useful vibrations and no sensation of sound is produced.
4. Frequency is the number of waves passing a point each second; 20 Hz means 20 vibrations per second and 20 kHz means 20,000 Hz.
5. The range of normal human hearing is from 20 Hz to 20 kHz.
6. This is why human hearing is limited to about 20 Hz to 20 kHz, and why the upper limit tends to fall with age as these parts become stiffer and respond less well to high frequencies.
7. If the frequency is too low or too high, the ear does not convert the sound into useful vibrations, so it does not produce the normal sensation of sound.