SoundEdexcel GCSE Physics: Revision notes

Sound travels as vibrations in air (a longitudinal wave). These vibrations can be converted into vibrations in a solid, and back again.

• Sound waves in the air make the eardrum (a thin membrane) vibrate
• These vibrations pass through tiny bones in the middle ear, and eventually cause vibrations that nerve signals can be generated from
• This conversion process — sound wave to solid vibration and back — only works over a limited frequency range, which is why the human ear cannot detect every possible frequency of sound

The human ear can typically only detect sound between about 20 Hz and 20 000 Hz. Sound outside this range still exists, but humans cannot hear it:

• Ultrasound: sound with a frequency greater than 20 000 Hz
• Infrasound: sound with a frequency less than 20 Hz

• Sonar: ships and submarines send out ultrasound pulses and time how long they take to reflect off the seabed or objects, calculating distance from the speed and time — used for navigation and finding shoals of fish
• Foetal scanning: ultrasound is directed into the body; it reflects differently off different tissues (e.g. the boundary between fluid and the foetus), and these reflections are used to build an image, without the risks of ionising radiation like X-rays
• Exploration of the Earth's core: infrasound (and seismic waves) travel through and reflect off boundaries deep within the Earth, and scientists use their travel times to work out the structure of the Earth's interior, including the core

When a sound wave crosses from one medium into another (e.g. air into water), its velocity and wavelength change, but its frequency stays the same (frequency is set by the source vibrating, not by the medium).

• If the wave speeds up in the new medium, its wavelength increases (since v = f × λ and f is constant)
• If the wave slows down, its wavelength decreases

This is the same refraction relationship that applies to all waves, applied specifically to sound.

This practical investigates equipment suitable for measuring wave properties in both a solid and a fluid.

1. In a solid (e.g. a metal rod or a vibrating string/spring): use a signal generator and vibration transducer to set up a standing wave; measure the wavelength directly using a ruler between nodes, and read the frequency from the signal generator; calculate speed using v = f × λ
2. In a fluid (e.g. sound in air, or ripples in a ripple tank): time how long a sound or ripple takes to travel a known distance using a stopwatch (v = x/t), or measure wavelength and frequency directly and calculate v = f × λ

Repeat readings and take an average to reduce the effect of random error; consider which measuring equipment (ruler, stopwatch, signal generator, oscilloscope) is most suitable and precise for each medium.