Sound waves and human hearing revision notes

Edexcel International GCSE Physics (4PH1), chapter 13: Sound, specification point 3.23, 3.24P. Free lesson.

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Sound is a longitudinal wave that travels through a medium, and its speed depends on the material. You can use an echo to calculate distance, but must account for the sound travelling there and back.

How sound waves move

A loudspeaker cone moves in and out, pushing and pulling the air in front of it. Air particles vibrate backwards and forwards along the same line as the wave travels, so sound is a longitudinal wave. The particles pass the vibration and energy to nearby particles, while each particle stays near its starting position. A compression is where particles are close together. A rarefaction is where particles are spread out. The amplitude is how far a particle moves from its starting position.

Why sound needs a medium

The material that sound travels through is called a medium. Sound is passed along as particles vibrate, so it cannot travel where there are no particles. A vacuum has no particles, which means sound cannot cross it. Sound can travel through solids, liquids and gases. Sound cannot travel through a vacuum.

How sound speed depends on the material

Sound travels at different speeds in different materials. Particles in solids are close and tightly linked, so sound travels fastest there. In gases, particles are far apart, so sound travels slowest. When sound enters a new material, its speed changes and it can change direction. This is called refraction. In steel, sound travels at about 5000 m/s. In water, sound travels at about 1500 m/s. In air, sound travels at about 340 m/s. Sound can also be reflected by hard surfaces. A reflected sound is an echo.

The range of human hearing

Frequency is the number of waves passing each second, measured in hertz (Hz). Humans can hear frequencies from 20 Hz to 20 000 Hz. Sound above 20 000 Hz is called ultrasound. It is still sound, but humans cannot hear it. Give both limits of human hearing with units: 20 Hz to 20 000 Hz.

How to calculate distance from an echo

Use the wave-speed equation, v = d/t. Here, v is speed in m/s, d is distance travelled in m, and t is time in s. For an echo, the sound travels to the surface and back, so the distance calculated from the equation is twice the distance to the surface.