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Cambridge IGCSE Physics · 0625

Chapter 3: Waves — Part 7

Topic 3.4 · Sound

How sound travels

Sound is produced by vibrating sources. It is a longitudinal wave.

A medium is needed — sound does not travel in a vacuum.

Speed of sound in air is about 330–350 m/s.

The approximate range of frequencies audible to humans is 20 Hz to 20 000 Hz.

Sound as a longitudinal wave: vibrating source, compressions and rarefactions in air, with a note that a medium is required.
Diagram 1: Sound as a longitudinal wave: vibrating source, compressions and rarefactions in air, with a note that a medium is required.

Loudness, pitch and echoes

Greater amplitude means a louder sound. Greater frequency means a higher pitch.

An echo is a reflection of sound.

Speed in air can be found by measuring a distance and a time (for example clap-and-echo: speed = 2d / t).

A student claps 80 m from a wall. The echo returns in 0.48 s. Calculate the speed of sound in air.

Worked example: echo speed is 2 times 80 over 0.48 equals 330 metres per second
The sound travels 2d. v = 2d / t = 330 m/s.

Ultrasound is sound with frequency higher than 20 kHz.

Compressions, speed in materials, ultrasound uses

A compression is a region of higher pressure (particles closer). A rarefaction is a region of lower pressure (particles further apart).

In general, sound travels faster in solids than in liquids, and faster in liquids than in gases.

Ultrasound is used in non-destructive testing of materials, medical scanning of soft tissue, and sonar. Depth or distance = (speed × time) / 2 for a pulse that goes to a reflector and back.

A sonar pulse is sent from a boat. The echo from the sea bed returns after 0.080 s. Speed of sound in sea water = 1500 m/s. Calculate the depth of the water.

Worked example: sonar depth is 1500 times 0.080 over 2 equals 60 metres
Depth = (v t) / 2 = 60 m. Do not forget the divide-by-two.
Echo method: clap or loudspeaker facing a wall, measured distance 2d and time t, speed = 2d / t; ultrasound pulse-echo for depth with the same idea.
Diagram 2: Echo method: clap or loudspeaker facing a wall, measured distance 2d and time t, speed = 2d / t; ultrasound pulse-echo for depth with the same idea.

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