Sound

O Level / IGCSE Physics · Waves. A short explanation of the idea, the rules to remember, the mistake to avoid, a worked example and practice questions with answers.

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The idea

Sound is made by something that vibrates, such as a guitar string or a loudspeaker cone. The vibrations push and pull the particles of the air, making a longitudinal wave. Places where the particles are squeezed together are compressions. Places where they are spread out are rarefactions.

Sound needs particles to pass on the vibrations, so it cannot travel through a vacuum. It travels fastest in solids, slower in liquids and slowest in gases.

A larger amplitude gives a louder sound. A higher frequency gives a higher pitch. An echo is a sound reflected from a hard surface, and it can be used to measure distance.

Rules to remember

Humans can hear frequencies from about 20 Hz to 20 000 Hz.
Ultrasound is sound with a frequency above 20 kHz (20 000 Hz).
Speed of sound in air is about 330 to 350 m/s.
speed = distance ÷ time. For an echo the sound travels there and back, so distance to the surface = speed × time ÷ 2.
Amplitude sets the loudness; frequency sets the pitch.
Two instruments playing the same note have the same frequency but different wave shapes on an oscilloscope. This is the quality (timbre).
Uses of ultrasound: cleaning, scanning an unborn baby, and sonar to find depth.

Common mistake

In echo questions students forget that the sound travels to the surface and back. Halve the time (or halve the total distance) to find how far away the surface is.

Worked example

A walker shouts towards a cliff and hears the echo 3.0 s later. The speed of sound in air is 340 m/s. How far away is the cliff?

Total distance travelled by the sound = speed × time = 340 × 3.0 = 1020 m.
This is the distance to the cliff and back.
Distance to the cliff = 1020 ÷ 2 = 510 m.

Answer: 510 m

Practice questions

Try each one, then open the answer.

1. A flute and a violin play a note of the same pitch and the same loudness, yet a listener can tell them apart. How do their oscilloscope traces differ?

A
the traces have different frequencies
B
the traces have different amplitudes
C
the traces have different shapes
D
the traces move across the screen at different speeds

Show answer

Answer: C. Same pitch means the same frequency and same loudness means the same amplitude. The different quality (timbre) shows as a different wave shape.

2. A sound becomes higher in pitch. Which property of the wave has increased?

A
amplitude
B
frequency
C
speed
D
wavelength

Show answer

Answer: B. Pitch depends on frequency; loudness depends on amplitude.

3. A ringing bell is placed inside a jar and the air is pumped out. The sound fades although the bell still vibrates. Why?

A
the glass absorbs all sound
B
sound needs a medium and there are no particles to carry it
C
the bell stops vibrating in a vacuum
D
the sound becomes ultrasound

Show answer

Answer: B. Sound is carried by vibrating particles. Without air there is nothing to pass the vibrations to the jar and out to the listener.

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