Sound

What the syllabus demands

• —Describe sound as a longitudinal wave produced by vibrating sources, requiring a medium
• —Relate pitch to frequency and loudness to amplitude
• —State the audible range: 20 Hz to 20,000 Hz; define ultrasound as above 20 kHz
• —Describe an echo method for measuring the speed of sound in air (~330–350 m/s)
• —Describe uses of ultrasound: sonar, medical imaging, cleaning

Definitions that earn marks

Clear definitions to practise — check your course mark scheme

Sound wave

A longitudinal wave consisting of compressions and rarefactions, produced by a vibrating source, requiring a medium to travel through.

Ultrasound

Sound with a frequency above 20,000 Hz (20 kHz) — beyond the upper limit of human hearing.

Echo

A reflection of sound from a hard surface, heard after the original sound.

The equations

More equations to practise: the IGCSE formula sheet.

Where the marks die

Common mistakes to check

1. 01

   Forgetting the factor of 2 in echo calculations. The sound travels to the wall and back, so the distance is 2d — or the time is halved. One or the other, never both.

2. 02

   Saying sound travels through a vacuum. It cannot — sound needs particles. The bell-in-a-vacuum-jar experiment exists to test this sentence.

3. 03

   Mixing up pitch and loudness: pitch is set by frequency, loudness by amplitude. On an oscilloscope trace, more waves per screen = higher pitch; taller waves = louder.

4. 04

   Reversing the media order: sound travels fastest in solids, slower in liquids, slowest in gases — because the particles are closer together, energy passes between them faster.

One worked example, done properly