Cambridge IGCSE Physics · Syllabus 0625 · Waves
Sound
What is Sound?
Sound is a longitudinal mechanical wave produced by a vibrating source: the particles of the medium vibrate parallel to the direction the wave travels. Sound requires a material medium and cannot travel through a vacuum.
This definition is part of the Waves chapter in Cambridge IGCSE Physics.
Sound in context
A wave is a disturbance that travels, carrying energy away from a source while the material it passes through only oscillates about fixed positions. Describe the disturbance with wavelength, frequency and amplitude; link them with \(v = f\lambda\). Every wave can reflect, refract and diffract. Light is an electromagnetic wave, so mirrors, blocks, prisms, fibres and lenses are all the same three behaviours applied to rays. Sound is a longitudinal mechanical wave, so it needs a medium, its pitch follows frequency and its loudness follows amplitude, and its echoes measure distance.
The electromagnetic spectrum is one continuous family of transverse waves. They all transfer energy, they all travel at the same high speed in a vacuum — far faster than sound — and they all need no material medium. In order of increasing frequency and therefore decreasing wavelength the main regions are: radio, microwave, infrared, visible, ultraviolet, X-ray, gamma. Each region is used for the jobs its particular frequency and wavelength suit, and the hazards change as you move along the spectrum: heating at the low-frequency end, and ionising damage to cells at the high-frequency end.
Sound is produced by a vibrating source and travels as a longitudinal wave: the particles of the medium oscillate back and forth parallel to the direction the wave travels. Because the disturbance is handed on from particle to particle, sound cannot travel through a vacuum. Its frequency sets the pitch and its amplitude sets the loudness. Humans hear roughly 20 Hz to 20 000 Hz; above 20 kHz the sound is ultrasound.
Common mistakes with Sound
- 4. “Frequency changes when a wave crosses a boundary.” Core 3.1.7 Why wrongIf fewer crests left the boundary each second than arrived at it, crests would have to pile up there without limit. They do not. CorrectFrequency is fixed by the source. Speed changes at the boundary, and since \(v = f\lambda\) with \(f\) fixed, the wavelength changes in the same ratio. Say this“The frequency is unchanged because it is determined by the source; the speed and wavelength both change.” CheckA 500 Hz sound passes from air into water where it travels about four times faster. State what happens to \(f\) and to \(\lambda\).
- 5. “Refraction occurs because the amplitude changes.” Core 3.1.7 Why wrongAmplitude controls how much energy the wave carries. It has no influence on direction; a very quiet and a very loud sound refract identically. CorrectRefraction is caused by a change of speed. A wavefront arriving obliquely pivots because one end slows before the other. Say this“Refraction occurs because the wave changes speed at the boundary.” CheckState the one quantity whose change causes refraction.
- 16. “Radio waves are sound waves.” Core 3.3.1 Why wrongA radio wave is transverse, electromagnetic, travels at the same high speed in a vacuum as every other electromagnetic wave, and crosses a vacuum. Sound is longitudinal, mechanical, travels at a few hundred metres per second and cannot cross a vacuum. CorrectA radio receiver converts the radio wave it detects into an electrical signal and then into sound. The two waves are different at every level. Say this“Radio waves are transverse electromagnetic waves; sound waves are longitudinal mechanical waves that need a medium.” CheckGive two differences between a radio wave and a sound wave.
- 19. “Pitch depends on amplitude.” Core 3.4.7 Why wrongPlay the same guitar string harder: it sounds louder, not higher. The note is unchanged. CorrectPitch is set by frequency. Higher frequency gives higher pitch. Say this“A higher frequency gives a higher pitch.” CheckTwo traces have the same height but one has twice as many waves. Which sound is higher in pitch?
- 20. “Loudness depends on frequency.” Core 3.4.7 Why wrongThe mirror image of the previous error. A high whistle can be barely audible and a low drum deafening. CorrectLoudness is set mainly by amplitude. Greater amplitude gives a louder sound. Say this“A larger amplitude gives a louder sound.” CheckWhich measurement on a waveform display tells you the loudness?
- 21. “Sound travels fastest in gases.” Supplement 3.4.11 Why wrongIt reverses the particle argument. Gas particles are far apart and interact only on collision, so a disturbance crosses a gas slowly. CorrectSound travels fastest in solids, more slowly in liquids, and slowest in gases, because closely packed and strongly bonded particles pass the disturbance on more quickly. Say this“In general, sound travels faster in solids than in liquids and faster in liquids than in gases.” CheckPut air, sea water and steel in order of increasing speed of sound.
- 22. “Ultrasound means very loud sound.” Core 3.4.9 Why wrongIt confuses the two axes of a trace. “Ultra” here refers to frequency, not amplitude. A dog whistle is ultrasonic and very quiet. CorrectUltrasound is sound of frequency above 20 kHz, beyond the upper limit of human hearing. Its loudness is a separate matter entirely. Say this“Ultrasound is sound with a frequency higher than 20 kHz.” CheckIs a 30 kHz sound necessarily loud? Explain.