Cambridge O Level Physics · Syllabus 5054 · Waves
Sound
What is Sound?
Sound is a longitudinal mechanical wave produced by a vibrating source, consisting of compressions and rarefactions travelling through a material medium; it cannot travel through a vacuum.
This definition is part of the Waves chapter in Cambridge O Level 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.
Sound is produced by a vibrating source and travels as a longitudinal mechanical wave: the particles of the medium oscillate back and forth parallel to the direction of energy transfer, creating compressions (particles closer together, pressure above normal) and rarefactions (particles further apart, pressure below normal). Because it needs particles to pass the disturbance on, sound cannot travel through a vacuum. Its frequency sets the pitch, its amplitude sets the loudness, and its waveform shape sets the quality or timbre.
Common mistakes with Sound
- 4. “Frequency changes when a wave crosses a boundary.” 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.” 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.” Why wrongA radio wave is transverse, electromagnetic, travels at \(3.0 \times 10^{8}\) m/s 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.” 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.” 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 an oscilloscope trace tells you the loudness?
- 21. “Sound travels fastest in gases.” 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.” 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.
Questions students ask about Sound
What is the difference between a transverse wave and a longitudinal wave?
In a transverse wave the particles vibrate at right angles to the direction of energy transfer, as in electromagnetic radiation, water-surface waves and seismic S-waves. In a longitudinal wave the particles vibrate parallel to the direction of energy transfer, producing compressions and rarefactions, as in sound waves and seismic P-waves. Both types transfer energy without transferring matter, and both obey \(v = f\lambda\).
Why can sound not travel through a vacuum?
Sound is a longitudinal mechanical wave: it travels only because the particles of a medium collide with and disturb their neighbours, passing the compressions and rarefactions along. A vacuum contains no particles to carry that disturbance, so sound cannot cross it. Electromagnetic waves are different — they are oscillating electric and magnetic fields and need no medium, which is why astronauts on a spacewalk can see each other but must use radios to talk.