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How sound is produced

Key idea: Sound starts with a vibrating source and needs a material medium. In air it travels as longitudinal compressions and rarefactions, while particles vibrate about their own positions.

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Pluck a guitar string and you can see it blur as it moves to and fro, and you hear a note. Every sound starts with something vibrating, and the vibration reaches your ear by passing from particle to particle through the air. Sound is a longitudinal wave, so it travels as a pattern of squashed and stretched regions of air.

Sound comes from vibrations and needs a medium

Every sound is made by something vibrating:

  • a guitar string moves rapidly to and fro after it is plucked;
  • a drum skin moves in and out after it is struck;
  • the prongs of a tuning fork vibrate; touch them to water and they splash;
  • a loudspeaker cone moves in and out;
  • your vocal cords vibrate when you speak. Touch your throat while you hum and you can feel them.

Stopping the vibration, for example by gripping a ringing bell, stops the source producing new sound waves. Waves already travelling through the medium can still reach a listener afterwards.

The vibrating object pushes and pulls on the particles next to it, and they push and pull on their neighbours. So sound needs a medium, a material made of particles, to travel through. It travels through gases such as air, liquids such as water, and solids such as wood and steel.

Remove the air while the bell keeps vibratingA glass bell jar seals onto a base. Two electrical leads pass through a sealed stopper to the vibrating electric bell inside. An external supply powers it. A pipe through the base leads to a vacuum pump. Removing air weakens the air transmission path, while a faint signal may still travel through remaining air or the solid supports.Watch the hammer; listen as the pump removes air.ElectricalsupplySealed wire feed-throughGlassbell jarHammerGongVibrating electric bellVacuum pumpAir is removed through the base.The source keeps vibrating while the air path transmits less sound.
Remove the air while the bell keeps vibratingA glass bell jar seals onto a base. Two electrical leads pass through a sealed stopper to the vibrating electric bell inside. An external supply powers it. A pipe through the base leads to a vacuum pump. Removing air weakens the air transmission path, while a faint signal may still travel through remaining air or the solid supports.Watch the hammer; listen as the pump removes air.ElectricalsupplySealed wire feed-throughGlassbell jarHammerGongVibrating electric bellVacuum pumpAir is removed through the base.The source keeps vibrating while the air path transmits less sound.
Schematic apparatus, not to scale. Observe both the visible hammer motion and the sound heard as air is removed and then let back in.

Evidence: the bell jar. An electric bell rings inside a glass jar. As a pump removes the air, the ringing gets fainter until it can hardly be heard, although you can still see the hammer striking the gong. Let the air back in and the ringing returns.

  • The bell is still vibrating, so the source is still working.
  • With most of the air removed, there are too few particles to pass the vibration on. A faint sound may still travel through the remaining air or through the solid supports.
  • You can still see the bell because light does not need a medium. Sound cannot travel through a vacuum.

This is why astronauts on a spacewalk talk to each other by radio: radio waves travel through space, sound does not.

Check your understanding 1 Has the bell stopped?

The bell jar is almost completely emptied of air. A learner says, “The bell has stopped ringing.” What is wrong with this statement?

Show answer

The hammer still strikes the gong, so the bell still vibrates. What has changed is the medium: with almost no air in the jar, very little sound is passed on. The vibration continues even though you can hardly hear it.

Sound in air is longitudinal: compressions and rarefactions

Watch a loudspeaker cone. When it moves out, it pushes the air in front of it together, making a region of higher pressure and density: a compression. When it moves back, it leaves the air in front of it more spread out, making a region of lower pressure and density: a rarefaction. A rarefaction is not an empty gap or a vacuum. The cone keeps vibrating, so it sends out a series of compressions and rarefactions, which travel away from it.

The air particles vibrate backwards and forwards along the direction the sound travels, about fixed average positions. They do not travel from the speaker to your ear; only the pattern of compressions and rarefactions does. Because the vibrations are parallel to the direction of travel, sound in air is a longitudinal wave.

Longitudinal sound from a vibrating loudspeakerA loudspeaker cone faces rows of air particles. Three crowded compression regions alternate with spread-out rarefactions. A marked particle has a local horizontal vibration arrow, parallel to the rightward arrow for wave propagation. One wavelength is marked between neighbouring compression centres. This is a schematic organised wave displacement; random thermal motion is not shown.Sound in air: particles move locally; the pattern travels.Vibratingspeaker coneSound travels rightMarked particle vibrates to and froCCCRROne wavelength λC: compressionR: rarefactionCompressions have higher pressure and density; rarefactions have lower values.
Longitudinal sound from a vibrating loudspeakerA loudspeaker cone faces rows of air particles. Three crowded compression regions alternate with spread-out rarefactions. A marked particle has a local horizontal vibration arrow, parallel to the rightward arrow for wave propagation. One wavelength is marked between neighbouring compression centres. This is a schematic organised wave displacement; random thermal motion is not shown.Sound in air: particles move locally; the pattern travels.Vibratingspeaker coneSound travels rightMarked particle vibrates to and froCCCRROne wavelength λC: compressionR: rarefactionCompressions have higher pressure and density; rarefactions have lower values.
The pattern and energy travel right, while particles oscillate locally. The molecular row is a schematic, not a literal photograph or a transverse sound wave. Random thermal motion is omitted.

The wavelength of a sound is the distance from the middle of one compression to the middle of the next, or from one rarefaction to the next. A compression and the neighbouring rarefaction are only half a wavelength apart.

Check your understanding 2 A curve on a graph

A recording plots air pressure vertically against time horizontally as a smooth up-and-down curve. A learner concludes that air particles move up and down. What is the error? For sound travelling horizontally to the right, sketch some air particles and their vibration arrows on paper, then compare your arrows with the answer.

Show answer

The curve is a pressure–time graph: its vertical coordinate is pressure, not particle height. Compare your particle sketch: show a separate propagation arrow pointing right and two-headed horizontal vibration arrows at the particles, parallel to that direction. The particles oscillate about their positions; they do not move steadily to the listener. In air, the particles vibrate along the direction the sound travels, so sound is longitudinal. Read the axes before you interpret a trace.

Each vibration of the source sends out one compression, so the frequency of the sound is the frequency of the source, and the wave equation links it to the spacing of the compressions.

Worked example 1

Sound speed from neighbouring compressions

Problem

A loudspeaker vibrates at 170 Hz. The centres of neighbouring compressions are 2.0 m apart. Find the speed of sound in the air.

Show full solution

  1. Identify the wavelength and frequency

    Method

    Use the spacing of neighbouring compression centres for wavelength and the source vibration rate for frequency.

    Reason

    The pattern repeats from one compression to the next; each source vibration produces one cycle.

    Working

    Answer

    λ = 2.0 m and f = 170 Hz.

  2. Calculate the sound speed

    Working

    Answer

    v = fλ = 170 Hz × 2.0 m = 340 m/s.

Check your understanding 3 Compression spacing

A source makes a 425 Hz sound in air, where wave speed is 340 m/s. Find the distance between neighbouring compression centres.

Unit: m
Show answer

Neighbouring compressions are one wavelength apart: λ = v/f = 340/425 = 0.80 m.

Check your understanding 4 Identify what the measured interval represents

A source vibrates at 200 Hz. A compression centre is 0.85 m from the neighbouring rarefaction centre. Find the wavelength and wave speed. Do air particles travel with this pattern?

Unit: m
Show the steps Optional working
Unit: m
Unit: m/s
Particle motion
Show answer

A compression to the neighbouring rarefaction is half a wavelength, so wavelength = 2 × 0.85 = 1.70 m. Wave speed = 200 × 1.70 = 340 m/s. Each air particle vibrates locally backwards and forwards; the compression and rarefaction pattern travels through the air. Neither 0.85 m nor 1.70 m is the distance an air particle travels along with the wave. The given spacing describes the pattern, not particle displacement.

How loud and how high a sound is are read from its trace in Loudness and pitch, and reflected sound is used to measure distances in Echoes and sound measurements.

Common mistakes

  • Saying sound can travel through a vacuum. It needs particles to pass the vibration on.
  • Saying air travels from the speaker to your ear. Each particle vibrates about a fixed position; only the pattern of compressions travels.
  • Taking an up-and-down sound graph as proof that sound is transverse. Read its axes: pressure on the vertical axis is not a height.
  • Calling the distance from a compression to the next rarefaction a wavelength. It is half a wavelength.
  • Calling a rarefaction a vacuum. It is a region of slightly lower pressure.

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