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Production & Propagation of Sound

Key idea: O Level sound waves: vibrating sources, compressions and rarefactions, why sound needs a medium, and calculations using v = fλ.

  • SEC G3 Physics 2027
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Learning objectives

  • Describe wave generation by vibrating sources, ropes and springs
  • Describe ripple-tank waves using wavefronts
  • Explain that waves transfer energy
  • Explain that wave energy transfer does not transfer matter
  • Use amplitude, frequency and wavelength to describe wave motion
  • Define and use wave speed and period and interpret wave graphs
  • Recall and apply wave speed = frequency × wavelength
  • Compare transverse and longitudinal waves and give examples
  • Explain sound production by vibration and the need for a medium
  • Describe sound using compressions and rarefactions
  • Relate sound loudness to amplitude and pitch to frequency
  • Explain reflected-sound echoes and use them to measure distance
  • Explain ultrasound use in sonar and soft-tissue scanning
  • Use the normal, angle of incidence and angle of reflection
  • Apply the law of reflection in constructions, measurements and calculations
  • Use the normal, angle of incidence and angle of refraction
  • Apply sin i divided by sin r as a constant for a fixed pair of media
  • Define refractive index as vacuum light speed divided by medium light speed
  • Explain the critical angle
  • Explain the conditions for total internal reflection
  • Apply total internal reflection to optical fibres and state advantages
  • Describe how a thin converging lens acts on a light beam
  • Define the focal length of a converging lens
  • Construct real and virtual image ray diagrams for a thin converging lens
  • Describe lens images as real or virtual, magnified or diminished, and upright or inverted

1. Definition

A. Sound wave

Sound is a mechanical wave produced by a vibrating source. It transfers energy through a material medium as a longitudinal wave (compressions and rarefactions).

2. Key Ideas

  • Sound is produced by vibrating sources (e.g. tuning fork, loudspeaker, vocal cords).
  • Sound needs a material medium (solid / liquid / gas) to travel.
  • In air, sound travels as a longitudinal wave:
    • compression: higher pressure region
    • rarefaction: lower pressure region
  • Particles of the medium oscillate; they do not move along with the wave.
  • Sound waves obey the wave equation: v = fλ

3. Detailed Explanations

A. Examples of vibrating sources

  • drum skin, guitar string
  • loudspeaker cone
  • tuning fork

B. Propagation of sound in air (compression and rarefaction)

When a source vibrates, it pushes and pulls on the surrounding air:

  • when it moves outward, it pushes air molecules closer together → compression
  • when it moves inward, it leaves a lower pressure region → rarefaction

These alternating compressions and rarefactions travel outwards. The air molecules themselves just vibrate back and forth about their equilibrium positions.

A vibrating tuning fork produces alternating dense compressions and sparse rarefactions that travel outward. Air particles oscillate back and forth parallel to the wave direction; the distance between compression centres is one wavelength.
The disturbance and energy travel outward, but each air particle only oscillates about its equilibrium position. Particle oscillation is parallel to wave travel, so sound in air is longitudinal.

C. Why does sound need a medium?

Sound transfers energy by particle-to-particle interactions in the medium. In a vacuum there are no particles to vibrate, so sound cannot be transmitted.

D. Sound wave quantities (use v = fλ)

Sound waves use the same wave equation as other waves:

v = fλ

where:

  • v = speed of sound (m s⁻¹)
  • f = frequency (Hz)
  • λ = wavelength (m)

For a sound wave, the distance between successive compressions (or successive rarefactions) is one wavelength, λ.

Link

Loudness and pitch are covered in Loudness & Pitch.

E. Speed of sound in different media (O Level idea)

Sound generally travels:

  • fastest in solids
  • slower in liquids
  • slowest in gases

Propagation speed depends on the medium’s elastic stiffness as well as its density. Solids are generally much stiffer than liquids and gases, so their restoring forces transmit a disturbance rapidly. Particle spacing or density alone is not a sufficient explanation.

4. Common Mistakes

  • Saying sound can travel in a vacuum (it cannot).
  • Saying air molecules “move with the sound wave” (they oscillate about fixed positions).
  • Mixing up frequency and wavelength (higher f usually means smaller λ for the same medium).
  • Claiming that greater density by itself always means greater sound speed.

5. Exam Tips

  • Use keywords: vibrating source, medium, longitudinal, compression, rarefaction.
  • If asked “why no sound in space?”, state: “no particles → no compressions/rarefactions”.
  • For calculations:
    • write v = fλ first,
    • convert units (cm → m),
    • final answer with unit.

6. Worked Examples

Modelled example 1

Explaining how sound is produced

Core

Problem

Explain how a vibrating tuning fork produces and transmits a sound wave through air.
Study the worked solution

  1. Create pressure variations

    Method

    Use the fork’s back-and-forth motion to push and pull nearby air.

    Reason

    Outward motion crowds particles into compressions; inward motion leaves rarefactions.

    Working

    Vibrating source → alternating high- and low-pressure regions.

  2. Transmit the disturbance

    Method

    Move compressions and rarefactions outward while air particles oscillate parallel to wave travel.

    Reason

    Particle-to-particle interactions transfer energy without net transport of the air.

    Working

    Longitudinal wave: oscillation direction parallel to propagation.

Guided practice 2

Wavelength of a sound

About 4 min

Problem

Sound travels at 340 m s⁻¹ and a note has frequency 170 Hz. Find its wavelength.

Rearrange the wave equation

Unit: m

Hints

Hint 1: start from the common equation
v = fλ.
Hint 2: make wavelength the subject
Divide speed by frequency.
View solution step by step

  1. Calculate wavelength

    Method

    Divide sound speed by frequency.

    Reason

    Each cycle occupies one wavelength along the travelling disturbance.

    Working

    λ = 340/170 = 2.0 m

Common misconception 3

Frequency from wavelength

Find and correct the mistake

Learner response

A sound wave has wavelength 0.85 m and speed 340 m s⁻¹. A student calculates frequency by multiplying vλ. Locate the rearrangement error.

Use dimensions to test the rearrangement

Unit: Hz

View solution step by step

  1. Make frequency the subject

    Method

    Divide wave speed by wavelength.

    Reason

    Multiplication would produce m² s⁻¹, not cycles per second.

    Working

    f = v/λ

  2. Calculate frequency

    Method

    Substitute the consistent SI values.

    Reason

    Metres cancel, leaving inverse seconds or hertz.

    Working

    f = 340/0.85 = 400 Hz

Examiner practice 4

Wavelength from frequency

3 marks

Examination question

Sound travels at 340 m s⁻¹. Find the wavelength of a 680 Hz note. [3 marks]

Show equation, rearrangement and result

View solution step by step

  1. Calculate wavelength

    3 marks

    Method

    Use v = fλ, rearrange and substitute.

    Reason

    Wavelength equals distance travelled per cycle.

    Working

    λ = v/f = 340/680 = 0.50 m

Challenge 5

Frequency from wave speed and wavelength

Minimal support

Changed-medium transfer

A sound wave travels at 330 m s⁻¹ with wavelength 0.66 m. Find the source frequency.

Use the medium's stated speed

Hints

Hint 1: do not assume 340
Use the speed stated for this sound wave.
Hint 2: divide by wavelength
f = v/λ.
View solution step by step

  1. Calculate source frequency

    Method

    Divide the stated wave speed by wavelength.

    Reason

    The source frequency is shared by the propagating wave.

    Working

    f = 330/0.66 = 500 Hz

7. Mind Stretchers

Mind stretcher 1: Hearing through a tableExtension

Why can you sometimes hear a faint sound more clearly by putting your ear against a table or a wall?

Show Answer

Sound often travels faster and can couple strongly through a solid because its stiff structure provides strong restoring interactions that transmit vibrations rapidly. Particle spacing or density alone is not a sufficient explanation.

Mind stretcher 2: Vacuum jarExtension

A ringing bell is placed inside a jar. Air is pumped out. The bell can still be seen vibrating, but the sound fades. Explain why.

Show Answer

As air is removed, there are fewer particles to transmit compressions and rarefactions, so less sound energy reaches the outside. In near-vacuum there is essentially no medium, so sound cannot travel.

8. Practice and next step

Write a three-link explanation using vibrating source → compressions and rarefactions → particle-to-particle energy transfer. Continue to loudness and pitch.

Continue with the next resource in this course.

Course and syllabus information
Course
SEC G3 Physics
Edition
SEC G3 Physics 2027