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School Physics Notes: SOUND 2. Reflection, refraction and diffraction

GCSE level Physics exam revision notes on SOUND

SOUND  Part 2. The reflection, refraction and diffraction of sound waves explained using longitudinal sound wave model diagrams

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[KEY POINTS and learning objectives for this page, after initial notes]

INDEX of physics notes on SOUND

2. The reflection, refraction and diffraction of sound waves and a sound wave model

(a) Reflection of sound waves

reflection refraction of sound waves at boundary gcse physics igcse diagram explain explanationWhen sound waves meet a barrier between two different media they can be reflected just like any other wave (diagram of wavefronts on the right). The angle of incidence will equal the angle of reflection (with respect to the normal at 90o to the boundary). Any solid surface will reflect sound, though soft material will tend to absorb the sound wave energy.

A flat hard smooth surface is the best reflector of sound waves - think of echoes (reflected sounds) that permeate an empty house with no carpets laid down.

A soft rough surface is the best absorber of sound wave energy - the idea is used in recording studios to minimise unwanted sounds and in ear muffs to protect you from ear damage due to very loud sounds.

In reflection there is no change in speed, frequency or wavelength, only the direction of movement of the wave changes.

(b) Refraction of sound waves 

(above right diagram, same diagram as wavefronts in reflection)

When waves, including sound, meet a boundary between two mediums, through which they can pass, there is a change in speed related to the difference in density.

The frequency stays the same, but both the speed and wavelength of the sound wave changes.

If the incident wave fronts are parallel to the boundary surface, there is no change in direction

 → IIIIII│IIIIII→

BUT, if the incident angle is NOT 90o, at the same time as reflection, and the sound waves can penetrate the 2nd medium of different density at a boundary, the wave will change speed, wavelength and direction, but no change in frequency.

Therefore if the sound waves have changed direction, the sound waves are refracted.

The number of waves passing through each medium per second is the same.

Since speed = wavelength x frequency (and frequency is constant),

if speed increases, wavelength must get larger,

if speed decreases, wavelength must get smaller.

Although they are longitudinal sound waves, this is normal wave behaviour just as you see with experiments with transverse light waves or water waves.

(c) Diffraction of sound waves

Diffraction is the effect of waves spreading out when passing through a gap or passing by a barrier. In effect, waves bend round corners into the 'shadow zone'! and it doesn't matter if its sound, light or water waves - they all diffract and bend round corners!

You should appreciate that significant diffraction of sound waves only occurs when the wavelength of the sound wave is of the same order of magnitude as the size of the gap or obstacle.

 

 

A: There is a relatively small diffraction effect when sound waves pass through a wide gap that is much bigger than the wavelength of the sound wave - but the sound waves still bend round the 'corners' into the 'shadow zones'.

B: You get the maximum spreading or diffraction when the waves pass through a gap of similar size to the wavelength of the incident sound waves.

INDEX of physics notes on SOUND

Key points of the physics of sound: The reflection, refraction and diffraction of sound waves

Information sources for Doc Brown's key points: IGCSE-GCSE physics are based on textbooks & syllabus-specifications for students taking the UK AQA, Edexcel, OCR 21st Century Science, OCR Gateway science suite, WJEC, CCEA and CIE GCSE physics 9-1 level science examinations

Here's a concise and exam-focused set of summary revision notes on reflection, refraction, and diffraction of sound waves, explained using the longitudinal wave model, and tailored to GCSE/IGCSE Physics specifications across WJEC, CCEA, CIE, AQA, Edexcel, and OCR.

Sound Waves as Longitudinal Waves

  • Longitudinal wave: Particles vibrate parallel to the direction of wave travel.
  • Key features:
    • Compressions: Regions of high pressure.
    • Rarefactions: Regions of low pressure.
  • Medium required: Sound cannot travel in a vacuum.

Reflection of Sound Waves

  • Definition: Sound waves bounce off surfaces.
  • Law of reflection:
    Angle of incidence = Angle of reflection
  • Examples: Echoes, sonar, ultrasound imaging.
  • Model: Compressions reflect off surfaces, maintaining wave direction symmetry.

Refraction of Sound Waves

  • Definition: Change in direction due to change in speed across media.
  • Occurs when: Sound enters a new medium at an angle.
  • Behaviour:
    • Speeds up in denser materials (unlike light).
    • Bends away from normal if speeding up; towards normal if slowing down.
  • Model: Compressions and rarefactions change spacing due to speed variation.

Diffraction of Sound Waves

  • Definition: Spreading of waves around obstacles or through gaps.
  • More pronounced when:
    • Wavelength is large compared to gap size.
    • Sound is low frequency (long wavelength).
  • Examples: Hearing around corners, sound through doorways.
  • Model: Compressions bend and spread out after passing through gaps.

Common Misconceptions about the reflection, refraction and diffraction of sound waves

  • Sound behaves like light in refraction
    → Correction: Sound speeds up in denser materials, unlike light.
  • Diffraction only occurs with light
    → Correction: Sound diffracts more easily due to longer wavelengths.
  • Reflection only happens on smooth surfaces
    → Correction: Sound reflects off any surface, though rough surfaces scatter it.
  • Sound waves are transverse
    → Correction: Sound is a longitudinal wave.

Exam Tips for reflection, refraction and diffraction of sound waves

  • Use ray diagrams to show reflection and refraction (with wavefronts or particle motion).
  • Describe wave behaviour using compressions and rarefactions.
  • Link wave speed to material density.
  • Explain diffraction using real-world examples (e.g. hearing someone in another room).
  • Know the conditions for echoes (e.g. time delay > 0.1 s).

Typical Board-Specific Highlights about the reflection, refraction and diffraction of sound waves

Focus Areas
Required practicals on wave behaviour, echo calculations.
Emphasis on wavefront diagrams and boundary behaviour.
Detailed treatment of wave interactions and longitudinal models.
Real-world applications and sound wave properties.
In-depth wave behaviour, including diffraction and refraction in solids/liquids.

Keywords, phrases and learning objectives for sound waves

Know that sound waves can be reflected, refracted and diffracted and the behaviour of sound waves can be explained using a longitudinal sound wave model.

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INDEX of physics notes on SOUND

INDEX of all notes on waves, radiation, astronomy etc.

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INDEX of physics notes on SOUND

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