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Unit 6: Sound and space

Interference Of Sound — HIA International - Grade 8 Revision Notes & Topical Questions

5 min read 8 parts Lesson 8 of 14 2 quizzes

📚 HIA International - Grade 8 Hub / Topical Revision
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Lesson summary

Interference of sound — HIA INTERNATIONAL notes: Explain how waves reinforce or cancel and apply this to loudspeakers and noise-cancelling headphones.Interference = the effect produced when waves meet each other.Same type — only waves of the same type interfere; sound waves can i…

  • Easiest to detect
  • Two effects
  • Water analogy
  • Requirement
  • Rarely complete
  • Quiet places

Part 1 of 6Interference

Interference
Interference
Interference the effect produced when waves meet each other.
Same type
  • Same type only waves of the same type interfere; sound waves can interfere with sound but not water waves.
Easiest to detect
  • Easiest to detect when the waves have the same frequency and amplitude.
Two effects
  • Two effects the waves can reinforce or cancel.
Water analogy
  • Water analogy two nails dipped into a tray make a pattern of interference.

Part 2 of 6Reinforcing

Reinforce
Reinforce to make stronger.
Condition
  • Condition waves meet with peaks together and troughs together.
Result
  • Result the amplitudes are added; the frequency does not change.
Sound
  • Sound the amplitude increases and the sound is louder.
Loudspeakers
  • Loudspeakers two speakers either side of a stage create louder places where the waves reinforce.

Part 3 of 6Cancelling

Cancel
Cancel adding together to make zero.
Condition
  • Condition a peak of one wave meets a trough of equal amplitude.
Result
  • Result the amplitude is zero and there is no sound.
Requirement
  • Requirement equal frequency and equal amplitude.
Rarely complete
  • Rarely complete reflections add many other waves, so complete silence is unlikely.
Quiet places
  • Quiet places there are places in front of loudspeakers where the sound is quieter.

Part 4 of 6Noise-cancelling headphones

1
A small microphone picks up the sound from the surroundings.
2
The headphones analyse the wave.
3
They create a wave with the same amplitude and frequency but out of phase.
4
This wave cancels the sound from the surroundings.

Part 5 of 6Examples with numbers

SituationResult
Two waves of amplitude 2 in stepCombined amplitude 4
Two waves of amplitude 2 out of stepCombined amplitude 0
Equal 600 Hz waves, one 0.25 mmThe other must be 0.25 mm and out of step to cancel

Part 6 of 6Reflected waves reinforcing

Tube over water
  • Tube over water a tuning fork held over a pipe is loudest when the length of pipe above water matches the sound, as the reflected wave reinforces.
Trend
  • Trend higher-frequency forks need shorter pipes, which explains why shorter wind instruments give higher notes.

Key terms

TermMeaning
InterferenceThe effect when waves meet each other.
ReinforceTo make stronger, as when peaks meet peaks.
CancelTo add together to make zero.
Out of phasePeaks of one wave meet troughs of another.
AmplitudeMaximum displacement of particles in a wave.
FrequencyThe number of vibrations per second.

Avoid these mistakes

1
Waves cancel only by being quiet — waves cancel when peak meets trough of equal size.
2
Sound waves can interfere with water waves — only waves of the same type interfere.
3
Reinforcing changes the frequency — the frequency stays the same; the amplitude increases.
4
Sound is always completely silent where waves cancel — reflections usually stop total silence.

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