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13.4Chapter 13: Sound Waves

Adding up and cancelling out — Alhussan Physics - Stage 9 Revision Notes & Topical Questions

5 min read 8 parts Lesson 4 of 5 2 quizzes

📚 Alhussan Physics - Stage 9 Hub / Topical Revision
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Lesson summary

Physics 13.4 Adding up and cancelling out — Constructive and Destructive Interference — Alhussan International Academy — Physics (Stages 7–9) notes: Describe what happens when sound waves interact and how interference is used.Interference = what happens when waves interact: they …

  • Superposition
  • Constructive interference
  • Complete cancelling
  • Two loudspeakers
  • Tuning fork
  • Reflected waves

Part 1 of 6Superposition and interference

1Interference
Interference what happens when waves interact: they can add together or cancel out.
2Superposition
  • When two waves meet in the same place, their effects add up.
3Constructive interference
  • The waves add up to make a bigger wave.
4Destructive interference
  • The waves cancel out to make a smaller wave or no wave.

Part 2 of 6Comparing constructive and destructive interference

FeatureConstructiveDestructive
Waves areIn stepOut of step
PeaksAll in the same placePeaks of one wave meet troughs of the other
Total soundLouderQuieter or silent
  • Complete cancelling
    • Waves must have exactly the same amplitude and frequency to cancel out completely;
      • otherwise the sound is just quieter.

Part 3 of 6Path difference

Constructive
  • The difference in position of the two waves is zero or a whole number of wavelengths.
Destructive
  • The waves are out of step by half a wavelength, one and a half wavelengths and so on.
Two loudspeakers
  • If you walk in front of two speakers playing the same note, the sound is loud in some places and quiet in others.

Part 4 of 6Tuning musical instruments

Principle
  • Tuners use the interaction of sound waves to check that the right note is played.
Tuning fork
  • Striking it near a string gives a loud steady note if the string is in tune.
  • If it is not in tune, the waves add up at some times and cancel at others.
  • The tuner hears loud and soft sounds, called beats.
Adjusting
  • The string is adjusted until the loud note no longer changes, which means the frequencies are equal.

Part 5 of 6Noise-cancelling headphones

Purpose
  • They block out noise in noisy places.
Action
  • A microphone detects the background noise.
  • The headphones produce a sound exactly out of step with it.
  • The two waves cancel by destructive interference.

Part 6 of 6Microwave ovens

Reflected waves
  • Microwaves reflected from the walls produce places where waves add up (hot spots) and places where they cancel (cold spots).
Turntable
  • It moves the food through both so it heats evenly.

Key terms

TermMeaning
InterferenceThe adding together or cancelling out of waves that meet.
Constructive interferenceWaves adding to make a bigger wave.
Destructive interferenceWaves cancelling to make a smaller wave or none.
In stepHaving peaks that meet peaks.
Out of stepHaving peaks that meet troughs.
BeatsThe loud-soft changes heard when two similar frequencies combine.

Avoid these mistakes

1
Thinking waves that interfere destructively disappear for ever — they cancel only where they meet.
2
Thinking destructive interference always gives silence — the amplitudes must be equal.
3
Thinking waves in step cancel — they add up.
4
Thinking tuning means making a string louder — it means matching frequencies.

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