PHYSICS · Stage 9 · Chapter 13: Sound Waves
Physics 13.4 Adding up and cancelling out — Constructive and Destructive Interference
5 min read 6 parts Lesson 4 of 5
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
Section 1 of 6Superposition and interference
- Interference = what happens when waves interact: they can add together or cancel out.
- Superposition
- When two waves meet in the same place, their effects add up.
- Constructive interference
- The waves add up to make a bigger wave.
- Destructive interference
- The waves cancel out to make a smaller wave or no wave.
Section 2 of 6Comparing constructive and destructive interference
| Feature | Constructive | Destructive |
|---|---|---|
| Waves are | In step | Out of step |
| Peaks | All in the same place | Peaks of one wave meet troughs of the other |
| Total sound | Louder | Quieter or silent |
- Complete cancelling
- Waves must have exactly the same amplitude and frequency to cancel out completely; otherwise the sound is just quieter.
Section 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.
Section 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.
Section 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.
Section 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.
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