# Explain why two loudspeakers connected to the same signal generator and placed a small distance apart produce regions of loudness and silence when a listener walks slowly along a line parallel to and several metres from the loudspeakers.

> IB DP Physics Higher Level (2023 syllabus) — C.2 Wave model · Explain · 4 marks

> Two loudspeakers are connected to the same signal generator so that they emit sound waves of identical frequency and amplitude in phase with each other.

## Mark scheme (4 marks)

1. The two loudspeakers act as coherent sources, emitting waves of the same frequency and with a constant (zero) phase difference.
2. At different positions along the line, the path difference from each speaker to the listener's ear varies.
3. Where the path difference is a whole number of wavelengths (nλ), the waves arrive in phase and interfere constructively, producing a region of maximum loudness.
4. Where the path difference is an odd number of half-wavelengths ((n + ½)λ), the waves arrive in antiphase and interfere destructively, producing a region of silence (or minimum loudness).

## Related

- [Revision notes for IB DP Physics Higher Level (2023 syllabus)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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