A horizontal glass tube, closed at one end, has a layer of dust laid inside it on its lower side. Sound is emitted from a loudspeaker that is placed near the open end of the tube.
The frequency of the sound is varied and, at one frequency, a stationary wave is formed inside the tube so that the dust forms small heaps.
The distance between four heaps of dust is 30 cm.
[Figure showing glass tube with loudspeaker and dust heaps]
The speed of sound in the tube is 330 ms⁻¹.
What is the frequency of the sound emitted by the loudspeaker?

Q: What topic does this question test?

This question tests Waves in A-Level Physics (syllabus code 9702). It is worth 1 mark.

Q: Which exam paper is this question from?

This question appeared in the Cambridge A-Level Physics Oct/Nov 2012 examination, Paper 1 Variant 2 (9702_w12_qp_12.pdf).

Question

A horizontal glass tube, closed at one end, has a layer of dust laid inside it on its lower side. Sound is emitted from a loudspeaker that is placed near the open end of the tube.
The frequency of the sound is varied and, at one frequency, a stationary wave is formed inside the tube so that the dust forms small heaps.
The distance between four heaps of dust is 30 cm.
[Figure showing glass tube with loudspeaker and dust heaps]
The speed of sound in the tube is 330 ms⁻¹.
What is the frequency of the sound emitted by the loudspeaker?

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The correct answer is A: 1650 Hz.

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