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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