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AS Daily A Level Physics question

2025-09-06 OCR A Waves & Optics (M4) 4.4.1(d)

Which statement must be true when a loudspeaker in room‑temperature air is driven at double its original frequency? Take the speed of sound in air as about 340 m/s. Initially the tone is 170 Hz.

  1. A The wavelength doubles to about 4 m because higher frequency makes the wave travel faster.
  2. B The wavelength halves from about 2.0 m to about 1.0 m because the speed in the same air is unchanged. (correct)
  3. C Both wavelength and speed double, so the wavelength stays about 2.0 m.
  4. D The wavelength decreases slightly (to around 1.6–1.8 m) because air cannot respond instantly at higher frequency.

Answer

The correct answer is B.

Correct: B — The wavelength halves from about 2.0 m to about 1.0 m because the speed in the same air is unchanged. A is wrong because it assumes the wave travels faster at higher frequency; in a given medium the speed is set mainly by the medium (e.g. temperature), not by frequency. B is correct because doubling frequency at essentially constant speed halves the spacing: about 340/170 ≈ 2.0 m becomes 340/340 ≈ 1.0 m. C is wrong because the speed of sound in the same air does not double when you change the driving frequency. D is wrong because there isn’t a significant ‘lag’ effect; within normal conditions the sound speed is effectively constant with frequency, so the change is not slight but close to a factor of one-half.

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