A2 Daily A Level Physics question
Two identical loudspeakers S1 and S2 are driven in phase at a single frequency on a lab bench. At a point P in front of them the sound level is at a pronounced minimum. Reflections are negligible and air conditions are uniform. Which statement about the path difference δ = |PS2 − PS1| at P must be true?
Answer
The correct answer is D.
Correct: D — δ is an odd multiple of (λ/2). For in-phase sources of similar amplitude, a path difference of (m + 1/2)λ gives a 180° phase difference, leading to destructive interference and a minimum. A is wrong because a whole-number multiple of λ makes the waves arrive in phase, giving a maximum. B is wrong because not any non-zero fraction gives a minimum; only specific half-odd multiples cancel strongly, most others give partial loudness. C is wrong because an even multiple of (λ/2) equals an integer multiple of λ, again in phase and therefore a maximum. D is correct because odd half-wavelength path differences produce out-of-phase arrival and cancellation at P.