A2 Daily A Level Physics question
Two identical loudspeakers S1 and S2 are driven by the same signal generator so they emit in phase. At a point P in front of them, the distances to S1 and S2 differ by 0.50 m. The wavelength of the sound is 0.20 m. Which statement must be true at P?
Answer
The correct answer is A.
Correct: A — The sound intensity at P is a minimum because the path difference is an odd multiple of half a wavelength. A 0.50 m path difference with λ = 0.20 m is 2.5λ = (5/2)λ, so the waves arrive in antiphase and cancel. B is wrong because 2.5λ is not a whole number of wavelengths; it gives antiphase, not in-phase addition. C is wrong because starting in phase at the sources does not guarantee arriving in phase; the path difference sets the phase difference at P. D is wrong because with equal amplitudes, destructive interference at odd half-wavelength path differences can produce a deep minimum, not the same intensity as one source.