Explain why two loudspeakers emitting sound of the same frequency and amplitude, connected to the same signal generator, can produce regions of silence at certain positions in front of them.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Two loudspeakers are placed side by side, both connected to the same signal generator so that they emit sound waves of identical frequency, amplitude, and phase.
Model answer (4 marks)
The two loudspeakers are coherent sources, so the waves they emit have a fixed phase relationship. At a point in front of the speakers the path difference between the two waves can be a half‑integer multiple of the wavelength (λ/2, 3λ/2, …). In that case the waves arrive out of phase. When the crests of one wave coincide with the troughs of the other, the two waves interfere destructively and the resultant amplitude is zero, producing a region of silence.
Examiner tips
- State that the speakers are coherent sources. Explain the condition for destructive interference (half‑integer wavelength path difference). Show that the resultant amplitude is zero. Use correct terminology: ‘coherent’, ‘path difference’, ‘destructive interference’.
Common mistakes
- Confusing constructive with destructive interference. Forgetting to mention the fixed phase relationship. Using vague terms like ‘cancel out’ instead of ‘interfere destructively’.
Mark scheme (4 marks)
- The two loudspeakers act as two coherent sources, emitting waves with a constant phase relationship.
- At certain positions the path difference between the two waves is a half-integer number of wavelengths (e.g. λ/2, 3λ/2, …), causing the waves to arrive out of phase.
- The two waves superpose / interfere destructively at those positions.
- The crests of one wave coincide with the troughs of the other, so the resultant amplitude is zero and no sound is heard.
Key terms in this question
Related
- All IB DP Physics Standard Level (2023 syllabus) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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