Intelligent assessment and revision tools for OCR A Physics

Two identical loudspeakers are driven by the same signal (so they are coherent and in phase). At a microphone P, the path difference from the two speakers is exactly one-third of the wavelength of the tone. Which statement about the sound intensity at P must be true, compared with using only one speaker?

• A It is the same as with one speaker alone, because the resultant amplitude equals that from a single source for a 120° phase difference.
• B It is about twice that with one speaker, because the equal intensities simply add.
• C It is four times that with one speaker, because the amplitudes add to double.
• D It is about half that with one speaker, because partial cancellation significantly reduces the amplitude.