Figure 1 and Figure 2 show a version of Quincke’s tube, which is used to demonstrate interference of sound waves. A loudspeaker at X produces sound waves of one frequency. The sound waves enter the tube
and the sound energy is divided equally before travelling along the fixed and movable tubes. The
two waves superpose and are detected by a microphone at Y.
(a) The movable tube is adjusted so that d1 = d2 and the waves travel the same distance from
X to Y, as shown in Figure 1. As the movable tube is slowly pulled out as shown in Figure
2, the sound detected at Y gets quieter and then louder.
Explain the variation in the loudness of the sound at Y as the movable tube is slowly pulled out.
Initially the path difference is zero/the two waves are in phase when they meet/the (resultant) displacement is a maximum.
As the movable tube is pulled out, the path difference increases and the two waves are no longer in phase, so the displacement and loudness decrease.When the path difference is one half wavelength, the two are in antiphase and sound is at its quietest.As the path difference continues to increase, the two waves become more in phase and the sound gets louder again.