The diagram depicts two identical loudspeakers, both driven in phase by a shared audio-frequency source. As a student walks along line XY, he observes changes in the sound loudness. The sound is heard in alternating loud and quiet regions, as shown on the diagram. How may the separation between the loud regions be reduced?
- Adecreasing the distance $a$ between the speakers
- Bincreasing distance $d$
- Cincreasing the frequency of the audio-frequency source
- Dincreasing the power output from the audio-frequency source