(II) Two loudspeakers S1 and S2 are at a distance d from each other. Point Q is located at y = 2.3 m above loudspeaker S2 while point P is located at x = 4.2 m in front of loudspeaker S1. The two loudspeakers emit a sound of frequency f = 99 Hz in phase. It is assumed that the intensity does not decrease with distance from the loudspeakers, and that the modulus of the sound speed is 340 m/s. What is the minimum value of d for which the sound intensity is zero at each of the following points: (a) P; (b) Q?