Two Loudspeakers S₁ and S₂, 3

Multiple Choice

Two loudspeakers S₁ and S₂, 3.0 m apart, emit the same single-frequency tone in phase at the speakers. A listener L directly in front of speaker S₁ notices that the intensity is a minimum when she is 4.0 m from that speaker (see figure) . What is the lowest frequency of the emitted tone? The speed of sound in air is 340 m/s.

A) 85 Hz
B) 0.17 kHz
C) 0.26 kHz
D) 0.34 kHz
E) 0.51 kHz