How far should speaker B behind Speaker A for Alice to hear a maximum loud tone? (Do not assume the two speakers are in phase)
 

Two speakers are in line with each other. Alice stands 4.8 m in front of Speaker A. The speakers are connected to the same 139 Hz sound source, and Alice hears nothing when Speaker B is 0.7 m behind Speaker A. Another person slowly moves Speaker B further away from Speaker A and Alice until Alice hears a maximum loud tone with intensity sound level of 90.1 dB. The temperature of the air is 50F. [Answer in 3 significant figures] 0.7 m 4.8 m