Moe, Larry, and Curly stand in a line with a spacing of d = 1.1 m. Larry is 3 m in front of a pair of stereo speakers 0.8 m apart. The speakers produce a single-frequency tone, vibrating in phase with each other.

What are the two lowest frequencies that allow Larry to hear a loud tone while Moe and Curly hear very little?

I thought one of them might be 2.2 (double d=1.1), but I don't know how to find the other. Can someone help? The answer in in terms of kHz.