Two stereo speakers are separated by

Two stereo speakers are separated by a distance of $2.40 \mathrm{~m}$. A person stands at a distance of $3.20 \mathrm{~m}$ directly in front of one of the speakers as shown in figure. Find the frequencies in the audible range $(20-2000 \mathrm{~Hz})$ for which the listener will hear a minimum sound intensity. Speed of sound in air $=320 \mathrm{~m} \mathrm{~s}^{-1}$.