Two loudspeakers, $4.0 \mathrm{m}$ apart and facing each other, play identical sounds of the same frequency. You stand halfway between them, where there is a maximum of sound intensity. Moving from this point toward one of the speakers, you encounter a minimum of sound intensity when you have moved $0.25 \mathrm{m}$.
a. What is the frequency of the sound?
b. If the frequency is then increased while you remain $0.25 \mathrm{m}$ from the center, what is the first frequency for which that location will be a maximum of sound intensity?