Two loudspeakers emit identical sound waves along the $x$ -axis. The sound at a point on the axis has maximum intensity when the speakers are $20 \mathrm{cm}$ apart. The sound intensity decreases as the distance between the speakers is increased, reaching zero at a separation of $30 \mathrm{cm} .$
a. What is the wavelength of the sound?
b. If the distance between the speakers continues to increase, at what separation will the sound intensity again be a maximum?