Varying sound level

In the above diagram, two speakers making the same sound of frequency $500 \text{ Hz}$ are facing each other. Calvin, standing on a straight line connecting the two speakers, walks from speaker $A$ to speaker $B$ measuring sound intensity with a sound level meter. (It looks like a cellphone, but it is actually a brand new sound level meter.) After a lot of trials he finally finds that the sound level goes up and down with a pattern like a sine curve. What is the distance between the position where the sound level is the largest and the subsequent position where the sound level is the smallest?

The speed of sound in the air is $340 \text{ m/s}.$