SIN Bin

1993, problem 8

A technical spider laid out her large plane web on a Cartesian co-ordinate system, as shown below. She located two identical small speakers, one at (X=-3m,Y=8m) and the other at (X=3m, Y=-8m).
She drove both speakers with the same electrical signal, so they sent out identical sound waves of frequency 85Hz, attractive to flies.
A bold fly walked slowly along the X axis from X=-100m to X=100m, listening to the resulting sound.
How many local maxima did the fly detect? Use 340m/s for the speed of air in sound if you need it.

A cartesian coordinate system showing the positions of the speakers, the plane of the spider web (XZ), and the fly walking on the web

(a) 1

(b) 2

(c) 3

(d) 4

(e) 5