Snoopy, a dog with excellent hearing, walks slowly along a straight path which we may call the X-axis. Behind him on this axis is a small speaker, which we may choose as the origin. A second identical speaker is located 4.5 m from the first one measured along a line perpendicular to the X-axis. Let's call that line the Y-axis, just to be original. A single tape recorder is used to drive both speaker in phase with an identical audio signal. The record is of a horn blown by Charlie Brown, Snoopy's owner, at a constant frequency and sound level. Snoopy notices that as he moves slowly out the X-axism the sound he hears is very loud at some places, and nearly zero at others. He records one minimum level at X = 5.5m and the next minimum exactly 2 m further out. From these observations, and his knowledge that sound waves travel through the air with a speed of 340 m/s, the cunning canine correctly calculates the frequency of his master's call. (Answer in Hz (cycles per second.))
(a) 170
(b) 340
(c) 510
(d) 940
(e) 1900
