Figure shows an air-filled, acoustic interferometer, used to demonstrate the interference of sound waves. Sound source S is an oscillating diaphragm; D is a sound detector, such as the ear or a microphone. Path SBD can be varied in length, but path SAD is fixed. At D, the sound wave coming along path SBD interferes with that coming along path SAD. In one demonstration, the sound intensity at D has a minimum value of 100 units at one position of the movable arm and continuously climbs to a maximum value of 900 units when that arm is shifted by 1.65cm. Find

(a) The frequency of the sound emitted by the source and

(b) The ratio of the amplitude at D of the SAD wave to that of the SBD wave.

(c) How can it happen that these waves have different amplitudes, considering that they originate at the same source?