In Fig. 17 - 45, a sound of wavelength 0.750 m is emitted isotropically by point
ID: 1991468 • Letter: I
Question
In Fig. 17 - 45, a sound of wavelength 0.750 m is emitted isotropically by point source S. Sound ray 1 extends directly to detector D, at distance L = 11.3 m. Sound ray 2 extends to D via a reflection (effectively a bouncing) of the sound at a flat surface. The reflection occurs on a perpendicular bisector to the SD line, at distance d from the line. Assume that the reflection shifts the sound wave by 0.500lambda. For what least value of d (other than zero) do the direct sound and the reflected sound arrive at D (a) exactly out of phase and (b) exactly in phase?Explanation / Answer
(11.32 + 4d2) - 11.3 = 0.75 m
127.69 + 4d2 = 145.2025
4d2 = 17.5125
d2 = 4.378
d = 2.092 m
(11.32 + 4d2) - 11.3 = 0.75/2 m
127.69 + 4d2 = 136.3056
4d2 = 8.616
d2 = 2.154
d = 1.468 m