Two speakers emit the same pure tone (sound of a single frequency) of wavelength 4.0 m and are in phase. The speakers are 3.0 m apart. Beginning at the midpoint between the speakers, how far must an observer move toward one of the speakers before encountering a quiet spot (completely destructive interference)?
a) 0.4 m
b) 0.6 m
c) 0.8 m
d) 1.0 m
e) 1.2 m