Two speakers are driven by the same oscillator whose frequency is f. They are located a distance d from each other on a vertical pole. A man walks straight toward the lower speaker in a direction perpendicular to the pole, as shown in figure.
(a) How many times will he hear a minimum in sound intensity?
(b) How far is he from the pole at these moments? Let v represent the speed of sound, and assume that the ground does not reflect sound.