Two identical loudspeakers are supplied with identical sinusoidal signals. They are placed close to one another in a room and it is noted that there are regions of increased and decreased sound intensity in the vicinity of the speakers. Which of the following actions will not change this pattern?
(a) Moving one of the speakers.
(b) Changing the amplitude of the signal.
(c) Changing the frequency of the signal.
(d) Replacing the air in the room with Helium gas.