Relative phase and path distance determine whether correlated waves cancel or reinforce in space.

Evidence

We saw earlier that when sound waves come from correlated sources, their pressure and associated velocity components simply add. This means that the pressure amplitude could vary between zero and the sum of the pressure amplitudes of the waves being added together, as shown in Example 1.9. Whether the waves add together constructively or destructively depends on their relative phases, and this will depend on the distance each one has had to travel. Because waves vary in space over their wavelength, the phase will also spatially vary. This means that the constructive or destructive addition will also vary in space. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 359

Categorization

• DEFINE: Clarifies the mechanism by which phase differences from path-length differences translate into spatial variation of loudness.