At low frequencies, loudspeaker signals add coherently so delay and amplitude trade off.

Evidence

Of course, different microphone layouts may be used to introduce phase shifts. Due to the effect of coherent addition of the two speaker signals at the ears at low frequencies, there is some trading between delay and amplitude. This can be used to achieve a variety of creative effects to the stereo image. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 2212

Categorization

• QUESTION: Why does coherent addition only matter at low frequencies — what is the crossover where phase stops dominating the stereo image?