Identical signals at both ears produce a centered phantom image.

Evidence

Both speakers driven equally: This is shown in Figure 7.2(b). In this case, both speakers are providing identical signals to both ears. The composite wave due to the sum of the two signals from the speakers at each ear are identical. Thus the phase shift between the two ears is zero. This is identical to the phase shift due to a sound source that is central between the two speaker’s positions. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 2201

Categorization

• DEFINE: Defines the zero-interaural-phase baseline case against which all other stereo configurations are compared.