Amplitude stereo produces directional images only when both ears hear both loudspeakers

Evidence

The net result is that amplitude stereo can give a convincing directional image of a set of sound sources because it provides both intensity and delay cues. Critically, this requires that both the loudspeakers are heard by both ears at low frequencies. This is something that does not occur when listening to conventional stereo recordings on headphones and will be discussed later. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 2209

Categorization

• XREF: Links to the interaural time and level difference cues covered in earlier chapters on spatial hearing.
• DEFINE: Nails down why stereo over speakers works but binaural-panning tricks over headphones don’t.
• corroborated: 2 independent passages support this claim — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 2209