THX dipole surrounds were bidirectional out-of-phase pairs, not true dipoles

Evidence

The dipole mode needs explanation. Back in Section 8.1.1, when discussing the evolution of multichannel audio, there was a period in which we lived with four matrixed channels, L,C,R and one channel for surround, Dolby Surround and Dolby ProLogic. In cinemas with many loudspeakers around the sides and rear of the large auditorium there was a pleasing effect. But when reproduced through two side loudspeakers in a small room it was not as rewarding. One of attempts to alleviate the problem was the “dipole” surround, a bidirectional out-of-phase (polarity-inverted) spaced pair of transducers. A true dipole has no separation of the opposite polarity radiating surfaces, as in front and back of a diaphragm or thin film radiating surface. These were not true dipoles. The goal was to attenuate the direct sound from the side loudspeakers and send more sound into the reflected, “diffuse”, sound field in the room. — Toole, Floyd E, Sound reproduction- the acoustics and…, 826

Categorization

• DEFINE: Distinguishes the marketing-brand dipole surround from an acoustically true dipole (opposite-polarity surfaces with no separation).
• SHARE: Good for an audio-engineer friend who still owns dipole surrounds - the true-dipole vs. marketing-dipole distinction settles an old argument.