Inverted-polarity (antiphasic) stereo signals cancel lows and smear image

Evidence

A special case of phase difference in stereophonic reproduction is the use of antiphasic signals in the loudspeakers, where the same signal is applied to both loudspeakers but with the polarity inverted at one loudspeaker, producing a constant 180◦phase difference between the signals at all frequencies. This phase difference changes the perceived sound colour, and also spreads the virtual sources. Depending on the listening position, low frequencies may be cancelled out. At higher frequencies this effect is milder, since when the wavelength becomes shorter, the listening area where the loudspeaker signals cancel each other shrinks, and beyond some frequency one of the ears will be outside this listening region where the signals cancel. This effect is also milder in rooms with longer reverberation. The directional perception of the antiphasic virtual source depends on the listening position. In the best listening position, the high frequencies are perceived at the centre and low frequencies in random directions. Outside the best position, the direction is either random or towards the closest loudspeaker. In the language of audio engineers, this effect in the sound is called ‘phasy’, or ‘there is phase error in here’. — Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 1912

Categorization

• DEFINE: Defines the engineer’s term ‘phasy’ and gives the underlying mechanism of phase cancellation in antiphasic reproduction.