Specifications

Myth: "Di-Pole surround speakers were
used because of Dolby Pro Logic's mono
surround channel and are obsolete with
Dolby Digital 5.1."
Reality: Di-poles are used to emulate the
multi-speaker arrays of movie theaters. It
is Adaptive De-Correlation (see THX
Controller section above) which addresses
mono surround channel issues.
The THX crossover consists of an 80 Hz, 4th order Linkwitz/Riley filter alignment, and it was not chosen lightly
or without serious consideration. As the crossover frequency increases, it becomes harder to blend the
subwoofer with a satellite, and the subwoofer becomes more difficult to audibly "hide". At the same time, as
the crossover frequency increases, distortion from the satellite decreases, the total dynamic range increases,
and loading the room for the flattest response becomes much easier. The logic of the 80 Hz crossover point is
that it's high enough to ease demands on the speakers and amplifier, but low enough to make the whole setup
work without a tremendous headache. The 4th order (24dB/octave roll-off, 6dB/octave/pole) Linkwitz/Riley
alignment not only offers a steep slope, but one that immediately transitions to that slope, maximizing the
benefits of that slope near the crossover point. The high-pass side minimizes excursion and power
requirements, while the low-pass minimizes more localizable content at higher frequencies.
To correctly achieve this, THX satellite speakers are sealed systems with an 80 Hz –3 dB low frequency cutoff
(preferably with a Qtc of 0.71). The electronic high-pass filter applied to them is an 80 Hz, 2nd order
(12dB/octave) Butterworth alignment. The speaker and the filter sum to a 4th order Linkwitz/Riley roll-off
which matches the electronic filter applied to the subwoofer, and an excellent crossover is achieved.
Surround Channel Speakers
The surround channels in movie theaters are
reproduced by arrays of speakers, anywhere from four
to sixteen speakers on either side and the same again
across the back. What is the best way to reproduce
that surround sound effect at home? The answer would
be to use twelve or more surround speakers of course!
THX knew that no one would even entertain such a
suggestion so for their surround speakers they mandate
a dipole design.
A dipole, or dipolar speaker is one which fires sound from two opposing sides, or poles, where each pole
is out of phase with the other (meaning that while one side is moving outward, the other is moving
inward). In a home theater, they are generally placed to the side of the main listening area with the
poles firing to the front and back of the room, never directly at the listener. The result is that the sound
bounces off the walls of the room, successfully emulating the speaker arrays of the movie theater.
Movie theaters and large dubbing stages use
speaker arrays for uniform, enveloping
surround sound.
Dipoles in a home theater do an admiral job of
emulating that sound field using just two
properly placed speakers.