Specifications

Myth: "THX Surround speakers are ok for
movies but not music"
Reality: THX Surround speakers, with their
flat total power response, provide
excellent, pleasing results for BOTH movies
and music.
THX dipoles must meet a strict requirement of flat total power output, which means the sum of the
sound coming spherically from all around the speaker must be smooth, as opposed to the sound coming
from just one of the poles. This is not easy for a designer to do, and there are plenty of examples of
poorly designed dipoles (none of which are THX-Certified).
In general no one will deny that dipoles do an admirable job of emulating the multi-speaker arrays of
movie theaters, but many a journalist has gone on record as saying they are not suitable for
multi-channel music and that monopole (a.k.a. conventional) speakers must be used.
Research done by Tom Holman in 1986 (involving both technically savvy audio engineers as well as
laypeople) revealed that while some sound engineers preferred mono-poles in certain situations only for
their ability to "expose" defects such as pops and dropouts, when it comes to actually listening, all
persons showed a clear preference for a diffuse ambient surround sound field when the test involved
properly designed dipoles level matched to the monopoles they were being compared to. The "bad-rap"
dipoles get is often due to evaluations clouded by the use of improperly designed dipoles and a failure to
level-match them with monopole counterparts.
There are many ways of getting diffuse sound, including
strategically positioned and angled monopoles, but
dipoles are simply the most practical solution for a
consumer (once again, THX is dealing with the realities
of the market).
Ultimately, the argument of monopole vs. dipole
surrounds is one of inevitable compromise, with THX
and others selecting the dipoles as the preferable of
the two. These days, THX certifies mono, di and bipole surrounds, recognizing that each has its
appropriate use. To achieve that elusive balance of envelopment with some directionality of special
effects or game sounds in wildly different rooms, it's necessary to choose the design that works best.
THX still puts dipoles first as great problem solvers in many typical rooms, but they acknowledge that
other designs have their uses.
Subwoofers
THX-Certified subwoofers embody the usual tenets of bass, which is both high in output and low in
distortion. Back to Reference Level, a THX subwoofer has to be able to reproduce the bass from all the
channels of a sound track at Reference Level (within a room size limitation) without distress or calling
undue attention to itself. Beyond that, like regular power amplifiers, the built-in amplifiers of THX
subwoofers must conform to an I/O spec that matches the THX controller. In particular, voltage levels
are much higher than for the other line-level signals, several times higher in fact! This gives the
subwoofer the 20 dB headroom it needs over any other channel (because it carries the bass from all
channels, plus the LFE channel, all summed together).
In terms of depth, THX subwoofers are traditionally anechoically flat to 35 Hz with a shallow roll-off
thereafter, allowing room-gain to make up the difference for a perceived flat in-room response. That
has changed somewhat with Ultra2, which we'll get to in a moment.
Projection Screens and Other Things
THX also has a specification and certification for perforated, acoustically transparent screens, which
obviously involves ensuring they really are acoustically transparent, or at least that the loss incurred is
predictable and uniform such that it can be easily compensated for. Light loss must also be minimized.
Although of little interest now, THX at one time had certification of LaserDisc players.
Rane and AudioControl had THX-Certified equalizers, which are still prized pieces.
THX Select vs. Ultra
In 1999, THX launched THX Select, and renamed what had previously been called just THX to THX Ultra.