Feature Article

THX-Certification: What It Means and How to Use It

Part IV

January, 2006

Brian Florian



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.

Whereas THX/THX Ultra was specified and designed for rooms "up to" 3000 ft3, THX Select took that requirement down to 2000 ft3.  Both the requirements of the amplification and the output of the speakers were scaled back appropriately, placing THX in the hands of a whole new audience who could not have otherwise afforded it.

When it comes to amplification, continuous output tests are run on up to one, four, and five channels (simultaneously) of an Ultra product, but only one at a time on Select.  With all products, the dynamic amplifier tests are done on up to all available channels.  Ultra amplifiers must be stable on all channels to 3.2 ohms and swing an 18A peak, while Select products must be stable into 4 ohms (front channel) and 8 ohms (surrounds), and swing peaks of 12.5A and 6.2A respectively.

So, right off the bat, Select amplifiers have a lower bar to reach, but ultimately in meeting it, they will still cleanly drive any reasonably designed speaker to reference level in a Select-size room. What we are talking about here is the idea that the lower powered equipment can get a THX certification that will assure consumers that the really affordable stuff has met certain standards like the high-end equipment.

The THX Controller section features are no different between Select and Ultra, which is why you'll never see a Select preamp/processor (SSP), only Select Receivers.

THX Select speakers, other than having reduced output requirements as compared to Ultra, do not have the same requirement for a narrow vertical listening window, because in a Select-size room you are apt to be close to the speakers and floor/ceiling reflections are that much less of an issue.  While THX Select surround speakers are still recommended to be dipole in design, conventional monopole designs are permitted for a few reasons, the main being that dipoles are, by their nature, expensive (having twice as many drivers as a conventional monopole), which goes against Select's mandate for a more affordable system.

Select subwoofers of course have reduced output requirements as compared to Ultra.

Ultra 2:  The Second Age

In 2001 THX revamped their Ultra program into Ultra2.

Ultra 2 Processing

One of the catalysts of the revamp was the enthusiastic consumer embrace of THX Surround EX and the 7.1 speaker layout that it implies, but Surround EX decoding only "works" well if the sound track was implicitly encoded for it, otherwise the surround sound filed tends to collapse to the center surrounds.  At the same time, multi-channel music was becoming more of a presence in the market, and speaker arrangements for music vs. movies were at odds with each other, with movies favoring a very diffuse sound field produced from the sides, and music favoring more in-your-face surround, with the source being more "behind" you.

With Ultra2, THX came up with a single system and speaker configuration which would work for everything.

Whereas THX Surround EX simply called for two more surround speakers at the back of the room for the then new sixth channel, Ultra2 replaced them with a pair of monopole speakers specifically placed right next to each other.  Somewhat like the way two-channel stereo can "position" sounds between two speakers, THX with their new process they call ASA, or Advanced Speaker Array, is able to "position" virtual surround speakers between the side surround speaker and the corresponding rear speaker (it's not really that simple, but it is the best way to visualize it).  So while Ultra2 controllers still offer THX Cinema and THX Surround EX modes, THX introduced three new THX modes with Ultra2 which use ASA:

THX Ultra2 Cinema.  This mode is identical to THX Cinema, except it does a "soft EX decoding", giving us some output from the rear, but does not allow the sound to collapse there.  Surround sound predominantly comes from the side dipole speakers.

THX Music.  This mode differs from the above in two ways.  First, using ASA, the surrounds are virtually positioned between the side and corresponding rear speaker, a position usually favored by multi-channel music setups.  This mode also disengages Re-Eq because music tends to be enjoyed at a lower median level than movies, and as such does not need it.

THX Game.  This mode basically puts all surround speakers on full duty, giving you a very lively, "exciting" sound experience which is what people want when playing an active video game.

The THX Ultra2 Layout For THX Surround EX, ASA places "virtual" speakers in the requisite position.


For Ultra2 Cinema, ASA "softens" the rear channel, keeping it active, while eliminating the "collapse" associated with Surround EX decoding of non-EX encoded material. For Ultra2 Music, ASA places "virtual" speakers in the location favored by multi-channel music enthusiasts, while maintaining an overall spacious surround space.

Its important to note that the Ultra2 controller has a setting for the distance between the two rear speakers, the default being <1ft as depicted above, yielding the most spacious sound field.  The alternate setting of 1-4 ft can be used to "favor" the music playback mode.  By moving the physical rear speakers apart, there is less of a "virtual speaker", with more of the surround sound in Music mode coming from the monopole rear speakers.

Click Here to Go to Part V.

Copyright 2006 Secrets of Home Theater & High Fidelity

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