All You Need To Know About THX.


What is THX

THX system is not a recording technology and it does not specify a sound recording format: all sound formats, whether digital (Dolby Digital, SDDS) or analog (Dolby Stereo, Ultra-Stereo), can be “shown in THX.” THX is mainly a quality assurance system. THX-certified theaters provide a high-quality, predictable playback environment to ensure that any film soundtrack mixed in THX will sound as near as possible to the intentions of the mixing engineer. THX also provides certified theaters with a special crossover circuit whose use is part of the standard. Certification of an auditorium entails specific acoustic and other technical requirements; architectural requirements include a floating floor, baffled and acoustically treated walls, non-parallel walls (to reduce standing waves), a perforated screen (to allow center channel continuity), and NC30 rating for background noise (“ensures noise from air conditioning units and projection equipment does not mask the subtle effects in a movie’s soundtrack.”)

History of THX


The first theater THX was used in was the University of Southern California’s Eileen L. Norris Cinema Theatre, a part of USC’s film school.

THX Processing

THX Block Diagram

Timbre Matching
Your ears hear different tonal qualities in sounds coming from different directions. Timbre Matching restores the frequency balance between your front and surround speakers—ensuring seamless and smooth panning between the front to back of the room.

Many movie soundtracks are mixed in studios for playback in large cinema auditoriums with an array of speakers. When played on home systems, they may appear abrasive and edgy. Re-EQ establishes a more accurate tonal balance when enjoying movies in your home.

Adaptive Decorrelation
Adaptive Decorrelation changes one surround channel’s time and phase relationship with respect to the other surround channel, expanding the listening position and creating the same spacious audio experience at home as in a movie theater.

and since too many speakers are being set too close to the wall (especially subwoofer), in order to get rid of the boominess due to boundary gain of any given room, THX applies Boundary Gain Compensation (BGC)
Sitting near a wall may result in distorted bass response. BGC corrects the way low frequency sound is perceived when seated near a wall. This results in a more true and accurate bass response.

THX Loudness Plus
THX Loudness Plus™ is a new volume control technology featured in THX Ultra2 Plus™ and THX Select2 Plus™ Certified Receivers, pre-amps and amplifiers. With THX Loudness Plus, home theater audiences can now experience the rich details of surround sound at any volume level.

THX Loudness Plus employs two technologies to make it work:
– THX Multi-channel Spectral Balancing
adjusts frequency response to counter the perceptual loss of low and high frequency sound in all channels. Begins with a flat response at Reference Level, and continually adjusts the response as the user selects lower listening levels. The equalization is applied to all channels in a multi-channel listening environment.

– THX Dynamic Ambience Preservation
Surround channel processing preserves the spatial detail that is lost when listening below Reference Level. It automatically shapes the output on surround channels to maintain the perceived balance in the original mix.

THX Certifications in Home Theatre Environment

Depending on the size of your room, THX have a solution for every consumer needs.


Ultra2 Plus

THX’s Ultra2 certification is given for home theater components said to be good enough for a large home cinema of 3,000 cubic feet or more.

Select2 Plus

THX’s Select2 certification is given for home theater components said to be good enough for medium sized rooms, up to 2,000 cubic feet in size, with a 10–12 feet viewing distance from the screen.

I/S Plus Systems

THX’s I/S Plus Systems include an AV Receiver + Speaker Bundle and are certified to fill a small home theater or dorm room where the viewing distance from the screen is 6–8 feet (1.8–2.4 m).

Multimedia Products

THX Certified Multimedia Products are designed and engineered for PC gaming and multimedia on the desktop.


THX-certified video displays (plasma display and LCD flat panels and projectors) include a THX mode which allows users to see a program or movie as the creator intended.

and the latest addition, is the THX Compact System certification

Until now, smaller, more compact speaker systems have been about convenience and form factor rather than performance. Well, times are changing, and recent advances in technology and design have challenged this precedent. Today, it’s quite possible for smaller speaker systems to deliver excellent sound experiences.


For many people, even a THX Select2 Certified system (speakers designed to power rooms up to 2,000 cu/ft) would be overkill given the size of their room. However, up until now the market for smaller spaces has been saturated by underperforming, highly promoted products – leading many people to believe that the best they could get was tinny sound, no bass, and limited upgrade options.

The goal at THX is, and has always been, to bring the best possible sound and video quality to the consumer. Speaker designs, build materials, components, amplifier power, and other factors have prevented smaller, less expensive systems from being able to pass the strict certification performance benchmarks.

With recent technology advancements in the audio world, THX has been able to develop a certification program, without lessening the criteria, for smaller systems that perform well above what their diminutive size would imply.

The Room
Now of course no small system could fill a big room, nor should it ever be asked to. Rather, the THX Compact Speaker System certification program is designed and engineered to power rooms up to 1,000 cu./ft. – for example, rooms that are approximately 13.5-feet long by 10-feet wide with 8-foot ceilings.

Just because the speakers power smaller rooms doesn’t mean the certification criteria isn’t asking them to do a lot. THX demands high output (up to 105 dB) with low distortion. In a radical change from many other small speakers, THX also requires a flat frequency response, just like all THX Certified speakers.

Understanding not all small rooms can fit a full surround sound system, the THX Compact Speaker System Certification program is flexible in that it will be found on 2.1 systems (two speakers and a sub) as well as up to 7.1 systems.

THX Reference Level

All movies are mixed in the post production mastering suite at 85dB average loudness with 20dB headroom. On a properly calibrated theatre (be it home or commercial) this level is reached when the volume dial is set to 0dB (meaning zero attenuation of the output signal). However, since this level is very loud for home applications, most people will listen to -10dB to -20dB lower than reference. By moving the volume level down, the details of the soundtrack will change and not be as intended by the sound engineer. To solve this problem THX invented the THX Loudness Plus algorithm as mentioned earlier in this article.

Loudness Plus

Is THX Certification Can Be “Bought”?
NO! For a receiver, for example, 2,000 tests with 14,000 data points are all needed to be passed by a receiver before it can receive the THX certification badge. For a display, 200 tests covering 400 data points need to be passed by any given display before it can receive the THX certification badge.

So does it mean that once you’ve purchased everything “THX” that your system don’t need any calibration? No. Having every unit to be THX certified means the units you’ve bought have passed (and or surpassed) the THX minimum standard requirements. You will then need to hire a THX Certified Professional to calibrate your system to the THX standards.


5 responses to “All You Need To Know About THX.

  1. Hi there. I own an M&K x12 THX Ultra certified sub, and watching Star Wars TFA today, when Kylo Ren uses the force, the super deep bass actually caused a couple of small pops in the bass. I read somewhere that THX standard is 20hz at -6db. Does this mean that when I want smooth deep 20hz bass, I need to set my level for my sub on my amplifier to -6db. I had it at -2 db, and may have been pushing it.
    Thankyou very much.

      • The processor is an Onkyo TXNR838, originally set up by me. Wasn’t too complicated. The other speakers are driven by an ATI power amplifier. I use the Onkyos pre-outs, except for the sub which outputs directly from the Onkyo.

      • I should have also mentioned that the Onkyo is a THX certified amplifier.

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