How THX Works

By: Jeff Tyson
THX written in a comic speech bubble.
THX is more than a movie sound format. tttuna / Getty Images

If you go to the movies frequently, you've probably encountered THX at some point. The lights dim, the screen lights up and you see the THX logo, accompanied by earthshaking sound. The phrase "The audience is listening" appears and you know you are in for an aural, as well as visual, experience.

The common assumption is that THX is simply another movie sound format, but it turns out THX is not a sound format at all but something else entirely. In this article, you will learn just what THX is, how it works and how theaters acquire it.


Short History

THX, named after THX 1138 (George Lucas' first feature film), was developed at Lucasfilm in the early 1980s. George Lucas wanted a way to ensure a movie would look and sound exactly the same no matter where it was played. Tomlinson Holman, former corporate technical director for Lucasfilm, and a team of THX engineers took on the challenge of developing a baseline set of standards.

They noted several theater shortcomings that might negatively affect an audience's enjoyment of a film:


  • Outside noise (from the lobby or other auditoriums)
  • Inside noise (from the projector or air conditioning)
  • Audio distortion
  • Obstructed or uncomfortable viewing angles
  • Reverberation in the auditorium
  • Insufficiently bright images
  • Unequalized or poor audio

The essence of THX is a set of guidelines that resolve these problems. To display the THX logo, a theater must adopt this set of standards and then be certified by Lucasfilm's THX division. The first movie to be shown in a THX-certified auditorium was Return of the Jedi in 1983. There are now more than 2,000 auditoriums around the world that have been certified.

Before a theater can be certified, a number of things must happen:

  • Licensing and evaluation
  • Certification
  • Design
  • Approval
  • Construction/renovation

We'll take a look at these steps in the following sections.


Licensing & Evaluation

The first step in the process is for the theater owner and THX to take care of the licensing agreement. The theater owner agrees to lease the THX crossover equipment and pay for the right to use the THX logo. Once the theater owner signs the agreement, THX reviews either the architectural plan of the auditorium or, in the case of existing auditoriums, a survey of the current conditions.

The theater owner and a THX team then develop a design for implementing the infrastructure needed to meet THX specifications. After the theater owner has incorporated the design changes, THX will assist him or her in selecting equipment from the THX equipment list. The final design and equipment list is submitted to THX for approval. Once THX approves the list, the owner can begin actual construction or renovation. CES: TXH Tech Pages shows some of the architectural design details involved in constructing a THX theater.



Projector alignment with screen

Let's take a look at the THX certification process. THX looks at a number of theater qualities, which fall into four basic groups:

  • Physical structure
  • Projection system
  • Seating arrangement
  • Sound system

The auditoriums in many theaters do not provide adequate barriers to keep out unwanted noise. How many times have you sat in an auditorium listening to the gunfire and explosions from the movie next-door? You may also have been distracted by the constant hum of HVAC equipment. THX-certified auditoriums cannot exceed a noise criteria level of NC-30 per octave. Noise criteria is a measure of background noise in a room -- in this case, noise from outside sources. NC-30 is comparable to the level of outside noise you would hear from inside a church.


The THX team also measures reverberation and echoing in the auditorium. They do this by counting the number of seconds it takes for a 60-decibel (dB) tone to completely fade away. For THX certification, this time must fall within a certain limit determined by the auditorium's size. You should not hear any echoes in a THX theater.

The projection system and the screen's brightness level must meet guidelines established by the Society of Motion Picture and Television Engineers (SMPTE). The projector must align with the center of the screen within five percent, and preferably three percent, of the screen's width and height.

When watching a movie with a standard 2.35:1 aspect ratio, people sitting in the end seats on the back row must have a viewing angle of at least 26 degrees (36 degrees is considered optimal).

This theater has a viewing angle of 26 degrees.
This theater has a viewing angle of 36 degrees.

No seat can have a partially obstructed view. Additionally, the movie's sound must be clear and distinct from any seat.



THX crossover sound processor

Certainly, high-quality sound is at the heart of the THX process. In developing the criteria for THX sound, THX engineers looked at two distinct parts of a sound system:

  • A-chain
  • B-chain

The A-chain comprises the components that decode the sound from whatever format it is in and process that signal to remove noise. This includes the audio pickup on the projector and the cinema sound processor. Digital sound formats, such as DTS, Dolby Digital and SDDS, and noise-reduction systems, such as Dolby A and Dolby SR, are systems that improve the A-chain.


The B-chain is basically the rest of the sound system (THX crossover, power amplifiers and speakers). The THX engineers realized that no matter how good the sound coming out of the cinema processor was, a weak B-chain sound system could ruin it. To prevent this problem, THX patented a sound system design consisting of a special crossover and components from a list of THX-approved equipment. The engineers at THX subject all of the professional audio equipment on the list to a battery of tests to ensure that it meets the specifications they have established.

The THX crossover is a proprietary sound processor that separates the low and high frequencies to provide cleaner sound over the theater's sound system. Theater owners do not buy the crossover, but instead lease it as part of the THX licensing arrangement. The crossover uses 24-dB-per-octave high-pass and low-pass filters. The filters keep all of the high frequencies from going to the subwoofer (low-pass) and all of the low frequencies from going to the main speakers (high-pass). This page demonstrates a high-pass filter, and this page demonstrates a low-pass filter.

During the 1990s, THX spread to home theater. VHS and DVD movies that bear the well-known logo have gone through the THX Digital Mastering Program. This is a professional services group that works with studios and post-production facilities to ensure that the highest standards are applied for sound, video, compression, and replication. Check out Certified Products for a list of manufacturers that offer THX-approved home theater components. Just remember, unless you set up your home theater in a room that meets the same demanding specifications required for theaters, home THX probably won't sound the same as it does at the movies.

For more information on THX and related topics, check out the links on the next page.