Paul Theodoropoulos

THX - what it is and is NOT

25 May 1993 18:56:49 -0700

Newsgroups: rec.arts.movies

The following is a repost of an article written by laura ackley -
formerly of THX - that i first posted a couple of years ago, and that
i recently posted to laura was kind enough to
write this for me in response to the numerous erroneous statements
concerning the nature of THX that are routinely posted in newsgroups
such as this.

i hope this clears up some of the common misconceptions about THX.


Hi, I'm Laura Ackley, the THX System Designer.  I'm responsible for the
design of all professional and commercial THX Sound Systems, so I thought
I'd try to clear up some of the confusion regarding THX.

THX is a set of implemented design and quality control standards for
theatre sound presentation.  First, the actual physical theatre must meet
certain acoustic criteria:

It must have low background noise, (no loud air conditioners or
projectors), good isolation (so the lucky patron doesn't get to hear the
gunfight in the theatre next door), and specific reverberant
characteristics based on the volume of each individual theatre.  Also,
the theatre can't have any nasty slap echoes or reflections.

When an individual theatre meets these standards, it means that the
dialogue will be more intelligible, the high frequencies will reproduce
clearly, the bass will not build up excessively within the room, etc.

Most of the sound one hears in the movies comes from behind the screen,
from 3 channels usually.  (Occasionally 5, but THAT'S a different

Because of this, the theatre must also have a good length/width ratio.
(i.e., No bowling alleys.)  From the visual standpoint, this is of course
desirable, and from the audio standpoint, it means having a good "stereo
image" between the left and right channels, so that the patron is
actually hearing in stereo -- in every seat in the house!

Once a theatre is accepted for the aforementioned preliminary THX
criteria, the owner must choose the equipment to be used from a list
provided by us.  This list includes amplifiers, processors, screen and
surround speakers.  They can choose from a number of different products
by various manufacturers, but they all the products have been objectively
tested here to conform to very high standards.

These high-quality components assure a wider dynamic range and more
faithfully reproduced sound, plus adequate power for the room size.

Incidentally, the ONLY Cinema Processors which were accepted until quite
recently were made by Dolby, so any THX theatre was by default a "Dolby"
theatre... and ever-so-much more!

Thus, we're not in competition with Dolby, we're a COMPLEMENTARY
TECHNOLOGY to Dolby Processing.  Recently, Ultra-Stereo also had a
processor approved for use in THX Theatres.

Also, we're not in the business of selling any equipment, just assuring
high quality hardware in THX theatres.  The ONLY piece of hardware that
is specifically "THX" is an electronic crossover network, which is
LEASED, not purchased by the theatre.

Our design office takes the architectural plans for every theatre, and
designs the placement of the screen speakers and surrounds.  The screen
speakers are set in a special baffle structure behind the screen.  This
structure has certain acoustic properties, and the speaker placement
within the structure is coordinated to best match the projected film

The surround speaker array is designed to provide optimum sound pressure
and audience coverage.  (and to integrate with the interior design
whenever possible).

We coordinate all this with the architects, owners, and equipment
installers.  Once the theatre has been completed or successfully
retrofitted, our field technicians visit the theatre to align and "tune"
the sound system, then they measure the acoustical characteristics to
make sure that they meet our requirements.  This test data is then
evaluated, and if the theatre passes on all counts, it becomes a
"certified" THX site.

This, of course, entitles the theatre to use the THX name and show the
THX trailer.  (You know, "The Audience is Listening.")  The theatre is
re-tested every 6 months to ensure that the quality of sound remains

Our "philosophy" is to recreate the film soundtrack mixing environment as
closely as possible so that the audience hears the soundtrack as the
film-maker intended.  (This is all tied up in a DIFFERENT lecture on the
history of film sound....)

In conclusion, we're NOT a method of recording, encoding, OR processing
movie soundtracks!  We're a design and quality control system for the

This means that EVERY FILM will sound better in a THX theatre, not just a
select few.  The premium quality of a THX theatre is often
well-demonstrated by "special-effects-extravaganza" films, because they
are mixed with a very wide dynamic range and a lot of "loud" or "low"
sounds that might distort when played back through a lesser system.

THX theatres are especially well-equipped to handle the demands of the
new digital theatre soundtracks (like "Cinema Digital" and "SR-D")
because of their high performance and headroom.

MIXING ROOM system.  The new Home THX system consists of a family of
licensed products that various licensees have developed to meet specs.
designated by our engineers.  The pricing of the home system is dependent
on the licensee.  These are sold through dealers like regular home A/V
equipment, and we do not design these systems.

Thanks for reading, and I hope this helps clarify things!