Rane THX Room EQ User Manual
Home THX Equalization Manual Rev. 1.5
Home THX Audio System
Room Equalization Manual
Rev. 1.5
This document is Copyright 1995 by Lucasfilm Ltd
THX and Home THX are Registered Trademarks of Lucasfilm Ltd
Dolby Stereo, Dolby Pro Logic, and Dolby AC-3 are trademarks of Dolby Laboratories Licensing Corporation
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Home THX Equalization Manual Rev. 1.5
Table of Contents
Introduction and Background: Why Room Equalization? ..................................... 3
Test Equipment Requirements ........................................................................... 4-5
The Home THX Equalizer ....................................................................................... 5
Equalization Procedures ................................................................................... 5-22
Section 1: EQ Procedure Using the THX R-2 Audio Analyzer ............................ 6-13
1.1 - 1.3 Test Equipment Set-Up ................................................................... 6-9
1.4 - 1.7 LCR Equalization ......................................................................... 10-11
1.8 - 1.10 Subwoofer Equalization ............................................................... 11-13
1.11 Listening Tests ................................................................................. 13
Section 2: EQ Procedure Using a Conventional RTA ...................................... 14-21
2.1 - 2.3 Test Equipment Set-Up ................................................................ 14-17
2.4 - 2.6 LCR Equalization ......................................................................... 17-18
2.7- 2.9 Subwoofer Equalization ............................................................... 19-21
2.10 Listening Tests ................................................................................. 21
Equalization Checklist .......................................................................................... 22
Appendix: “WOW!” A User’s Guide to the Laser Disc ..................................... 23-24
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Home THX Equalization Manual Rev. 1.5
Introduction and Background:
The goal of the Home THX Audio Program is
to reproduce in the home the film sound experience exactly as the director heard it on the film
dubbing stage. Even with all the elements of a
Home THX Audio System correctly installed, one
challenge remains to our ability to perfectly “close
the loop” connecting the dubbing stage to the
home environment. That challenge is the almost
infinite variability of room acoustics. To achieve a
consistency of performance under varying acoustical conditions, some form of room equalization is
necessary.
Why Room Equalization?
In the room volumes (under 6000 cu ft) for
which the Home THX Audio System was designed, several problems exhibit themselves no
matter how “flat” or “accurate” a speaker is designed. The first problem is the existence of room
modes or standing waves. These modes are a
function of each room’s shape and dimensions,
and cause uneven frequency response with
peaks and dips in the low frequency range. Since
these standing waves occur at fixed locations for
fixed frequencies within a room, there is no way to
completely avoid these artifacts. However, since
the audience for a home theater remains generally in one specific area of a room, prudent equalization can usually level these peaks and troughs
and produce smoother response in the listening
area. The proper positioning of the Subwoofer
elements of a Home THX Audio System can do
much to minimize these standing wave artifacts
(see the Home THX Newsletter #2), but even with
careful placement, bass equalization is usually
necessary to restore a flat and accurate bass
response.
The second problem which room equalization is designed to correct is that of speaker/room
boundary interactions. These boundary interactions are the same ones that make a loudspeaker
appear to have more bass when placed in a
corner versus the center of a room. As you can
see, the placement (for example) of the screen
LCR loudspeaker asymmetrically in a room can
have serious repercussions. The speakers placed
nearer to the room’s boundaries will have a
different tonal balance to those placed more
centrally. The result of unequal tonal balance is
that sounds can vary dramatically as they are
panned across the three front channels, and
dialogue from boundary-close loudspeakers may
have poor intelligibility.
In both circumstances, with room modes or
with room boundary problems, a properly set up
room equalizer can restore the accurate spectral
and inter-channel tonal balance of a home theatre
system. Remember that these effects will vary
from room to room and installation to installation.
As valuable as room equalization is in restoring correct spectral balance, the measurements
required to obtain consistent and repeatable
response are very tricky. During the 60’s and
70’s, graphic equalizers were introduced by the
home audio industry. Unfortunately, these products were used for everything from tone controls
to a means of forcibly obtaining flat frequency
response from grossly inaccurate loudspeakers.
One test method of the period was to use records
with tracks separated into 1/3 octave pink noise
bands and to plot a speaker’s output band by
band with an uncalibrated SPL meter! In most
cases, the results were highly unsatisfactory. The
concept of equalization to correct for room modes
and boundary effects was, without accurate test
procedures, almost forgotten in the consumer
audio field.
However, within professional audio circles,
and in particular the motion picture industry, the
use of equalization to improve the accuracy of
sound reproduction has been continuously refined
and perfected. Over the past decade the Professional THX Theatre program has equalized over
600 motion picture auditoriums world-wide on a
yearly basis. Since the inception of the THX
Sound System program, records have been
maintained of the thousands of auditorium analyses and equalizations that we have performed. It
was from this experience that the standards for a
Home THX Room Equalizer and the enclosed EQ
procedure were developed.
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Home THX Equalization Manual Rev. 1.5
Test Equipment Requirements
1.) Real Time Analyzer
This procedure requires the use of a realtime (spectrum) analyzer and a pink noise source.
The analyzer approved for use is the R-2 THX
Audio Analyzer. The R-2 analyzer contains the
following:
• a 4 input real-time analyzer with measurement
bands at ISO one-third octave and ISO octave
intervals
• four calibrated omnidirectional microphones
• spatial averaging through microphone multi-
plexing
• averaging over time (10 seconds up to 2
hours)
• a calibrated internal pink noise source
Along with real-time analysis, the R-2 Analyzer
can measure room reverberation (RT-60) and
background noise (NC levels).
If the R-2 Audio Analyzer is unavailable, the
following equipment may be used with care:
• A real-time analyzer with measurement
bands at ISO one-third octave intervals and a
display range of ± 5 dB (minimum)
• A calibrated omni-directional microphone, or
microphones.
• The analyzer must be capable of defeating
any weighting which may be applied to the
real-time display
• The real-time analyzer must be also capable
of correctly storing and averaging a minimum
of four measurements and have a slow response mode.
The use of a single RTA, a large number of
multiple measurements, and the averaging of
these measurements is a time consuming
process and can be subject to a high degree
of operator error. It is therefore highly recommended that R-2 be employed whenever
possible.
2.) Pink Noise Sources
Pink Noise can be obtained from one of the
following sources:
• the R-2 analyzer
• the “Wow!” laser disc Chapters 8-10
• the Delos/Stereo Review Surround Sound
Test CD
• any calibrated true pink noise source (this can
be verified by measuring the noise source into
the line input of the analyzer for flat response)
Why Pink Noise
What is Pink Noise and why choose it over
White Noise? Simply put, white noise is a random signal with equal amplitude per frequency,
and pink noise is a random signal with equal
energy per octave. Let’s look at two octave
bands; say from 500 Hz to 1 kHz and 1 kHz to 2
kHz. If each of these bands had equal amplitude
per frequency, it’s apparent that the 1-2 kHz band
would contain more energy than the 500 Hz to 1
kHZ band because it contains twice the number of
frequencies. Consequently white noise sounds
very bright. Pink noise, however, containing equal
energy per octave, closely reflects our psychoacoustic expectations of flat response. Because
of this perception of flat tonal balance, pink noise
is a very useful tool when using a spectrum
analyzer with 1/3 octave or octave measurement
intervals, and when comparing loudspeakers for
spectral similarity by ear .
One element of caution is necessary,
though. Because pink noise has a random element to it, when you measure pink noise using a
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Home THX Equalization Manual Rev. 1.5
peak level meter or some RTAs you will notice
peaks far above the average. This is more
noticeable through a Subwoofer than through an
LCR speaker. This is because a random bass
peak can last for a longer time (lower frequency =
longer period) than most RTAs or SPL meters
average for. Higher frequency peaks last for a
shorter period. This is why most measurements
using pink noise are averaged for a long time or
are made by averaging multiple measurements.
That way these instantaneous peaks won’t throw
your readings off.
3.) The Home THX Room Equalizer
The Home THX Room Equalizer meets the
exacting specifications of the Lucasfilm Home
THX Audio program. It is specifically designed to
have the wide dynamic range, low noise, and low
distortion required by the demands of motion
picture soundtracks. Careful attention was also
paid to musical transparency.
The frequency centers of each channel’s
controls are carefully chosen to provide the
precise control necessary for accurate room
equalization, and the “constant Q” nature of each
control assures the operator that corrections to
one band don’t “spill over” into adjacent bands.
Parametric controls (where provided) allow for the
pin-point correction of mid-frequency problems,
and every equalizer is provided with a security
cover to help keep a tuned system tuned.
Equalization Procedure
NOTE: THE FOLLOWING TEST PROCEDURES
ASSUME THAT A HOME THX AUDIO SYSTEM
HAS BEEN PROPERLY INSTALLED, AIMED AT
THE LISTENING AREA, AND LEVEL CALIBRATED. FAILURE TO CORRECTLY INSTALL
A HOME THX AUDIO SYSTEM MAY RESULT IN
INCORRECT ANALYZER READINGS, IMPROPER EQUALIZATION, AND AN ACTUAL
REDUCTION IN THE OVERALL PERFORMANCE OF THE SYSTEM.
Please refer to the Home THX Audio System
Installation and Operation Manual (available from
any Home THX Licensee) for details on system
design, setup, and calibration.
For your convenience, an Equalization Procedure Checklist is located on page 22 of this
Manual. We recommend that you use it as a
handy reference only after thoroughly studying this Manual.
Graphic Conventions: When referring to the THX
R-2 Analyzer, specific, numbered function keys
on the control computer are identified by the
following graphics:
F-7
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Home THX Equalization Manual Rev. 1.5
SECTION 1: Room Analysis Using the R-2 Analyzer
1.1) Define The Listening Area:
The first step in correctly equalizing a Home THX Audio System is to identify the listening area.
The equalized response of the system will be averaged over this area to provide a balanced sound
field for all listeners. Equalizing for a single position can result in poor performance at other points in
the listening area. However, calibration of SPL (Sound Pressure Level) may be done from a single
reference position using the internal test signals of the Home THX Controller. These bandwidth limited
signals minimize room mode effects.
You should pay particular attention if the listening area is particularly deep (several rows) or wide.
With some measurement positions very close to Left or Right screen speakers, care will be needed in
averaging the RTA measurements to prevent unintentional weighting.
1.2) Choose Measurement Positions:
1
2
4
3
Mic Positions
Suggested Microphone Positions for 1 Row Seating
Fig 1
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Home THX Equalization Manual Rev. 1.5
• Choose four positions that represent prime listening positions spaced equally throughout the
listening area (Fig. 1).
• Position your analyzer’s microphones at seated ear height (38" to 48" off finished floor). Place the
microphone(s) on a stand.
• Do not attach any microphone directly to the analyzer or hold it in your hand. Your body is an
acoustical object large enough to influence what is supposed to be a room measurement.
• Label in your notes each position and note any related information (e.g., Microphone 3 located
under loft overhang) which can affect your interpretation of the measurements.
• Do not point any microphone directly at a loudspeaker. Point it straight up. You are looking for a
room measurement, not just the direct field of the loudspeaker.
• If you are placing a microphone on any piece of furniture (i.e., a chair or couch), make sure that
the mic is away from any cushion or seat back by at least 1 foot. This will improve the accuracy of
measurements at that position above 800 Hz.
For multiple rows of seats, see Fig. 2 below.
2
1
3
4
Mic Positions
Multiple Row Seating and Microphone Positions
Fig 2
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Home THX Equalization Manual Rev. 1.5
1.3) Home Theatre and Test Equipment Set-Up
Home Theatre Equipment:
• Switch your Home THX Controller to the “Dolby Pro Logic Surround” mode. The Home THX
Cinema mode must be switched off for this procedure. Note: For Controllers featuring Dolby
AC-3 Decoders, there is no easy method to insert broad band pink noise into this signal path.
Equalization should be done through the Dolby Pro Logic mode on these controllers as well.
• Calibrate the individual channel levels as usual using the internal test signals and a reliable SPL
meter.
• Disconnect or disable the Subwoofer and the channels you are not measuring. You want to ana-
lyze each channel individually and disconnecting unused channels helps prevent assignment
errors. One installer spent a frustrating hour trying to EQ a Center Channel speaker only to find
that he had been playing pink noise through the Right Channel speaker.
• Set the System Volume at Reference.
Pink Noise Sources:
Pink Noise may be obtained from one of the following sources:
• The internal pink noise source of the R-2 Audio Analyzer
F-7
• The “Wow!” laser disc, Chapters 8-10.
• The Delos/Stereo Review Surround Sound Test CD
• External calibrated pink noise source (200 mV RMS) placed into each channel’s EQ input.
R-2 Setup:
Defeat any weighting on the RTA portion of R-2 (e.g., “C” weighting). Measurements are to be taken
with flat response.
F-3
F-4
,
F-4
,
Set the analyzer to Slow Response.
F-2
F-2
,
Set your analyzer’s scale to the appropriate SPL range, and the dB per division scale to 2 dB.
F-3
F-2
,
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