Feature Article

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

Part III

January, 2006

Brian Florian


Separate Amplifiers and the Amplifiers in Receivers

Metrics Tested on THX Amplifiers:

Reference Output Voltage
Voltage Gain
Output Current
Output Source Impedance
Overload Restoring Time
Stability with Capacitive Load
Harmonic Distortion and Noise
Modulation Distortion
Difference-Frequency Distortion
Noise Output Voltage
Phase Response
D.C. Offset at the Output
Acoustic Noise Level
Mechanical Noise
Input Sensitivity
Input Impedance
Output Impedance
Load Impedance Range
Voltage Output Capability
Current Output Capability
Transient Output Capability
Transient Overload Recovery Time
Asymmetrical Clipping
Frequency Response
Phase Response
Phase Margin
Total Harmonic Distortion
Intermodulation Distortions
SMPTE IM Distortion
IHF IM Distortion
DIM 30 Distortion
Radiated Interference
Conducted Interference

It's too easy for people, including us here at Secrets, to say "get as much power as you can", "you should have at least 100 watts per channel", and so on.  While having more power than you need, even lots more, is not a bad thing and in fact is much preferred by speaker manufacturers (in that more speakers are damaged from underpowered amps driven to clipping, than ones which were fed too much power), the reality is that power, REAL power, is expensive.  The question is, how much is needed to drive a reasonably efficient speaker in a reasonably sized room to reference level and still have adequate headroom left over?

THX has done something which, to my knowledge, no one else in the industry has done, at least on the scale which they have done it:  They have logged the dynamic content of virtually every piece of finished sound track they can get their hands on (we're talking hundreds of sound tracks here) and from that developed a practical "dynamic" requirement for multi-channel power amplification.

All of the THX amplifier tests use bursts in various combinations at various frequencies, at various lengths of time, repeated cycles and combinations of speaker loads.  This enables THX to uniquely qualify an amplifier, particularly a receiver, as capable of playing sound tracks to reference level (given of course a certain speaker efficiency and room size limit).

Make no mistake:  THX Ultra power amplifiers tend to be very powerful.  We've never seen one with less than 100 watts/channel continuous RMS, full band, all channels driven, BUT because of the "practical sum" THX has defined for themselves, even seemingly modest receivers under $1,000 can get THX Select2 certification and provide a satisfying experience for a lot of people who can't afford a stack of THX Ultra power amps (we'll talk about Select- vs. Ultra-Certification a little later).

But it's not all about raw power.  As we mentioned when talking about the THX Controller, the input and output levels and impedances of everything "THX" are within a certain tolerance so that all pieces "talk" to each other synergistically.  One can see from the metrics list above that everything down to the hum of the power supply transformer must be in check to be THX certified.  Yikes!

Speakers and Subwoofers

Metrics Tested on THX Speakers:

Axial Frequency Response Analysis
Directional Characteristics
Harmonic Distortion
Low Frequency Cut Off
Phase Angle
Stray Magnetic Flux
Maximum Output Level
Acoustic Noise Level

Back once again to the realties of the consumer market, and once again knowing that a living room will remain a living room, much of what makes THX speakers unique and special has to do with addressing this fundamental tenet.

Starting with the more pedestrian facets of speaker performance, it almost goes without saying that THX speakers must have no compromise in terms of neutral frequency response, power handling, output capability, and the often overlooked dynamic, or transient, response.

Any good speaker should be able to make such claims.  THX speakers are designed as such and can, without hesitation, be driven (by a THX amplifier) to reference level.  However, THX speakers go beyond this by designing in certain features and characteristics which put them in a position, as a system, to excel at faithfully reproducing the program in that "living room home theater" we keep talking about.

Myth: "THX speakers only sound good for one person in the room"

Reality:  The opposite is true: while THX speakers must have a narrow listening window in the vertical to attenuate reflections from the floor and ceiling, they must simultaneously provide a VERY wide horizontal listening window, ensuring good sound for everyone in the room.

Main/Front Channel Speakers

THX Speakers have to meet very specific design goals in terms of their radiation pattern:  In the horizontal, they must have a very, VERY wide listening window so that everyone across the couch hears good sound.  At the same time they must have a limited, or narrow, listening window in the vertical because reflections off the floor and ceiling can smear and distort the sound in the time domain.

Again, back to THX realizing people will not likely acoustically treat their ceilings so that conventional speakers can be used, recently (as of Ultra2) these requirements have changed in terms of emphasis (less on vertical roll-off, more on off-axis linearity), but we'll talk about that when we cover Ultra2 a little later.

The other major design characteristic of a THX speaker is that it is a dedicated satellite speaker which REQUIRES the support of a subwoofer.  The THX speaker system therefore is categorically a sub/sat system.

Full-range speakers are nice.  I love full-range speakers, but they have no place in a multi-channel sound system if we want to have any chance of realizing a flat, uniform reproduction.  Again, this goes back to acoustics:  it is very difficult to get a similar low end response from five speakers spread out through a room, or even just three across the front of it, even if all the speakers are identical, because their different physical positions in the room are going to result in different acoustical loading (i.e., the bass response will not be the same from speaker to speaker).  By summing all the bass in the sound track and sending it to a subwoofer, or set of subwoofers (all getting the same signal), the system's reproduction of bass from each channel will be uniform.

The other benefit of a sub/sat system, known by experts such as Ken Kreisel long before THX and home theater, is dynamics.  By asking one amp and speaker to cover the upper audible range (the main speaker) and a completely different one the bottom, both do a better job than either could if it was trying to do the whole shebang.

Remember we said bass management was integral to the THX controller, long before a time when it was common in consumer equipment?  Now we're getting to the heart of that.  These days, all processors and receivers offer bass management, but what slopes do they use?  What crossover frequencies are offered?  Will it all work with your speakers?  If you have a THX Controller with THX speaker, you don't have to worry about this.  Your stuff will work together famously because it was designed as such from the ground up.

For you Techies:

The THX crossover consists of an 80 Hz, 4th order Linkwitz/Riley filter alignment, and it was not chosen lightly or without serious consideration.   As the crossover frequency increases, it becomes harder to blend the subwoofer with a satellite, and the subwoofer becomes more difficult to audibly "hide".  At the same time, as the crossover frequency increases, distortion from the satellite decreases, the total dynamic range increases, and loading the room for the flattest response becomes much easier. The logic of the 80 Hz crossover point is that it's high enough to ease demands on the speakers and amplifier, but low enough to make the whole setup work without a tremendous headache. The 4th order (24dB/octave roll-off, 6dB/octave/pole) Linkwitz/Riley alignment not only offers a steep slope, but one that immediately transitions to that slope, maximizing the benefits of that slope near the crossover point. The high-pass side minimizes excursion and power requirements, while the low-pass minimizes more localizable content at higher frequencies.

To correctly achieve this, THX satellite speakers are sealed systems with an 80 Hz �3 dB low frequency cutoff (preferably with a Qtc of 0.71).  The electronic high-pass filter applied to them is an 80 Hz, 2nd order (12dB/octave) Butterworth alignment.  The speaker and the filter sum to a 4th order Linkwitz/Riley roll-off which matches the electronic filter applied to the subwoofer, and an excellent crossover is achieved.

Myth:  "Di-Pole surround speakers were used because of Dolby Pro Logic's mono surround channel and are obsolete with Dolby Digital 5.1."

Reality:  Di-poles are used to emulate the multi-speaker arrays of movie theaters.  It is Adaptive De-Correlation (see THX Controller section above) which addresses mono surround channel issues.

Surround Channel Speakers

The surround channels in movie theaters are reproduced by arrays of speakers, anywhere from four to sixteen speakers on either side and the same again across the back.  What is the best way to reproduce that surround sound effect at home?  The answer would be to use twelve or more surround speakers of course!  THX knew that no one would even entertain such a suggestion so for their surround speakers they mandate a dipole design.

A dipole, or dipolar speaker is one which fires sound from two opposing sides, or poles, where each pole is out of phase with the other (meaning that while one side is moving outward, the other is moving inward).  In a home theater, they are generally placed to the side of the main listening area with the poles firing to the front and back of the room, never directly at the listener.  The result is that the sound bounces off the walls of the room, successfully emulating the speaker arrays of the movie theater.

Movie theaters and large dubbing stages use speaker arrays for uniform, enveloping surround sound. Dipoles in a home theater do an admiral job of emulating that sound field using just two properly placed speakers.

THX dipoles must meet a strict requirement of flat total power output, which means the sum of the sound coming spherically from all around the speaker must be smooth, as opposed to the sound coming from just one of the poles.  This is not easy for a designer to do, and there are plenty of examples of poorly designed dipoles (none of which are THX-Certified).

In general no one will deny that dipoles do an admirable job of emulating the multi-speaker arrays of movie theaters, but many a journalist has gone on record as saying they are not suitable for multi-channel music and that monopole (a.k.a. conventional) speakers must be used.
Myth: "THX Surround speakers are ok for movies but not music"

Reality:  THX Surround speakers, with their flat total power response, provide excellent, pleasing results for BOTH movies and music.

Research done by Tom Holman in 1986 (involving both technically savvy audio engineers as well as laypeople) revealed that while some sound engineers preferred mono-poles in certain situations only for their ability to "expose" defects such as pops and dropouts, when it comes to actually listening, all persons showed a clear preference for a diffuse ambient surround sound field when the test involved properly designed dipoles level matched to the monopoles they were being compared to.  The "bad-rap" dipoles get is often due to evaluations clouded by the use of improperly designed dipoles and a failure to level-match them with monopole counterparts.

There are many ways of getting diffuse sound, including strategically positioned and angled monopoles, but dipoles are simply the most practical solution for a consumer (once again, THX is dealing with the realities of the market).

Ultimately, the argument of monopole vs. dipole surrounds is one of inevitable compromise, with THX and others selecting the dipoles as the preferable of the two.  These days, THX certifies mono, di and bipole surrounds, recognizing that each has its appropriate use. To achieve that elusive balance of envelopment with some directionality of special effects or game sounds in wildly different rooms, it's necessary to choose the design that works best.   THX still puts dipoles first as great problem solvers in many typical rooms, but they acknowledge that other designs have their uses.

Click Here to Go to Part IV.

� Copyright 2006 Secrets of Home Theater & High Fidelity

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