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THX Certification General Questions Continued

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THX Speaker Type & Placement General Questions

Q: Please provide illustrations and guidelines for proper THX 5.1 and 7.1 speaker placement.

A: See the illustrative figures #1-3 below.

Since there are so many variables in residential environments, having a clear goal is every bit as important as the speaker layout recommendations. The goal is to envelope the seating area with sound that is the same for each listener and strikes a good balance between being immersed in the movie/performance and effective localization of movie special effects or discrete musical events. One should start with the recommended layout and make adjustments or positional changes as dictated by the room boundaries, seating locations, speaker types, etc.

Q: It is often asked by consumers why THX recommends placing the back channel surround speakers only a few feet apart. Can you please comment?

A: How one places the speakers depends on a couple of things. Our prime recommendation for consumers using components with THX Ultra2 Certification and looking for a single speaker arrangement for cinema and music is the following: two dipole speakers to the side and two direct radiators in the back touching side by side with the ASA circuit engaged at a distance less than one foot. (Figure 2). The ASA circuit takes the single back channel signal and using the two speakers makes it sound like one timbrally correct wall-to-wall sound. In THX cinema mode, the balance is as recorded. In THX Music mode, the sound is biased toward the back to mimic the ITU recommendations for 5.1 speaker placement in music recording situations.

Customers who are primarily interested in 5.1 music can use a slightly different approach. For example: two dipoles to the side and two direct radiators in back at plus/minus 150 degrees (Figure 3). For 5.1 movies, use the dipoles only. For EX or ES movies, use the dipoles plus the backs and if you have Ultra2, put ASA in the "back speakers > 3ft apart" mode. For 5.1 music, turn off the dipoles and use the back speakers only.

For movies, only use four dipoles as in Figure 1. With any of these systems, be careful that no one is sitting directly on axis and close to any of the speaker faces. Loud on-axis sound above 500Hz is very tiring to listen to when it's constant and outside of the field of vision.

Q: There are currently no true 7.1 discrete surround formats commercially available to the public. DTS ES is discrete 6.1 while DD EX is matrixed 6.1, what type of post processing does THX offer to expand these formats to 7.1?

A: You probably already know this, but it's good to keep in mind that matrixed back channel in EX is capable of the same frequency response and dynamic range as any of the 5 other discrete channels and needs to be treated to the same bass management and other types of post processing as the rest of the channels. THX directs this single source signal to the two back speakers equally or mixed with information from the side surrounds, or run through the ASA circuit depending on whether you have Select or Ultra2 products and the speaker configuration and choice of THX Cinema or THX Music. Once the system is installed, the consumer can choose between the two when listening to 5.1 material.

Q: Based on your responses above, we would like you to evaluate a new speaker type that we have had custom designed for us as applicable for achieving optimal performance while engaging THX post processing modes for 7.1 channel audio.

This design is based on a Bi-Polar/Di-Polar speaker and modified so that each woofer/tweeter section are independent to each other via two dedicated crossovers, and wired in phase. Each woofer/tweeter pair receives a dedicated amplifier and essentially takes the role of two speakers in one common cabinet.

Essentially this speaker is two 41-SE's in a common cabinet firing slightly off angle from each other. In the THX guidelines for back channel speaker placement commonly found in THX certified receiver/processor user manuals, they recommend placing the back channel speakers on the back wall slightly apart from each other. The RBH 44-SEB achieves this quite nicely, but manages to do so in one box

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The RBH 44-SEB is a custom designed variant of their Bipole/Dipole 44-Signature SE speaker designed for Audioholics .com. It is electrically isolated and in-phase two channel speaker system in one enclosure making it easier to integrate 7.1 in a constrained room. RBH may offer this product for sale upon special request in the near future.

A: This approach would definitely give customers another reasonable choice in their search for solutions that balance envelopment and localizability in a wide variety of rooms. A customer using Ultra2 with ASA could feed each half of this speaker with one of the SB channels and have a very compact way to get a great sense of envelopment. Also, check out the Harman Dual Drive technology that uses a very similar approach. Citation and Fosgate have used it in commercial products.

Post Processing

Q: It is often more the case than not, that we find THX processing modes soften the front soundstage depending on the program material. Why doesn't THX provision for the option to disable or adjust the Re-EQing feature on the front three channels?

A: The ability to turn ReEQ on and off independently from the rest of the THX features is optional with the licensee. Numerous products offer it. The question one must always ask is "what did this sound like on the dubbing stage?" Technically and philosophically, only the original sound design as heard on the dubbing stage is "correct." Of course, the consumer should be free to have it any way they please, including too soft or too bright-but how would they know? Another question must be "how much rope does the average consumer need before they hang themselves up in confusion and take what may or may not be a reasonable presentation?" Different licensees make different choices.

Q: Why is THX so adamant about having only one source in a multi channel system producing bass below 80Hz while also having a fixed 80Hz global crossover setting? Our blind listening tests revealed that placing a subwoofer near a listener is localizable when crossed over above 60Hz (24dB/octave). In addition, many users enjoy large bass capable main speakers and prefer the advantages of stereo bass while also supplementing their speakers with a subwoofer and adjustable crossover setting. Higher crossovers limit where the subs can be placed to properly cross to the satellites and not be localizable.

Reference: Mating a Subwoofer with Bass Capable Speakers

A: Here's another of those "how much rope" questions. 80Hz is a very thoughtfully and scientifically chosen compromise, but is indeed compromised for some people in some situations. An interesting and useful experiment is to listen to a sub by itself, playing program material at various levels, moved from place to place with the crossover changed from point to point-all done blind. The listener cannot know ahead of time what he's listening to. Other crossover points can work just fine but place more restrictions on the system than 80Hz.

Some of the observations/issues are as follows:

Unless you're sitting right on top of it, a sub properly crossed at 80Hz, 24dB/octave and free of stray resonance, rattles, port chuffing, coil bumping and the like cannot be localized. 160Hz is the lowest frequency that is consistently localizable by most people in most situations.

Lower crossovers require larger screen speakers that load the room's bass modes at inflexible locations to the left and right of the screen. This makes it more difficult to get smooth bass in all the seats in the theater.

Larger speakers are sometimes more difficult to place properly in relation to the picture, particularly the center speaker, which is the most important speaker in movie sound.

Q: What are the differences between Cinema and Music Modes of THX?

A: THX Cinema mode is SL to SL, SB to SBL and SBR equally, SR to SR.

THX Music mode biases SL and SR to SBL & R. Mimicking the balance in an ITU 5.1 music setup.

Q: Why is the "Boundary Gain Compensation" feature limited to a 55Hz high pass filter, as opposed to an adjustable parametric EQ for higher flexibility and system integration where particular speaker placements and room modes adversely affect performance?

A: Again, it's the "how much rope" issue. In this case, you'll find some of the high-end manufacturers have heard your request and jumped into the fray with so-called "Room EQ" or "Room Calibration" auto or manual. In THX land, the BGC is a carefully chose compromise. It is our overwhelming experience that proper EQ of a system can be beneficial, but ONLY if the operator knows exactly what he's doing. Automated systems are mostly junk. Systems that provide EQ above 1K or so are mostly junk. Octave systems are junk. Lexicon and Meridian both have systems that are showing promise but we've not yet finished our evaluations. The Meridian system in particular has a prayer of improving the system rather than simply changing it.

Some rules of thumb follow:

  • Use EQ only as the last step in calibration. Good speakers, properly located in a good room with properly located seating, should all come first.
  • EQ in the bass region only. Usually < 200Hz
  • Parametric is best, but 12 th octave can be somewhat useful.
  • More than two sections of EQ per channel are usually too many.
  • Cut only no boost.
  • Steady state measurements such as RTA only reveal part of the problem and have only approximate correspondence to human perception.

Going Forward with THX

Q: What are the next steps for THX and how are they advancing and progressing with regards to the theatrical specification as well as the home theater specification?

A: We are deeply involved in Digital Cinema, but at this time we can't share our plans with you.

Q: Does THX have any plans to incorporate algorithms or standards for room correction.

A: We do have some new processing technologies, but not for room correction. Consistently successful room corrections are still best done in the room, not the electronics.

None of these are observations or guidelines are cast in stone rules to be followed blindly. They're the results of 12 years of experience in installing hundreds of systems and working closely with designers and installers who have installed thousands and thousand of systems. They work really, really well for the uninterested or uninformed customer.

Ultimately these are the THX performance goals for a good system:

  • Clear dialog
  • Precise sound localization
  • Spacious, enveloping surround
  • Even tonal balance
  • Full dynamics
  • Transparent system
  • Every seat a good seat

THX Loudspeaker Placement Suggestions

Figure 1: In a larger room where film is the dominant medium this is our recommended arrangement.

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Figure 1: Large Room Speaker Placement

Figure 2: In a room where the back speakers must be close to the listeners or where the listener wants one speaker set up for both movies and music Ultra2 electronics and the following speaker arrangement is best.

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Figure 2: Common Set-Up for Music & Movies Ultra 2

Figure 3: In a case where the customer is willing to manually switch between side dipoles for film and ±150 degree direct radiators for ITU style film the following can be used. The back surround speakers can also be used for Surround EX film material. Care must be taken that the rear speakers are not too close to the rearmost listeners.

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Figure 3: Multiple Configuration Mode for Cinema and Music

 

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