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Active Room Correction: Audyssey MultEQ Pro - page 2

By Patrick Hart

The Importance of Proper Speaker Placement: Optimizing Any Surround System

When I interviewed Audyssey's Chris Kyriakakis and Tom Holman back in late 2004, the Denon AVR-5805 was just about to become available. At that time, in the University of Southern California's original Immersive Audio Lab, Tom and Chris did a first demo for me of the Audyssey MultEQ XT software that was embedded within a portion of the AVR-5805's Texas Instruments Aureus DSP. Readers of this earlier article (see our Audyssey Labs' MultEQ XT Report ) will recall that the Lab's surround speaker system was set up in the classic THX (5.1) theater layout as prescribed in the ITU 775 standard.

Since that short, exhilarating first experience with Audyssey's MultEQ XT technology I've performed other MultEQ XT-assisted installations, often using Denon AVR-3806 receivers. Putting the systems through their paces after each of my installations has highlighted the extreme importance of adhering, as closely as possible, to the THX/ITU 775 speaker layout recommendations.

So in planning a custom installation I always start with the speakers and consider:

  • The chosen loudspeaker's on-axis linearity and the smoothness of their off axis radiation patterns (directivity index)
  • Adherence to proper horizontal and vertical placement and
  • Positioning (angling toward or away from the listening area)

These are the most vital set up factors necessary to extracting the optimum performance using the Audyssey technologies . If you're a former car audio installer coming over to the home side of the business like I did, these recommendations will probably already make a lot of sense. But if you're coming straight into the ranks of home theater without previous experience in speaker placement perhaps a review is in order.

CEDIA's 100 Core Level Class, "Fundamentals of Home Theater Design" teaches critical design elements for home theaters. Among the most basic of the elements is proper layout of 5.1, 6.1 and 7.1 speaker systems as well their more bass-capable .2 or .4 variations. To get accurate, realistic surround audio performance in any home theater environment it is always best to emulate the monitoring loudspeaker arrangement found at world class sound mixing stages. Here's how the best are set up.

The L/C/Rs: Left/Center/Right

Left and right front speakers should be positioned at ±22.5° to 30° from the 0° center-channel and pointed toward the listener(s). The dialogue-heavy center channel needs to be immediately above or below the video display (or behind a perforated screen) and toed-in toward the listeners' ears.

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ITU-R BS.775-1 (for 5.1)

Note: The 22.5° L/C/R inner angle (not shown) is an addendum to the ITU standard that was instituted by an agreement between THX and Dolby

Studies have shown that because of the brain/ears' head related transfer function (HRTF) our ability to accurately locate a sound source directly in front of us is an amazing ±1°. And with left-front or right-front sound sources at the prescribed ±22.5° to 30° (and toed-in toward the central listener) we are generally good to ±4°. ) Vertically our top-to-bottom localization acuity remains pretty consistent up to around ~+30°. Higher than +30° or so (as in ceiling mounted L/C/Rs) and the old HRTF starts working again and the left ear can begin to hear direct sounds from the right speaker (because it's no longer completely shielded by your head) and vice versa. Therefore:

  • Keep L/C/Rs as close as possible to the same horizontal plane . Panned cars going from left to right across a screen are easy confirmation of the veracity of this L/C/R recommendation.
  • L/C/Rs mounted horizontally across the top of the display device will fall under the +30° maximum angle (to the listeners ears) if the recommendations for the +15° max to the screen center (from the viewer's eyes) is maintained.
  • With bottom mounted L/C/Rs, point (up, if necessary) toward listening position and make sure that no coffee tables, legs, etc block the path toward the ears.

Mounting any speaker in a cavity, such as the enclosed A/V bookshelf-style units is like asking the speaker to perform from within a trash can. Instead, turn the cavity into a baffle which snuggly surrounds a bookshelf-style speaker. Otherwise cancellation dips, virtually always in the vocal range, will detract from vocal intelligibility. Yes the Sound Equalizer can take out the dips but it is a much more sound installation practice (pun) to build a fixed baffle around the speaker and run a thin bead of silicone around it to seal the bookshelf speaker and baffle to each other. If clients or their kids or their pets move a speaker the whole system can fall out of calibration. Due diligence in securely and properly mounting speakers will limit callbacks.

The Surrounds: Left and Right, Left Rear and Right Rear

Envelopment is a term Tom Holman (creator of THX and Audyssey co-founder) used to describe the mission of the surround speakers when he taught an (H)DTV Audio Seminar back in 1999 for budding surround television professionals. Back then Tom, as first editor for Surround Professional , (a recording industry magazine) stressed that properly executed 5.1 surround speaker placement followed the ITU 775 standard.

In the "optional ¾ loudspeaker arrangement" of the ITU 775 definition left and right (side) and rear surrounds are situated ±60° to 150° from centerline. Curiously, most articles I've seen show only the mid ±110° location for the (side) surrounds (the 5.1 system). Part of the wide, side-placement parameter allowance is again related back to the HRTF. When, for instance, a left surround speaker is placed within these angle parameters only the left ear hears the direct sound from that speaker. The ears/brain directional ability becomes severely degraded because of the loss of higher frequencies in the direct sound to the right ear (which are blocked by the head). So, fortunately for the envelopment goal, human localization ability to the sides is only accurate to ±15° front-to-back (but improves in the vertical direction). Many people have claimed that their side localization capabilities are much more accurate. But when those same people have participated in single-blind testing wherein the side surrounds cannot be seen then the naysayer's true capabilities have reverted back to the wide, ±15° window of localization uncertainty. This is much worse than the nearly ±1° horizontal and ±1.75° vertical localization ability in the front hemisphere.

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ITU-R BS. 775-1 (for 6.1 & 7.1)

My preference for surrounds, having done dozens of surround set-ups in both homes and recording spaces, is properly designed, true, full range dipoles. It has been my experience though that the use of dipoles requires that the room dimensions plus possibly obstructing (room) design elements and seating locations of the listeners all be determined beforehand. This is because dipoles perform their diffusive envelopment trick by very precise horizontal positioning along the sides (for 5.1) or rear wall (for 6.1/7.1). Dipoles need to project a non-localizable null space throughout the listening area while radiating a positive wavefront to toward an appropriately close front wall and a negative wavefront toward a rear wall. It is this opposing, out-of-phase-with-each- other characteristic of dipoles that, when executed correctly, can pull off a superbly convincing sense of envelopment. Thus, if a high dollar/performance Home Theater is to be constructed from scratch an acoustician experienced in both loudspeaker dispersion characteristics and passive room treatments should be consulted.

In my corner mounted set-up which I'll describe shortly I use bipoles or, to be more accurate, dual monopoles corner-mounted into a faux 90° bipole configuration. Lastly, monopoles pointed not at the listeners but bounced off side and/or rear wall surfaces or pointed over and not at listener's heads are at the bottom of my recommendations when high quality surround is the goal.

Optimal vertical placement of the left and right surrounds is also vitally important to maintain the illusion of envelopment. Vertically, surrounds should be placed at least two feet above the listeners' heads and not pointed toward the listeners but rather aimed so the main wavefront (for monopoles and bipoles) or lack of wavefront (for dipoles) is at a 90° perpendicular angle to the floor and over the listeners heads. Getting too close to the ceiling, which is usually past +15° elevation from the central listener tends to call too much attention to the upper direction of the left and right surrounds. As you'll see, a too-high surround mounting position was one of the major location compromises I had to make due to my particular room layout (and more significant other). In my system, submarine scene envelopment like U-571's depth charges sound just fine but an off camera door-knock sounds positively weird.

Finally, left and right rear surrounds (for 6.1 and 7.1 channel systems) should be placed at ±135° to 150 (ITU 775 rec.) to 170° (in a pinch). Therefore there should be both left and right rear surrounds spaced at least 6' apart, even if they must work from a single rear channel amplifier. When we get to pictures of my personal home set-up it will be seen that dual, 90° opposing monopoles mounted close to the ceiling were my only option. Were I to do it over again I would have spread the two rear surrounds at least four feet out on either side from the 180° rear centerline.

The Problem with Ceiling Mounted Speakers

Note: Only brief mention has been made of in-ceiling speaker mounting either for front L/C/Rs, left/right primary (side) surrounds or single or dual rear surrounds. Ceiling mounted speakers, by their very positioning violate both primary tenets of proper placement; ceiling mounted L/C/Rs (even those angled toward the listeners) cannot provide front localization audio to the video display. The difference between the listener's viewing angle and the ears listening angle is just too great. Plus, remember that ceiling mounting left fronts and right fronts will allow direct sounds from the left speaker to be heard by the right ear and vice versa. It then becomes easier to understand why the much coveted left-right stereo imaging effect goes down the tubes as well; your head isn't in the way. And having your head in the way is what allows the wide soundstage, L/C/R imaging effect work optimally.

Ceiling "surrounds" mounted both to the sides and the rear of the listening area fail to convincingly generate optimal envelopment for the same HRTF reasons. That is, ceiling "surrounds" is almost an oxymoron. They are overhead surrounds whose position is much more capable of being localized because both ears are "in the clear" enough to hear the first wavefront from all surround speakers. Thus, there is no rear separation.

There are of course many jobs which will be signed onto wherein ceiling speakers are an only option. The point in exposing the drawbacks of ceiling speakers here is to warn custom installers against hoping that either of the Audyssey systems can soften or ameliorate the problems that poor speaker placement imposes.

 

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