A/V Veteran

My last blog was limited to placement of the left and right speakers in 2-channel system, or just the left and right speakers in a home theater setup. There’s a lot more involved with a full surround sound setup.

An argument was made in a response to my previous blog that rigid wood walls are superior to more flexible sheetrock, the latter the most common material used in U.S. homes. While I don’t necessarily dispute this, it’s impractical for most people unless they’re building from scratch or have a huge remodeling budget. There’s also a possible concern about outgassing from many stiff, engineered wood materials, such as particle board, that might be chosen as a more economical and stable alternative to solid wood.

Extremely stiff walls do have one other downside. All rooms have bass modes, the major culprits producing the uneven bass afflicting virtually all rooms. Stiff walls don’t vibrate as easily as sheetrock, therefore they better preserve not only the desired bass response but also these modal deviations as well. (Published information on calculating room modes, found in many books on room acoustics, invariably assumes perfectly rigid walls). Sheetrock gives a bit, absorbing some (but far from all) of the energy produced by these room modes — along with some of the desired bass information! I’m not saying that non-rigid walls are necessarily better, but only that there’s no free lunch here. The best approach is first getting the speaker positions right, then add some domestically acceptable room treatment to help with the most persistent offenders (hard, bare floors, large uncovered windows) listen, then rinse and repeat as necessary, perhaps adding in some room EQ as a final sweetener. It takes time to get the best results (and sometimes money—acoustical panels aren’t cheap).

But I digress. The remaining speaker channels we haven’t yet discussed are the center, the surrounds, and possibly Dolby Atmos. I’ll limit this discussion to 5.X or 5.X.4 setups, both with and without four Atmos speakers (for newbies, the 0.4 in the 5.2.4 designation describes the number of Atmos speakers, and the X is a stand-in here for the number of subwoofers, most often one or two). There’s a wide range of additional possibilities, including front height speakers, two additional surrounds (7.X.4), more than four Atmos speakers (though we don’t know of any consumer Atmos sources that offer more than four discrete Atmos channels), or only two rather than four Atmos speakers. But I suspect that 5.1.4 or 5.2.4 (five main channels, one or two subs, and four Atmos speakers) will describe most readers’ setups.

Some home theater fans choose to omit a center channel speaker, preferring a “phantom” center (as in the image above). In the latter, the center channel information is split evenly in the AVR between the left and right front channels. It will then be perceived, by a center-seated listener, as coming from the center where the screen is located. That saves money, but in my experience doesn’t work as well, or sound as natural, as a competent center speaker offering a reasonable timbre match to the left and right front speakers. And if you move to the side with a phantom center setup, the center channel information will inevitably collapse into whichever front speaker you’re closest to. The dialogue will then no longer appear to come from the screen.

A discussion of center channel speaker design is beyond the scope of this blog, but a three way (with a vertically arrayed midrange and tweeter) is clearly the best choice if available. It should also be at least as robust as the left and right front speakers. It’s a common misconception that the center channel is just for dialogue, so any old center will do. That’s not even remotely true. According to some experts, as much as 70% of the audio in a multichannel soundtracks comes from the center speaker. That means not just dialogue, but also much of the music and effects as well. While that 70% value may be overstated, a good center speaker is perhaps the most important speaker in your home theater system (apart from listening to music in 2-channels).

Where the center speaker should be positioned is obvious…duh, in the center. But it should be as close to the bottom of the screen as possible with a flat panel TV or non-perforated projection screen — or just above it if that rarely used positioning works for you. If you use a perforated projection screen the center speaker can be placed just behind it and centered. In the latter case, if possible leave a few inches between the front of the speaker and the screen (the same goes for the left and right speakers if they’re also behind the screen).

For setting up surrounds, Dolby recommends positioning them to the left and right sides of the room, about 20-30-degrees behind the listeners, at or just above ear height, and aimed at the center of the seating area. This is where I’ve often deviated from the recommendation, mainly because it’s often impractical and can be distracting to a listener who isn’t seated dead center. My favorite heresy here is to locate conventional or bipole surrounds (I’ve used both) at the back of the room where they call attention to themselves only when genuinely needed. In my current setup this often puts the surrounds atop a row of filled bookshelves.

I’m not exactly sure who gave Dolby the imprimatur on positioning all of the loudspeakers in a surround system, but they’ve been at this longer than just about anyone so their advice is worth considering. This is certainly true for Atmos, which after all is Dolby Atmos). I’ve personally been avoiding the optimum in- or on-ceiling Atmos setup because of hassles involved, but will soon be breaking that fast. If you follow Dolby’s recommendations to the letter, you’ll position the in-ceiling speakers to fire straight down (not angling them toward the listening seats). All four of the speakers should be in line with the left and right speakers, with the front pair at the left and right midway roughly between the front speakers and the front row of listeners. The back pair should to between the listening seats and the back wall. Though Dolby is a bit vague on this where multiple rows of seats are involved, I’d position the rear Atmos speakers between the back row of a two-row seating area and the back wall, and consider more than four Atmos channels for three or more rows.

If your listening seats consist of a single row pushed hard against the back wall I’d recommend using only a front pair of Atmos speakers. In this situation a single pair of regular surrounds will likely be directly to the left and right of the listeners. This setup isn’t ideal for a wide range of reasons, not least of which is having a wall immediately behind your head (put some sound absorption on that wall if possible). But a less than optimum home theater system is better than no home theater system at all!

My room has eight-inch deep ceiling soffits around all sides if the room. These extend about a foot out from the walls. Above the soffits the ceiling slopes upwards on all four sides of the room, forming the underside of a shallow pyramid peaking at an estimated 10 feet. My intent is to position four small bookshelf speakers on the front of the soffits, secured by wall brackets that will allow for some aiming flexibility as they won’t be firing straight down. I’ll let you know how this works in a future blog.

But most Dolby Atmos setups don’t use ceiling speakers for the Atmos experience. They employ Atmos speakers that fire upward, hoping to simulate sounds that originate from above by using reflections from the ceiling. This method avoids the installment complexity of overhead speakers. It usually involves a maximum of four of these Dolby Atmos “modules,” one each atop the front left and right main channels and surrounds. While not quite as effective as a true overhead-speaker setup (in my opinion, but I’m not alone), this greatly simplifies achieving an Atmos setup. This ceiling-reflective method was almost certainly developed because Dolby likely recognized that the in-ceiling setup (which requires cutting holes for the speakers and routing wires behind the walls) would severely limit the format’s market acceptance and might even make it a non-starter.

You need a flat ceiling for this reflective setup to function as intended, and don’t put absorptive ceiling treatments where they might interfere with the reflections. Apart from that, these upward firing Atmos modules are widely available, many of them designed to cosmetically match your front and rear speakers.