Making Your Sound Hemispherical

Focus on Number of Speakers Needed, Not Available Channels
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Everywhere you go in audio circles these days, people are talking about Dolby Atmos. It’s the big decision. How many channels do you have? Did you go 5.1.4? 7.1.4? 9.1.2? True in-ceiling speakers? The reflected route? This is all very exciting, but I’m afraid that we’re losing sight of the forest for the trees. Some folks out there–maybe even a few well-meaning integrators–are getting the idea that Dolby Atmos is just a fancy name for 12-channel surround sound. It’s not.

The Dolby Atmos consumer format is designed to support up to 35 speaker outputs. Dolby Atmos is intended to be scaled based on room size, and of course, budget. There is no set configuration that is ideal for all rooms. No one expects you to put 35 channels in a 12x10x8 space; they’re simply not necessary. A 5.1.2 configuration may be perfect. But a large room may require 24.1.10. The point is that the channels are now there when you need them.

Plan for the Future. Today’s so-called “Dolby Atmos” processors only have 12 outputs. Common chip-based processors don’t have the decoding horsepower right now to break the 12-channel barrier. (Super high-end audio computers like the Trinnov Altitude do.) New chips are coming. Then you’ll see processors that increase the number of outputs, and you’re going to have channels coming out your ears. Your mission is to prepare your clients for that eventuality. Stop fretting over 7.1.4 versus 9.1.2 and start figuring out how many speakers you need in each room to create a perfect hemispherical bubble around the audience. Go ahead and wire them, and heck, install the speakers and amps! That way, when the processors do hit, your clients are off and running with exactly the right number of channels.

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There’s talk going around that you can put speakers anywhere you want and the processor will magically make sound go where it’s supposed to go. That’s not true.

Easier said than done. How are you supposed to figure out the ideal number of speakers? I can’t fully cover all of that information here, but will mention a couple of specifics to get you on the right path.

Put Speakers in the Right Places. First, there’s talk going around that you can put speakers anywhere you want and the processor will magically make sound go where it’s supposed to go. That’s not true. If you put all the speakers on the back wall and ceiling, then you’re going to have a bad time. General guidelines still apply. There will be speakers behind/around the screen, down the side walls, across the back wall, and on the ceiling. You just need to put them in the right place to create that perfect sonic bubble.

Make Each Speaker Loud Enough. Second, Dolby seems to be backing off its original statement that any “point” in the room should be able to produce a sound level of 105 dB. That’s good, because it’s super hard to achieve, and mixers aren’t realistically going to put a peak-level hit just over your left shoulder and slightly above you. However, a system still needs to be good and loud from any point in the room. There’s ongoing discussion about just how loud, but it looks like somewhere in 95–100 dB range is where they’ll land.

Don’t Skimp on Certain Speakers. Third, surround and top speakers are going to be producing a great percentage of the total acoustic output in the room. If you use cheap, crappy speakers in those positions, the sonic balance of the room is going to shift toward cheap and crappy, even if the fronts are really good. I’m not saying every speaker should be the same (that doesn’t work practically or acoustically), but don’t skimp on the quality of the surrounds and tops just because there are a lot of them.

Careful with Those Top Speakers. Fourth, generally speaking, the “top” speakers should really be in a line that is halfway between the center and L/R speakers. Wider than that, and the soundfield contrast falls apart. Also, I am big fan of the “wide” speakers, and they should generally be on the side walls, pointing at you, at around 45 degrees from center axis. And they do need to be decent-quality speakers to handle the sound levels that a film mixer may put there.

Design for Atmos, First. I realize that I’ve been talking specifically about Dolby Atmos. What about Auro and DTS? Let’s be frank and pragmatic. Like it or not, Dolby is a juggernaut in this business. Atmos has more traction than Auro among content producers and theater owners on the commercial side, which still ultimately drives what we see at home. Do you really want to design systems for the smaller number of Auro mixes? While it’s true that a 7.1.4 Atmos installation is not strictly ideal for Auro, it will be possible to remap Auro to an Atmos array with a larger number of surround and top speakers. According to DTS, its format should be able to map the mix to any speaker configuration, so, theoretically, DTS:X should play back fine on an Atmos system. Regardless, at this point I wouldn’t worry too much about the format war. Focus on getting the right number of speakers to create a seamless bubble. The channels will be there to feed them.

Chase Walton contributed to this column.