A Dolby Atmos system starts at 5.1.2: five speakers at ear level, one low frequency channel and two overhead. That is the floor, not a compromise. From there the format scales a long way, and Dolby's own installation guidelines describe home systems running as many as 24 speakers on the floor and 10 overhead. For most dedicated rooms on the Los Angeles Westside, 7.1.4 is where the money stops buying obvious improvement.
What do the three numbers mean?
The naming follows the older stereo, 5.1 and 7.1 convention and adds a third number for height.
The first number counts speakers at ear level: front left, centre, front right, and the surrounds.
The second number is the low frequency effects channel. It is written as .1 because it is one channel, not because you own one subwoofer. Two subwoofers in a room is still a .1 layout, and two is often the better choice, because a second sub is the cheapest way to even out bass between seats.
The third number counts height channels, and this is what makes Atmos different from everything before it. Content is authored as objects that move in three dimensions rather than as fixed channels, so the same soundtrack adapts to whatever layout you have.
One rule sits underneath all of it: speakers are added in pairs. There is no 5.1.3.
Which layout fits which room?
5.1.2 works in a living room or a media lounge where the ceiling allows one pair of height speakers. It is the minimum we will call Atmos.
5.1.4 adds a second pair overhead. This is the upgrade people notice most, because overhead pans stop jumping between two points and start moving smoothly front to back.
7.1.4 is the reference for a dedicated room. Side and rear surrounds are separate, four ceiling channels carry the height layer, and the layout matches what Dolby uses as its own reference for angles.
9.1.6 and larger belong in purpose built rooms with acoustic treatment, riser seating and a processor with the channel count to drive it. There is real headroom above that number, but the returns per speaker fall away quickly once the room itself is the limit.
Where do the overhead speakers actually go?
This is where most Atmos installations are lost, and Dolby is specific about it.
Height should sit between two and three times the height of your ear level speakers. The angle of elevation from the listening position to each overhead pair, in a 7.1.4 reference layout, should be 45 degrees, adjustable between 30 and 55 degrees where the room forces it.
Side to side, the overhead pairs should be about as wide apart as left and right front speakers sitting at plus or minus 30 degrees, which works out to roughly 0.5 to 0.7 of the total width of the layout. Keep the overhead group centred front to back over the seating, even when the screen sits further away than the surrounds do.
Aiming depends on the speaker. If the overhead speakers have wide dispersion, meaning roughly 45 degrees or more off the reference axis across the band from 100 Hz to 10 kHz, they can face straight down. Narrower speakers need aimable or angled drivers pointed at the main seat.
Timbre matters too. Overhead speakers should match the tonal character of the ear level speakers as closely as possible, because an object crossing from the front wall to the ceiling should not change colour on the way.
What matters more than the speaker count?
Placement and calibration, by a wide margin. A correctly placed and calibrated 5.1.2 beats a badly placed 7.1.4 every time, and it is not close.
The room comes first. Reflections from side walls and ceiling smear detail before any processor sees the signal, which is why treatment goes in before electronics. We covered where panels earn their keep in acoustic panel placement. Seating position decides both the picture and the bass, and that is set out in how far to sit from the screen.
Then the system is measured and set: levels, distances, crossovers, and a final voicing pass by ear. The full process is in how Atmos is calibrated in a private cinema.
Frequently asked questions
What is the minimum for real Dolby Atmos?
5.1.2. Dolby's home installation guidelines describe systems starting at five speakers on the floor and two producing overhead sound.
Is 5.1.2 enough, or should I go straight to 5.1.4?
If the ceiling allows a second pair and the budget allows it, take the second pair before you upgrade any single speaker. Four height channels change overhead movement more than better height channels do.
Do up firing speakers count as Atmos?
Yes. Dolby defines an enabled speaker that reflects sound off the ceiling as an alternative where installing in ceiling speakers is not possible or not wanted. The result is a slightly more diffuse overhead image than dedicated overhead speakers give.
Do I need two subwoofers?
Not to have Atmos, and it stays a .1 layout either way. But a second subwoofer is usually the most effective money in the room, because it evens out how bass reads from seat to seat rather than only at the best chair.
Can I add height speakers to my existing 5.1 or 7.1 system?
Sometimes. If the existing overhead speakers currently carry surround duty, they can be reassigned to height only if a new ear level speaker takes over the surround feed. There has to be separation between the ear level layer and the height layer for the effect to work.
Designing or calibrating an Atmos room in Los Angeles
Ermine designs, installs and calibrates private cinemas across the Los Angeles Westside, from a 5.1.2 lounge to a reference room. Call (888) 818 9996 or use the contact page at erminehub.com.