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Dolby Atmos: Dispersion Requirements for Ceiling Speakers
by ct_acoustic_frontierJuly 06, 2022
In this blog article we look at the dispersion requirements for ceiling speakers in a Dolby Atmos system. We consider the example of a system with 4 ceiling speakers and two seating rows and show how dispersion requirements change for:
Speakers firing straight down vs. angled at the listening area.
Different ceiling heights (9'6", 8'6" and 7'6").
What is speaker dispersion?
Dispersion is another word for coverage - the question we are asking is "does each member of the audience hear the same sound?".
The sound emitted by a speaker can be thought of as a cone that expands in area as distance from the speaker increases. Simplistically speaking if listeners are within the coverage "cone" then they will hear the same thing and if they are outside it they will hear something different. It's obviously not as simple as this, but it is true that high frequencies can fall off significantly even at small off axis angles. For example if a speaker only covers out to 30 degrees off axis then someone at 60 degrees off axis will hear something very different from someone on axis.
For a more detailed explanation of speaker coverage and the Acoustic Frontiers coverage targets please read our article on dispersion specifications and off axis response plots.
Dispersion requirements: straight vs. aimed
In our imaginary home theater we have two rows of ceiling speakers (a 7.1.4 system) and two seating rows. The first ceiling speaker is at an angle of 55 degrees from the perspective of the first row, and the second ceiling speaker at an angle of 115 degrees.
If you are an eagle-eyed reader then you might notice that these angles are slighly outside the official specifications. As explained in our article 10 speaker layout tips for Dolby Atmos, DTS.X and Auro, we think the specifications should be adapted for multi-row theaters, in particular to provide clear line-of-sight to all surround speakers. The impact of this are changes to the angles for the ceiling speakers, since the rear surrounds are higher than ear level.
The two drawings below show the coverage / dispersion requirements for speakers pointed straight down vs. angled at a point between the two seating rows.
With the speakers pointed straight the front Atmos ceiling speaker must have consistent coverage out to ~70 degrees off axis! If you understand typical speaker dispersion profiles then you'll know that very few speakers have such wide coverage.
[caption id="attachment_3509" align="aligncenter" width="700"] Required coverage / dispersion, speakers pointing straight down. 9 foot 6 inch ceiling.[/caption]
[caption id="attachment_3510" align="aligncenter" width="700"] Required coverage / dispersion, speakers aimed at point between two seating rows. 9 foot 6 inch ceiling.[/caption]
Angling the speakers significantly reduces the dispersion requirements, especially for the first ceiling speaker row.
Note that these drawings only consider coverage in one dimension; in reality you also need to consider coverage across the width of the seating area.
Dispersion requirements: different ceiling heights
The dispersion requirements for the 9'6" ceiling were shown above. Let's now look at those for 8'6" and 7'6" ceilings.
First let's see what losing a foot in room height means for coverage in the case of the speakers pointing straight down.
[caption id="attachment_3511" align="aligncenter" width="700"] Speaker coverage, speakers pointing straight down. 8 foot 6 inch ceiling.[/caption]
[caption id="attachment_3514" align="aligncenter" width="700"] 7 foot 6" ceiling.[/caption]
With 7'6" ceilings and speakers pointed straight down we need almost ~80 degrees of consistent off axis coverage. Even the back row of ceiling speakers needs ~65 degrees.
Now let's see what happens with angled speakers and lower ceilings.
[caption id="attachment_3512" align="aligncenter" width="700"] Speaker coverage, speakers aimed at point between two seating rows. 8 foot 6 inch ceiling.[/caption]
[caption id="attachment_3513" align="aligncenter" width="700"] 7 foot 6" ceilings.[/caption]
These examples clearly show that angling speakers significantly reduces the coverage requirements. The coverage of the second ceiling row will become the limiting factor in a two row home theater.
We can draw the following conclusions:
ANY room, even those with quite high ceilings, that uses ceiling Atmos speakers firing straight down requires that those speakers have very wide coverage.
Angling speakers significantly reduces the coverage requirements.
As the room gets lower the coverage requirements increase.
All this goes to show that a very critical part of any home theater design is ensuring the speakers selected can provide consistent coverage of the seating area. You really can't select a set of speakers unless you know what the coverage requirements are, and you also have to know what the speakers are capable of as many manufacturers provide no data or specifications.
The rabbit hole
Of course we have tried to keep things simple in this article but there is more complexity to consider with a Dolby Atmos speaker layout:
Coverage across the width of the room.
Differences in SPL due to some seats being closer / further away than others. This needs to be considered when the speakers are positioned.
Off axis suckouts. Most speakers (with exception of some coaxials) have a suckout somewhere off axis due to path length differences between the tweeter and midrange drivers. If you know where these suckouts are you can position the speakers accordingly.
Timbral matching. All the surrounds speakers should sound identical, and should also sound the same as the LCRs. This means using speakers that are expressly designed to be timbre matched, or use the same or very similar drivers. We're seeing many enthusiasts use ceiling speakers from a different manufacturer to their LCRs and surrounds.
Much confusion still exists about what a room correction product does, what problems it can (and cannot) solve and therefore its 'place' in a modern high quality sound reproduction system. Part of the challenge of understanding room correction is that it requires a reasonable level of understanding of sound quality, acoustic science, acoustic measurement and psychoacoustics (how humans perceive sound). The majority of the articles I have read online or in print magazines do not cover the fundamentals in enough depth to allow the curious and committed reader a chance to understand room correction on anything more than a cursory level. By the end of this article I hope that you will have learnt enough to judge for yourself what room correction can and cannot do and how best to apply it in the context of a world class music or home theater system.