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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.
Required coverage / dispersion, speakers pointing straight down. 9 foot 6 inch ceiling.
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.
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.
Speaker coverage, speakers pointing straight down. 8 foot 6 inch ceiling.
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.
Speaker coverage, speakers aimed at point between two seating rows. 8 foot 6 inch ceiling.
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.
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Nyal Mellor, Founder, Acoustic Frontiers
Nyal Mellor
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