Speaker placement dimensions for 5.1 and Atmos

Where the angles are measured from#

Every published surround layout is a set of angles, and an angle needs a vertex. The vertex is the ear of the primary seat: not the centre of the room, not the centre of the screen, and not the centre of the sofa. Those three points are rarely in the same place, and choosing the wrong one is the single commonest reason a system measured with a tape still sounds lopsided.

The second datum is at the other end of the line. Distances are measured to the acoustic centre, which for most speakers is the tweeter, not the front corner of the cabinet and not the grille. On a floorstander with the tweeter 2 in (5 cm) behind the baffle and 40 in (102 cm) up, measuring to the cabinet edge on the floor can be 6 in (15 cm) out, and 6 in of path error is 0.44 ms of delay error in a channel the receiver believes it has aligned.

Take the room dimensions before any of this, in the order set out in how to measure a room properly, and mark the seat position on the floor first. Everything below is drawn from that mark.

The circle that ITU-R BS.775 actually describes#

The reference layout is an arc, not a rectangle. ITU-R BS.775 places the centre channel at 0 degrees, left and right at 30 degrees either side, and the two surrounds at 110 degrees, with every speaker at the same distance from the listener. The equal distance is the part that gets dropped, and it is the part that the standard cares most about, because it is what makes a phantom image between two channels sit still.

At a reference radius of 10 ft (3.05 m), the arc turns into coordinates a tape measure can find.

Read the third column of the surround row again. Nine point four feet either side of the centreline means the two side surrounds want to be 18.8 ft (5.73 m) apart, in a room that also has to be long enough to hold a 10 ft viewing distance. That is a 20 ft (6.1 m) wide room, and it is the reason the next section exists.

Why the surrounds never land on the circle#

Take a 15 ft (4.57 m) wide room. The side wall is 7.5 ft (2.29 m) from the centreline, so 90 in (229 cm) is all the lateral offset available. Solving the 110 degree geometry backwards, a surround at 90 in of lateral offset sits at a radius of 90 divided by sin 110, which is 95.8 in (2.43 m), and 32.8 in (83 cm) behind the ear. It is at the right angle and the wrong distance: 8 ft (2.43 m) instead of 10 ft (3.05 m).

Two corrections follow, and both are arithmetic rather than opinion. The level difference from the inverse square law is 20 log (120 / 95.8), which is 2.0 dB, so the surrounds must be trimmed down by that much. The path difference is 24.2 in (61 cm), which at 13.56 in per millisecond is 1.8 ms, so the receiver needs the shorter distance entered, not the one the standard implies. In a 13 ft (3.96 m) room the same sums give a radius of 83 in (211 cm), a 3.2 dB trim and 2.7 ms of delay.

This is why automatic calibration exists and why it is worth running. It is also why the microphone position matters more than the microphone: the trims are only right for the seat the microphone occupied.

Tweeter height and the seated ear line#

The trade figure for tweeter height is 36 to 42 in (91 to 107 cm). Derived from body dimensions rather than from habit, it comes out higher.

The published band assumes a low, reclined seat and a shortish user. Our recommendation is to stop quoting a band at all: sit in the actual chair, have somebody measure from the floor to the tragus of your ear, and use that. It will usually land between 40 and 46 in (102 and 117 cm). The same measurement governs eye level and sightlines for the screen, and it is worth taking once and writing on the plan, because seat height and depth move it by more than any speaker stand will.

Then relax about the last 3 in (8 cm). A tweeter 6 in (15 cm) off the ear line at 10 ft (3.05 m) is 2.9 degrees off axis, which is inside the flat window of almost any decent speaker. The same 6 in error at 3 ft (91 cm), which is where a desktop or gaming setup lives, is 9.5 degrees and clearly audible. Height tolerance scales with distance, so the nearer the speaker, the more precisely it has to be aimed.

The overhead layer for Atmos#

Dolby's home guidance puts the overhead pairs at an elevation of 45 to 55 degrees from the seated ear, measured in the vertical plane, with the pair spaced laterally to match or slightly exceed the front left and right.

On an 8 ft (2.44 m) ceiling with the ear at 42 in (107 cm), the speaker is 96 minus 42 minus 2 for the flange, so 52 in (132 cm) above the ear. At 45 degrees the horizontal offset equals that height difference: 52 in (132 cm) in front of the seat and 52 in behind it, giving a front to back spacing of 8 ft 8 in (2.64 m). At 55 degrees the offset falls to 36 in (91 cm). At 65 degrees it is 24 in (61 cm).

Raise the ceiling to 9 ft (2.74 m) and the height above the ear becomes 64 in (163 cm), so the 45 degree offset grows to 64 in and the pair moves 10 ft 8 in (3.25 m) apart. Overheads spread out as ceilings get taller, which is the opposite of what most people assume when they mark out the ceiling, and it matters when the room is short.

Joists rarely agree with any of it. At 16 in (406 mm) centres the nearest bay can be 8 in (20 cm) from the ideal mark, which turns a 45 degree elevation into 41 or 50 degrees. Both are acceptable. What is not acceptable is moving a speaker two bays to keep the pair symmetrical about a light fitting instead of about the seat. If the joists run the wrong way, the honest options are height speakers on the front wall at 30 to 55 degrees of elevation or a dropped bulkhead, and the depths that costs are set out in ceiling beam and coffer dimensions.

Subwoofers: position before power#

A subwoofer's position changes its output by more than its amplifier does. In a rectangular room the axial modes sit at 565 divided by the dimension in feet: 31 Hz along an 18 ft (5.49 m) length, 38 Hz across a 15 ft (4.57 m) width and 71 Hz between an 8 ft (2.44 m) floor and ceiling. Every one of those modes has a pressure maximum at both boundaries, which is why a corner is the loudest place for a subwoofer and the least even.

The configurations that measure flattest across several seats, from Welti and Devantier's published work on multiple subwoofers, are two units at the midpoints of the two side walls or four at the midpoints of all four walls. Two at the midpoints cancels the odd-order width modes at the centreline and evens out the length modes, and in an 18 ft room the midpoint is 9 ft (2.74 m) back from the screen wall, which is usually clear of both the seats and the front speakers. Placing the pair diagonally in opposite corners is the fallback when the walls are full.

Two practical numbers go with it. Leave 8 to 12 in (20 to 30 cm) of clear air behind a rear-ported cabinet, measured from the port mouth rather than the case, and enter the measured distance for each subwoofer separately rather than averaging them.

Boundary distance and the null it puts in the response#

Any driver near a wall hears its own reflection. When the extra path is half a wavelength the direct and reflected sound cancel, and the null frequency is 1,130 divided by four times the distance in feet.

The null bites hardest between roughly 60 and 200 Hz, because that is where a woofer radiates in all directions and the reflection comes back at nearly full strength. In practice that means keeping the cabinet within about 12 in (30 cm) of the front wall or pushing it beyond 60 in (152 cm), and never leaving it at the 24 to 36 in (61 to 91 cm) that a shallow media wall naturally produces. The same reasoning applies to the wall to the side and to a media wall or fireplace surround that recesses the speakers into a niche.

Setting it out with a tape measure#

Everything above is checkable in half an hour with a tape, a laser and a chair. Tape the layout out at full size first if the room is not built yet, using the method in taping out a layout, because a 110 degree line on the floor makes the width problem obvious before anything is ordered. The room proportions themselves come first of all, and they are covered in home theatre room dimensions; what the treatment then has to do is in acoustic treatment placement, and the boxes that all of it plugs into are in media console and equipment rack dimensions. The wider set of published layouts and where they come from is indexed in the standards and codes reference, and the rest of the room is in the media room dimensions hub.