Home theatre systems

Surround and immersive speaker layouts

Where each loudspeaker goes in 5.1, 7.1 and immersive layouts such as 7.1.4 — the angles, the heights, and how the layout adapts to more than one row.

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Schematic

Angle zones measured from the reference seat. Front left and right at ±22–30°, centre on the screen axis, side surrounds at ±90–110°, rear surrounds at ±135–150°, and overhead pairs above the listening area at around 45° elevation.

Animated illustration of the principle. Not measured data.

A film mix is made in a room with loudspeakers at known positions. Reproduce those positions at home and sounds arrive from where the mixer put them; move them and the soundtrack moves with them. Speaker layouts are therefore not taste. They are published geometry, measured as angles from a reference seat.

Reading the names

5.1 means five main loudspeakers and one low-frequency effects channel. Immersive formats add a third number for the overhead layer: in 5.1.4, “the .4 indicates the four speakers that play overhead sound”, as Dolby’s installation guide puts it. 7.1.4 adds a pair of rear surrounds to that; 7.1.6 and 9.1.6 add more surrounds or overheads. The ITU writes the same idea as upper + middle + bottom layers — 4+5+0, 4+7+0 and so on.

Immersive formats are largely object-based: sounds are stored with positional data and the processor works out how to reproduce them on the loudspeakers actually installed. Dolby describes its content as “not tied to any specific playback configuration”, and DTS:X likewise is “not tied to prescribed speaker configurations”. More loudspeakers in the right places means the processor has more precise positions to render to — not that a different mix is played.

The ear-level layer

The international reference for 5.1 is ITU-R BS.775: front left and right at ±30° (an arc of 60°), centre straight ahead, and the surrounds between 100° and 120°, no closer to the listener than the fronts. AV receiver makers give the familiar home ranges as ±22–30° for the fronts, 90–110° for side surrounds and 135–150° for rear surrounds.

Height matters too. BS.775 places the acoustic centre of the front loudspeakers at roughly ear height, with the centre immediately above or below the picture — or behind an acoustically transparent screen. Dolby’s guide puts all listener-level loudspeakers at the same height, typically about 1.2 m, and its home guidance describes the listener level, or up to about 0.6 m above it, as optimal for surrounds.

Dolby does not recommend dipole surrounds for immersive playback. Direct-radiating loudspeakers aimed at the listening area are the norm.

The overhead layer

Overhead loudspeakers sit in pairs above the listening area. Dolby recommends an elevation angle of 45° from the reference seat, adjustable between 30° and 55°, with the pairs spaced about as wide as the front left and right. The ITU’s advanced layouts use the same 30–55° band for the upper layer. With two overheads, Dolby suggests mounting them slightly in front of the listening position; with four, roughly one third of the way in from the front and rear walls.

Auro-3D builds its height layer differently — a second ring of loudspeakers directly above the ear-level ones, elevated about 30° (25–40°), plus an optional single top loudspeaker directly overhead. A room designed for more than one immersive format needs those positions reconciled at the drawing stage.

Up-firing (“Dolby Atmos enabled”) loudspeakers bounce sound off the ceiling instead. They need a flat, reflective ceiling between 2.3 and 4.3 m high, should be no higher than half the wall height and at least 0.9 m (ideally 1.5 m) from the seats. In a dedicated room with a treated ceiling, ceiling-mounted loudspeakers are the better choice.

More than one row

Every angle above is measured from one reference seat. Move one row back, or one seat to the side, and every angle changes — the surround that was at 100° may now be at 80°. Layouts for a whole audience are therefore a compromise worked out in plan for every seat: wider rooms add surround pairs so no seat is far closer to one loudspeaker than the others, and Dolby’s guide shows larger layouts with additional side and rear surround positions for this reason.

The test is coverage: each seat should hear every channel at a similar level and from roughly its intended direction. That is checked on a drawing with the seats and the loudspeakers’ coverage angles, before any cable is pulled.

Matched loudspeakers

Sounds pan between loudspeakers, so loudspeakers that sound different make a moving sound change character as it travels. The EBU’s multichannel listening guidance calls for matched channels and stresses that close matching of the three front loudspeakers is especially important; ITU-R BS.1116 asks for front loudspeakers matched within 1 dB from 250 Hz to 2 kHz in critical listening rooms; and Dolby asks for overhead loudspeakers “timbre matched as closely as possible” to the main ones.

For integrators and architectsThe numbers, the standards, and what to specify

Ear-level: ITU-R BS.775-4: L/R ±30°, C 0°, surrounds 100–120°, side/rear no closer than fronts; front acoustic centres approximately at ear height. Home ranges (AVR makers): fronts ±22–30°, sides ±90–110°, rears ±135–150°.
Overheads: Dolby: 45° elevation, 30–55° permitted, pair width ≈ L/R width; four overheads ~⅓ in from front and rear walls. ITU-R BS.2051 upper layer 30–55°. Auro-3D height ring ~30° (25–40°) above the ear-level speakers.
Heights: Listener-level loudspeakers at the same height, typically ~1.2 m (Dolby); rear no more than 1.25× the height of the fronts.
Up-firing: Ceiling 2.3–4.3 m, flat and reflective; module no higher than half wall height; ≥0.9 m (ideally ≥1.5 m) from seats. Prefer in-ceiling in treated rooms.
Matching: BS.1116-3: front loudspeakers matched within 1.0 dB, 250 Hz–2 kHz. Same family across the screen channels; overheads timbre-matched (Dolby).

What to write into the brief

A plan and section drawing with every loudspeaker’s azimuth and elevation from the reference seat, the format(s) supported, and the coverage check for every seat.

Sources

External links open in a new tab. Figures quoted in this chapter are taken from these sources; a Science of Sound measurement is only ever presented as one, with its file, date and instrument.

Keep reading

Home theatre systemsScreen size, seating and sightlinesHow big the screen should be for the seats, how far back each row sits, how high the risers go, and what an acoustically transparent screen does to the sound.Home theatre systemsChoosing loudspeakers for a roomWhat actually distinguishes a good loudspeaker — directivity, matching and output — and how to check one will play loud enough for the room.Home theatre systemsSpeaker placement and the roomWhy the distance from a loudspeaker to the wall behind it changes the bass, and how placement is the cheapest acoustic treatment there is.

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