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A cinema room is an acoustic and optical instrument

A television in a dark room is not a cinema. What separates the two is that everything in a real cinema room — its dimensions, its surfaces, the distance from the seats to the screen, the angle of every loudspeaker — is the result of a calculation, not a preference. Get the room wrong and no amount of equipment will rescue it. Get the room right and even modest equipment sounds and looks extraordinary.

iHome has been designing and building private cinema rooms in the Algarve since 2006. What follows is the method we apply to every project, and the standards we hold it to. We publish it because the decisions that matter most are taken before anything is bought — usually before the walls are even built — and because a client who understands them makes better ones.

The room comes before the equipment

Every rectangular room has resonances. Between any two parallel surfaces, sound builds a standing wave at a frequency determined purely by the distance between them:

f = 343 ÷ (2 × distance in metres)

A room 5 m long resonates at 34 Hz. A ceiling 3 m high resonates at 57 Hz. At those frequencies, bass will be overwhelming in some seats and nearly absent in others — and no equaliser can fix a null, because you cannot amplify energy that cancels itself.

What can be fixed is the distribution of those resonances. When the three dimensions of a room are proportioned badly, modes pile up at the same frequencies and reinforce each other. Cubic rooms are the worst case: length, width and height all resonate together. Simple integer proportions — 1:2:3, 1:1.5:2 — are nearly as bad.

Where we have influence over the shell, we aim for proportions close to 1 : 1.4 : 1.9 (the Louden ratio), which spreads modes evenly across the bass region. Where the room already exists, we measure what we have and design the treatment and the loudspeaker positions around its specific behaviour.

The frequency that divides the two halves of the problem is the Schroeder frequency:

f_s ≈ 2000 × √(T60 ÷ volume)

For a 60 m³ room with 0.4 s of reverberation, that is about 163 Hz. Below it, the room behaves modally and is treated with bass traps and considered subwoofer placement. Above it, the room behaves statistically and is treated with absorption and diffusion. Two different problems, two different solutions — and a great many rooms are built having addressed only the second.

Acoustic treatment: how much, and where

Reverberation time is the first number we commit to. Our targets, by class of room:

• 0.3 – 0.5 s — THX reference for cinema rooms
• 0.25 – 0.4 s — CEDIA Tier 4
• 0.2 – 0.35 s — our reference specification, and the drier end of the range

Reaching those figures in a domestic room typically means 25 – 40% of total surface area as broadband absorption, plus a minimum of four corner bass traps in any room above 40 m³. Materials matter as much as quantity: bare plasterboard absorbs about 5% of the energy that strikes it at 500 Hz, carpet over underlay 30–60%, a proper broadband panel 85–100%, and a corner trap 70–95% below 200 Hz.

The rear wall is usually treated harder than the front. A reflection arriving from behind the seats a few milliseconds after the direct sound produces comb filtering, which is heard as a hollow, phasey quality in dialogue — and is one of the most common faults in rooms that were otherwise well equipped.

Screen size and seating distance are one decision, not two

Screen size is not chosen by budget or by wall width. It is derived from where people will sit, because what governs immersion is the angle the image subtends at the eye.

Standard	Minimum	Recommended	Maximum
THX	26°	36°	40°
SMPTE	28°	30°	36°
CEDIA	30°	36°	40°

Expressed as distance, the SMPTE reference seat sits at 1.26 × screen width, or three times the screen height, giving a 43.4° field of view. The furthest seat THX still recommends is at 1.54 × screen width — 36°. Beyond about 2.17 × screen width the image no longer fills enough of the visual field for the effect to hold, whatever the resolution.

For reference-grade rooms we work closer than the SMPTE figure, at roughly 2.2 × screen height, which approaches the field of view of a Dolby Cinema auditorium. That is a deliberate choice and not for every client — it is the single decision most worth experiencing before committing to, which is why it is worth sitting in a finished room before signing a drawing.

Geometry constrains the rest. The primary seat sits at roughly two thirds of the room length. There should be at least a metre of space behind the last row, both for acoustics and for the rear loudspeakers. The room should be about 30 cm wider than the screen on each side. Ceilings below 2.5 m compromise the overhead layer; 3 m is where we would like to be. Where there is more than one row, a 30 cm riser keeps both sight lines and listening lines clear.

Projection: throw, shift and brightness

Projector selection begins with a single ratio — throw distance divided by image width. A 4 m wide screen at 6 m gives a throw ratio of 1.5, which most standard cinema lenses cover. Short-throw and long-throw situations narrow the field considerably, and the lens is often the harder constraint than the projector body.

We place images optically, using lens shift, and avoid digital keystone correction wherever possible: keystone works by scaling pixels, which is a loss of resolution applied permanently to every frame.

Brightness is where specifications are most often misread. DCI commercial cinema calibrates to 14 foot-lamberts, about 48 nits — considerably dimmer than most people expect. HDR material, however, wants 100 to 300 nits at peak to show what it was graded to show. Required output follows from screen area and gain:

lumens = (screen area in m² × target nits × π) ÷ screen gain

A 4 × 1.7 m screen at 50 nits needs roughly 1,100 ANSI lumens; the same screen with headroom for HDR needs more than double that. Acoustically transparent screens — which allow the front loudspeakers to sit behind the image where they belong — have a gain near 1.0 and cost light, so they must be accounted for at specification, not discovered at commissioning.

Sound: the reference is a number, not an opinion

Cinema sound is calibrated to a standard. Each main channel reaches 85 dB SPL at the reference position for average level, with 20 dB of headroom above it — so 105 dB peak per channel. The low-frequency effects channel runs 10 dB hotter still, at 115 dB peak. A system that cannot reach those figures cleanly is not reproducing what was mixed.

That requirement drives amplification. Required power follows from loudspeaker sensitivity and listening distance, and the numbers are larger than most expect: a 94 dB/W/m loudspeaker at 4 m needs roughly 400 W of peak capability per channel to hit 105 dB with margin.

Loudspeaker angles are equally specified. Left and right sit between 22° and 30° off centre, matching the width of the image. Side surrounds fall between 90° and 110°, rear surrounds between 135° and 150°. The overhead layer is placed by elevation: front heights around 25°, top middle between 65° and 85° — effectively overhead — and top rears between 25° and 45° behind. The centre channel belongs directly behind the screen, which is only possible with an acoustically transparent surface.

Subwoofers are a room problem before they are a product problem. A single subwoofer in a corner will always produce large seat-to-seat variation. Multiple units, positioned by measurement rather than convenience and driven with individual correction, are what deliver consistent bass across every seat rather than only the middle one.

Calibration is where the room is finished

A cinema room is not complete when the last cable is terminated. Every loudspeaker is measured in place, levels and delays are aligned to the seating position, and the frequency and phase response of the system is corrected against a target curve. In multi-row rooms we measure multiple positions so that correction serves the whole seating area and not a single chair.

This is not a quick procedure. A single-position calibration takes about an hour. A full multi-position calibration of a reference room, including the measurement grid and target-curve work, takes the better part of a day. It is also the step most often omitted, and the one that most reliably separates a room that measures well from a room that merely looks expensive.

How a project runs

1. Survey and feasibility. Room dimensions, proportions, construction, services, and what the room has to do besides showing films.
2. Design. Acoustic model, seating geometry, screen and projector selection, loudspeaker layout, and the electrical and structural implications — issued as drawings that a builder can work from.
3. Coordination. We work alongside the architect and the main contractor. Cinema rooms fail more often from missed conduit and inadequate structure than from equipment choices.
4. Installation and commissioning. Fit-out, termination, configuration, measurement and calibration.
5. Support. Ongoing maintenance and remote assistance.

Working with iHome

We are an official Control4 dealer and a certified KNX partner, and have been integrating technology in Algarve homes since 2006. Our work has been recognised internationally by Control4, with European Best Lighting Project for Lote 21, Alma do Mar, and nationally by the Portuguese KNX Association, with the Prémio Projeto KNX 2020 for Dunas Douradas Beach Villas. More on our awards.

We have a dedicated home cinema showroom in Quinta do Lago, where a full Dolby Atmos installation can be heard and seen rather than described. Almost every decision on this page — how close to sit, how large a screen should be, what calibrated bass actually sounds like — is settled in twenty minutes in a finished room, and never settled properly on paper.

To arrange a visit or discuss a project, call (+351) 289 090 900 or email geral@ihome.pt.