By Andy Munro
[Editor’s note: Whether it is to check your own mixes, have a demo room for your clients, or for your home enjoyment, at some point you’re going to need the services of a home theater. Here, Munro Acoustics, a leading U.K. acoustical design firm (which explains the use of the metric system throughout the article), lends their expertise and gives you the important facts you need to know before building. These also work when it comes time to re-do your studio’s control room.]
If one is to presume that high performance means comparable to professional standards, then it is prudent to make some basic definitions that can be used as a benchmark for all concerned.
To qualify as a theater, any space must be suitable for both listening and watching filmed entertainment. It must therefore be both quiet and dark, at least for the duration of the performance. The space must also be large enough for the required audience and it must be acoustically neutral enough to allow the recorded sound track to dominate the soundfield as perceived by the listener. The screen size and sound system must be carefully matched to the space to avoid distortion and disappointment.
Image Size
An electronic screen is limited by both cost and technology to not much more than a meter wide, so the maximum viewing distance for a cinematic feel is about 3 meters [1 meter = 3.2808399 feet] at most. This is fine for many people, and small freestanding loudspeakers that can be arranged to suit the furniture best serve the sound matching for such a screen. Even at 3 meters the room acoustics will be sufficiently influential to warrant some damping of reflections and reverberation, so more on this later.
Anything bigger in terms of image size and we are talking projection, larger speakers and a serious attempt to re-create the atmosphere of a movie theater, albeit scaled to fit where required.
I use the simple rule that the maximum viewing distance should be double the screen width, so for a 7-meter room, the screen should be 2.5 meters wide and therefore 1.4 meters high for a 16.9 aspect ratio.
Listening Position
The ideal seating position subtends an angle of 45 degrees to the screen, and the speakers also need to be placed at the outer edges of the screen, except for the center unit, which should have its apparent acoustic center at 2/3 the screen height, to localize on-screen dialog to the most likely point. I like to use direct radiating loudspeakers in smaller theaters, as they sound so much more natural than horns. This means lower efficiency of radiation, which in turn means more amplifier power is required but as long as the room volume is under 250 cubic meters, this does not present a problem. I always calculate the exact power requirement for each room, for a given speaker system and acoustic absorption. Ironically, the fairly dry acoustic (typically T60=0.3 seconds) required for small film theaters means even more power is required. As the room size increases, so too does the power requirement until more is required than the speaker can handle, and so horns and multiple drivers must be employed. This is really the break point between what is essentially quasi hi-fi equipment and the more prosaic world of professional audio.
Acoustic Requirements
This is probably the least understood part of the residential theater market in that décor seems to takes precedence over science. If the room is larger than 250 cubic meters, then the listening position will be beyond the critical distance (Dc, where the direct sound and the reverberant energy of the room are equal in scalar terms). This is good in that everyone will listen at the same averaged sound level, but potentially unpleasant if the acoustics do not produce an even, diffused reverberation at all frequencies. We are now entering the zone occupied by the infamous “X” curve (based on SMPTE, etc.).
Imagine you are sitting outside, in mid air, 1 meter in front of a loudspeaker that has a flat response and capable of fairly high output. What you hear in the absence of any room is the natural axial balance of that speaker. Moving away from the speaker, but staying on axis, you should hear the same sound balance but quieter, –18 dB at say 8 meters. By increasing the power output of the amplifier, the sound spectrum will look identical to that at 1 meter.
Now place yourself in a room of about 200 cubic meters and everything changes; the loudspeaker spreads low-frequency energy around like confetti, but high frequencies are still mainly confined to a narrow beam on-axis. The room surfaces mop up what little HF there is, but do little to attenuate the very low frequencies. The all-important mid range is acting somewhere in between, and depends on the speaker design for the evenness of coverage. Put the whole lot together, and the combined sound spectrum will look a little like the “X” curve published in ISO2969.
Fig. 1 shows the direct sound, Ld, in octave bands, as a “perfect” response and the reverberant sound, Lr, in a room with typical small theater acoustics. Note how much louder the LF is from the room compared with the direct sound. The combined sound, Lp, looks a bit like the X curve and, if you add screen filtering and air absorption plus the usual cinema horn boost of mid frequencies in the direct sound, you are home and dry (ish). (See fig. 2.)
The main point is that one must be very careful not to apply general rules to any system response equalization in small rooms because a well-designed room with controlled LF absorption and good diffusion, with a wide-open speaker response, will not sound good if forced to follow the X curve. The whole point is to make all theaters sound “compatible,” and one simple curve will never do that whereas a carefully constructed algorithm can be applied to any room and a suitable X curve generated.
Bigger is Better?
It is a myth that large rooms always sound better than small ones — good acoustics can be achieved on almost any scale. However, the choice of equipment and the acoustic design is critical to the achievement of an accurate reproduction of the original soundtrack that probably was mixed in a large theater. Ironically, the surround sound channels can be used to even greater effect in a small room because they can be used to re-create large space acoustics more effectively than in a huge theater where they often struggle with localization and coverage problems.
Good design is more than just choosing and installing the right equipment. A theater of any size deserves correct acoustic design, including mechanical and electrical services, as well as the final seductive gloss of the interior designer.
When the lights go out you, too, could be wherever the movie takes you.
For more information, visit www.munro.co.uk.
