Wednesday, September 13, 2006

The CAVX Approach To Room Design

Home Theatre is about recreating the Cinematic Audio Visual eXperience in the home. To do this properly, one must understand at least the very basics of how a real cinema works.

Home Theatre follows the guidelines of a real cinema layout in that we use three speakers for the screen channels [Left Centre Right], but rather than an array of speakers on the side walls [extending to the back wall] we use just two surround speakers [located at the sides of the listening position]. Like real cinemas, the speaker layout also includes a sub-woofer [for the low bass extension as well as the LFE channel].
A Look At A Real Cinema
This photo was taken in a real cinema that is equipped for Surround EX playback. Your can see the multi-speaker arrays on both the side and rear walls. This cinema has a total of 18 surround speakers [6 on each wall] to form a very enveloping sound field. Note the sound absorption panels on the back wall.

With the introduction of Extended Surround systems in the home came an additional pair of speakers on the back wall. I am a believer that the four surround speakers should be the same make and model [the three LCR screen channel speakers should also be identical, but may differ from the surrounds], and should be spaced symmetrically from +/-90degrees to +/-150degrees so as to form three equal lateral triangles between each speaker back to the listening position.

Room Design
For new construction, it is best to use staggered proportions of Length/Width/Height where 1 is the height of the new room. The following are examples of "staggered" ratios that work.

1.0 : 1 x 1.3 : 1 x 1.6:1

1.0 : 1 x 1.6 : 1 x 2.3:1 [used in the diagram above]

1.0 : 1 x 1.6 : 1 x 2.6:1

The reasons you need staggered ratios is that each plain [L W H] will create and resonate at a particular frequency and [along with harmonics of the principal frequency] produce both peaks and dips in the frequency response. If these peaks and dips are too close to one another, they compound to create a very rough in room response and this will affect the sound you hear regardless how good your speakers are.

For existing construction, avoid rooms with L to W and W to H ratios of 1.1 : 1.0 or +/-5% and avoid rooms where the depth is greater then 3 times the height.

If you have to work with in such a room, you might consider building a false wall or changing the ceiling height. An infinite baffle like those used behind an A.T. Screen can help here as this new wall effectively becomes the rooms new "boundary". Other uses for this left over space might include dedicated EQ Rack rooms or even a projection booth. I recently completed my own home cinema using these concepts.

Viewing Angles
The image above is based on cinema viewing angles, but is ideal to work off for the home as well.

Mathematics In 3 Easy Steps

You want your viewing experience to be enjoyable, not fatiguing, so designing your room to give comfortable viewing angles is as important as the shape of the room itself. According to THX, the "preferred" viewing angle for "cinema scope 2.39:1" is 36 degrees. Over the years, this "preferred" viewing angle has also been used for the home. However, unless your using a "Scope" screen such as in a CIH set up, you should not be working off the width of the screen, but rather the height of the projected image. This is regardless of the resolution of the display, but more important to the Standard Definition resolution.

1. Take the Room Length and divide it by any number between 3.68 and 5.18 to get the Screen Height.
2. Take the Screen Height and multiply that by 2.0 to get the "Closest" Seating Distance.
Note: 3.68x the screen height equates to 36 degrees, which is the "preferred" viewing angle for CinemaScope, but you may sit as close as 2x the screen height.
3. Take the Screen Height and multiply that by the Aspect Ratio to get the Screen Width.

The 4 common Aspect Ratios for video are 1.33:1, 1.66:1, 1.78:1 and 2.37:1 and if the above formula is used, will allow the creation of the perfect Constant Image Height set up.

NB:The screen widths for "Scope" images using video are 16 / 9 x 1.33 = 2.37:1, not 2.35:1 or 2.39 [CinemaScope]...

Room Treatments

Real cinemas [like the one pictured above] are designed to have controlled reverberation and sound isolation. At a minimum, you need to find and treat the first reflection points in your room. These points will most likely be on the back wall (caused by sound from the LCRs), but may also extend onto the side walls.

Where possible, lining the complete front of the room (first 3rd) with sound absorbing material will work the best. The bottom half of the other 2/3rds of the room should also be treated. You can cover the treatments with fabric to give them a higher WAF. Treating a room properly is a science and therefore is outside the scope of this page and why I have not discussed bass traps and other specialized treatments.
CAVX Diffuser
So in the interest of improving the acoustics of my own room, I have developed an acoustic tile that acts as a diffuser. Basically it a series of non even surfaces that bounce sound in many directions at the same time. The tile is made from black ABS plastic that has been vacuum formed over a mold that I made. Here the diffuser has been placed between my back surround speakers and it had changed the way they sound in a good way.



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