Tech Notes – Boyce Audio Resources

Introduction
In our last article on home theater layout, we spent some time looking at the video issues affecting the room. In this article, we will look at the audio issues that can influence room layout and speaker configuration.

Room Modes
When it comes to audio, there are important relationships between the room’s length, width, and height, and the way your room sound. For home theater, some combinations should be avoided. This is because of what is known as standing waves. If the wavelength of a particular tone happens to be a multiple of the distance between two opposing walls (or ceiling and floor), that wave will be reflected back and forth between the two walls. It will be reinforced relative to other tones, and also tend to hang around long after other tones have died out. The frequency at which this happens is called a room mode. This particular case of standing wave is called an axial room mode.

If the wave is reflected around the room in a single plane via four walls (or two opposite walls, the ceiling, and the floor), this forms a tangential room mode when the total distance of this trip is a multiple of the wavelength of a tone. Tangential room modes also increase the relative level of affected tones, but only about half as much as axial modes.

Waves can also reflect around the room via each surface in the room, i.e. four walls, the ceiling and the floor. When the total distance of this round trip is a multiple of the wavelength of a tone, we get an oblique room mode. Oblique room modes reinforce affected waves only about a quarter as much as axial modes.

Room modes are not a big issue for higher frequency waves, because many thousand different frequencies will fall into this category, and they all tend to blend together to form a homogeneous sound. This is not the case with bass frequencies below about 300 Hz, however. At these frequencies, the room modes are pretty isolated, and once excited, can lead to certain frequencies sounding boomy and muddy. Now you can begin to see why certain dimensions of rooms should be avoided. A cube would be the worst, since the same few bass frequencies would be reinforced between each axis, leading to annoying boominess for sure. The next worse layout would be a room where any two dimensions were the same, e.g. a square room.

Dealing with Room Modes
There are few ways to deal with room modes. The first is through careful design. It turns out that there are certain dimensional ratios that result in a fairly even distribution of modes at the low frequency end of the audio spectrum. Sometimes referred to as golden ratios, these dimensions were researched by L. W. Sepmeyer. There are many ratios which give good results in this regard, but three popular ratios shown in Table 1. Table 1

In the post construction phase, we can use acoustical treatments to combat the harsh effects of room modes. These include broadband absorbers, such as foam absorbers, traps, and even furniture; and tuned absorbers, such as bass traps and Helmholtz resonators.

While it is possible to place acoustical treatments by guess and experience, it is best to have the room acoustically analyzed to first determine the extent of the need for treatment, and during the treatment process, to see exactly what effect the treatment is having on the room. Sometimes, even minor repositioning of a speaker can have drastic effects on the quality of the audio at the main listening position, and may stem off the need for elaborate room treatment.

Speaker Layout
Nowadays, the typical home theater speaker layout is either a 5.1 layout, or a 7.1 layout. Since 5.1 can be considered a subset of 7.1, we will mention it only in the context of discussing 7.1 layouts.

Dobly Digital Surround is considered to be a 5.1 surround format because it has five discrete channels of audio (front left, center, front right, surround left, surround right) plus a low frequency effects (LFE) channel. Dolby Digital EX is exactly like Dolby Digital Surround, with the addition of a matrixed mono surround back channel. Because this additional channel is not a discrete channel, calling it a 6.1 format has been discouraged. Many home theater receivers however, send this mono channel to two separate amplifier channels, named surround back left and surround back right. Thus we now have seven speakers plus a subwoofer to be placed in the home theater room. Some people call this arrangement 7.1.

Dolby Digital EX
Not surprisingly, there are different recommendations from the developers of the current home theater audio formats regarding speaker placement. Dolby Laboratories, (which developed Dolby Digital EX together with Lucasfilm THX) has the following recommendations for setting up the speakers for a “7.1” scheme.

The center speaker should be in the center of the screen. For a front projection setup, it is best to put it behind the screen. Otherwise, above the screen is better than below the screen.

The left and right front speakers should be between 22 and 30 degrees off the screen center, and at ear level.

The surround left and right speakers should be between 90 and 110 degrees from the listening position, and about 2 feet above ear level.

The surround back left and right speakers should be between 135 and 150 degrees from the listening position. They should be about 2 feet above ear level.

The low frequencies produced by the subwoofer are typically non-directional, so theoretically, the subwoofer can be placed anywhere in the home theater room. Practically however, note that a subwoofer that is placed on the floor along a wall will be 3 dB louder that if were not on the floor. And a subwoofer placed in a corner will sound 6 dB louder. This can lead to a boomy bass sound on certain frequencies.

THX EX
The THX recommendation for speaker placement is a little different from Dolby.

The left and right front speakers should be about 23 degrees off the screen center (i.e. 46 degrees included angle).

The surround back left and right speakers should be placed next to each other, directly behind the listening position, if the digital decoder has THX ASA (Advanced Speaker Array) decoding. Otherwise, these speakers should be placed as for Dolby recommendations. They should be about 2 feet above ear level.

THX recommendations for other speaker placement is the same as the Dolby recommendations. Note however, that THX insists on dipoles for the surround left and right speakers, and insists on monopole speakers for the surround back channels.

Conclusion
With the right set of modern home theater audio decoders and amplifiers, it is possible to get very good sound to accompany your video. Coupled with a properly laid out and treated room, and a set of high quality speakers configured to match your decoder, you will get awesome sound. Together with the proper video display, your home theater will give years of delightful entertainment.

Introduction
The best room for home theater is a dedicated room. But most people are not so lucky. For many, the home theater will be a shared family room, living room, or bedroom. This will be a compromise at best.

Shared Room
When the home theater is in a shared room, the “home theater” part of the equation is usually secondary. Prime consideration must be given to décor, lighting, seating arrangement, traffic, and so on. For the most part, the home theater equipment needs to be unobtrusive and fit the environment. In-wall speakers with grills painted to match the walls can be used to help solve integration problems. The room’s draperies and upholstery can be used to help solve acoustical problems. For a bright room, window blinds and a bright video display may be required. A thin Plasma or LCD display will be plenty bright for almost any application, and take less space than a rear projector. However, remember that LCDs tend to have more restrictive viewing angles that Plasma. If the room is big enough to use a front projector, a high contrast grey screen may be used to improve contrast and black levels. When Sony’s black screen that reflect only red, green, and blue light, but absorbs all other colors becomes available, it will offer even higher contrast and definition than grey screens.

Dedicated Room
A dedicated home theater room is the best route to take if you can afford it. With a dedicated room, you can be as creative as you want with the design and still adhere to the good design practices established for home theater.

Video Issues
Home theater is about audio and video. A lot of emphasis is placed on getting the audio of the home theater to be just right. However, preparing the room for good video presentation is just as important as designing for good audio. For the remainder of this article, let’s look at some of the things that affect the quality of the viewing experience.

Room Décor
The walls of the home theater should be painted in somewhat dark colors. This is to help control the amount and color of light that is reflected back onto the screen during a screening. Any light that falls on the screen and which does not come directly from the projector will washout and color the image somewhat. Dark walls help to minimize this. Similarly, drapes, blinds, etc. in the room should be chosen to minimize reflected light. Light fixtures that will be on during the movie should be designed and installed so that they do not cast a beam on the screen. There light output should be fairly confined, and they look better when the light output is diffuse.

The Display Screen
In a front projection system, often the screen is not given enough attention. However, it is one of the most important links in the video chain. The primary job of the screen is to reflect the light form the projector in a uniform way, so that a bright image can be seen from a wide viewing angle. However, besides reflecting light, screens also absorb some light, and transmit (i.e. allow light to pass through) the rest. Typically, we want a good screen that minimizes the latter two. In this discussion, we will consider flat screens only. The discussion also does not apply to Sony’s black screen technology.

The amount of light that a screen reflects is called the gain of the screen. The gain is determined by the manufacture by comparing the screen’s reflective strength to a standard magnesium carbonate reflector under standard lighting. A screen with a gain of 1 would reflect the same amount of light as the standard. A matte white screen has a gain of very close to 1. Brighter screens have gains bigger than 1. Darker screens will have gains less than 1. Typically, you will want to get a screen with a high gain. This will allow two things: 1) you can have a bigger image and it will remain bright. 2) You can lower the light output from your projector, extending component life. However, a screen with very high gain may suffer from hot spotting, where the center of the screen image is annoyingly bright compared to the rest of the picture.

The gain numbers presented by manufacturers are taken form directly in front of the screen. Except for matte white screens, the gain tends to fall off as the viewer moves off-center. Thus if your home theater has a wide seating arrangement, you will want to get a screen that has a wide viewing angle, i.e. maintains (close to) its gain over a wide angle.

The screen composition can also affect color purity. Some screens, e.g. some pearlescent, glass bead, and silver screens, cause a color shift in the image that is projected upon it. You want to get a screen that maintains the color balance of the original image.

The type of screen you get also depends on the placement of the projector. Glass bead screens tend to be retro-reflective. That is, these screens reflect much of the light from the projector back towards the projector. For a ceiling mounted projector, that would cause the image to appear dark to the audience. In this case, you would want a diffusive screen. For a coffee table mounted projector, a retro-reflective screen would be ideal however, since the audience is much closer to the centerline of projection.

Holes in the Screen?
Some screens are made of a micro-perforated material. That is, the screen is full of very tiny holes. This is to allow the mounting of front speakers behind the screen, and not lose audio quality. Because micro-perforated screens tend to be quite fragile, some manufactures simply make screens for similar usage with a loosely woven fiberglass fabric. In both cases, the gain of the screen suffers because less light is reflected by these types of screens.

Seating
How far should you sit form the screen? In the past, distance from the screen was determined only by picture size. Nowadays though, with HDTV content, the distance from the screen should also factor in picture resolution and aspect ratio. There are different recommendations and rules of thumb floating around. We’ll just look at just one of them. For HD content, THX recommends seating at 1.37 times the screen width (not diagonal), while for standard content, THX recommends seating at 1.87 times screen width. Furthermore, every viewer should have clear sightlines to the entire screen, and no viewer should have to look up more than 15 degrees to see the top of the screen. This means that second row seats would have to be raised when compared to front for seats. You can use this online Viewing Distance Calculator to calculate viewing distance.

Conclusion
We’ve looked at the home theater room mostly with respect to consideration of video issues in this article. Though there is no one simple answer to getting exceptional video experience in your home theater, we have seen that appropriate décor, controlled lighting, proper screen selection, and effective seating arrangement form a foundation which, if done properly, and mated to the right display technology will lead to the most satisfying viewing experience. In our next article, we will explore the other half of the home theater equation – the listening experience.

Home Theater Layout Part 2

Home Theater Layout Part 1