UNDERSTANDING HOME THEATER ROOM ACOUSTICS



Your decision is finally made. After weeks of reading reviews and driving from store to store, you finally decide on the speaker system that's going to make your home theater room come alive. It's a little more money than you wanted to spend, but your local specialty retailer has offered you a deal that's hard to refuse. The next thing you know you're back home pulling speakers out of cardboard boxes, and draping speaker cables all over the place. Finally, when all the connections are made, you plop yourself in the front row center seat, hit the play button and....Hey, wait, what's going on. Where's the center channel detail I heard in the store! And where's that thundering bass!

Welcome to the mysterious world of room acoustics. The reason that your speakers sounded different in the dealer's demo room most likely had to do with the difference between the acoustical characteristics of his room vs. yours. Audio experts have known for years that the physical attributes of listening rooms are every bit as important as the audio equipment involved. As matter of fact, some would argue that room acoustics are far more important than popular equipment features such as gold connectors, polypropylene capacitors and the like. Now don't get us wrong, it's not that these details are unimportant; it's just that room acoustics are far more important. And often overlooked.

One of the first scientists to investigate the role of room acoustics in high quality sound reproduction was Roy Allison. Roy's seminal research in this area was revolutionary and led to a new way of looking at speaker and equipment design. The basic premise is that the acoustical makeup of a listening room can play a significant role in the total system of sound reproduction. Roy explains: "The acoustical parameters of a room have a tremendous effect on the quality of sound you hear. The problem is that acoustics as a science is poorly understood by many audio enthusiasts. However, it should be known that one does not need a PhD in the field to properly treat their own listening room, a few modest room treatments can go a long way. The only trick is knowing which ones are best to use".

Back to school.

Before we can dig deeper into the subject of listening room acoustics, a basic review of the physics of sound is prudent. By understanding the way that sound waves travel in listening environments, we can more fully understand how to keep them out of trouble.

Although the exact acoustical sound field that occurs in an enclosed space can be quite complex, there are three components that predominate. The first is the direct sound from the speakers themselves. These are the sound waves that travel in a straight line directly from the speaker drivers to the listener's ears. These direct sounds are considered the most significant component of sound reproduction because of their relatively large amplitude and direct transmission characteristics. Their successful propagation merely requires an unencumbered line-of-sight path from the speakers to the listeners.

Next are the first reflections. These are the sound waves that bounce off surfaces flanking the speakers and the listeners. As our diagram illustrates, these sound paths typically bounce off nearby walls, ceiling and floor surfaces. Acousticians feel that these early reflections are important to the perception of the "sound stage" and a failure to properly attenuate them can result is a marked reduction in "breadth" of the recording.

The last component is reverberation. Reverberation consists of the countless random reflections that bounce off other surfaces in the room and eventually arrive at the listener's ears. These sound reflections reinforce the feeling of room size and ambiance. When you are in a large room with hard surfaces, such as a gothic stone church, the reverberatory echoes bouncing off the stone walls are the components that give you that sonically cavernous feeling.

Listening Room Physics.

So how do these sound components effect your listening room? When you fired up your new home theater speaker system, all the sounds bouncing around the room combined to form a unique acoustical room signature. This acoustical signature effectively becomes superimposed on the primary recording and modifies it. It is almost as if there are two separate sound systems in the room playing at the same time.

Acousticians agree that the best way to design a listening environment is to strive for sonic balance of these "systems". For example, you do not want an overemphasis on reverberatory energy components or it sounds like you are listening in a tiled bathroom. On the other hand, you do not want a room void of reflections or the room sounds dead and lifeless. The ideal listening room allows some of the first reflections and some of the reverberatory components to appear at the listeners ears. Just enough of each to balance the resultant sound and make it sound natural.

There are two ways to control the first reflections and room reverberations to achieve sonic balance; absorption and diffusion. Absorptive surfaces consist of materials that dampen sound energy so that only a fraction of the energy is reflected. The portion that isn't reflected is actually converted into a tiny amount of thermal energy which dissipates into the air. Acousticians use "Absorption Coefficients" to indicate how well a material absorbs sound. The scale ranges in value from 1 to 0. A material with a absorption coefficient of "1" means it absorbs sound energy completely. A material with a "0" coefficient reflects it entirely. As you might imagine, real world materials lie somewhere in between. The chart on the next page shows the different absorption coefficients of typical surfaces found in residential structures.

The other technique used to control sound is diffusion. The principal here is to take the sound waves directed at the diffusive surface and break them into many small components. This resultant sound field is then scattered around the room at greatly reduced magnitude. The diagram below illustrates this. This technique can also be augmented in a listening room via natural room components like bookcases, furniture or with specially engineered diffusion panels.

How to do it.

There are dozens of methods and hundreds of products available to whip your listening room into shape. Rather than catalog all the products and techniques available, we will take a more pragmatic approach. We will consider each room surface individually and suggest appropriate treatments.

First, lets look at the floor and ceiling surfaces. These can be the worst offenders in a listening room because they are often constructed of extremely reflective materials. The absorption coefficient, for example, of your standard plaster/gypsum board ceiling is approximately .05 (at 1kHz). In other words, sounds bounce right off this stuff. And hardwood and tile floors aren't much better with absorption coefficients of .01 and .07 (at 1kHz), respectively. Fortunately, though, there is a standard household building material that does a wonderful job of soaking up sound waves; carpeting. Both wall to wall carpeting and area rugs do a wonderful job of absorbing incident sound energy. And fortunately carpeting goes quite well with most listening room decors.

Ceilings, however, can be more problematic. One could glue carpeting to the ceiling but we doubt your interior decorator (read: spouse) would approve. The next best thing is to install acoustic ceiling tiles. The least expensive option is to use the standard 1' x 1', fiber-based, tongue and groove tiles sold by building supply stores. Typically, these are installed 12" on-center, on 1"x 3" strapping that is firmly screwed into the ceiling joists. Proper installed, these fiber-based tiles actually do a pretty good job of absorbing incident sound waves, particular in the higher frequencies.

Even better sound absorption can be obtained via drop ceilings, and most building supply stores offer a wide variety of acoustical tiles designed for this purpose. The standard "worm-hole" pattern tile is common in commercial applications, because it provides a good mix of absorptive and diffusing properties. One thing to note: if you plan to listen at very high sound levels, drop ceilings can rattle. This could be quite annoying but several companies offer small rubber hangers to isolate the drop ceiling structure and thus reduce this problem.

You can also consider the professional acoustical ceiling materials. Various companies sell materials that are decor friendly and specifically designed for ceiling use. IIllbruck, for example, offers their famous Sonex panels made from a new, Melamine absorptive foam. These panels are available in a number of different surface textures, and in a variety of colors. For those who are opposed to the commercial "anechoic" look of Sonex, there are plenty of flat-faced products available. These are typically constructed of fiberglass or foam materials. Acoustical Solutions, for example, offers a number of panels that can be applied to the ceiling. Their Alphasorb panels are constructed of fiberglass sheets wrapped with woven decorator fabrics and are offered in 65 different colors. Several companies also offer special ceiling diffusing panels. These panels come in a multitude of shapes and sizes and designed with intricate diffusing surfaces.

Listening room walls often promote sonic misbehavior because they are typically constructed of hard substances such as gypsum board or wood. Since the wall surfaces located near the listener are directly involved in first reflections, it is especially important that they be addressed. For the economy approach, ordinary drapes can go a long way. You can also place furniture, plants or other items in the way to diffuse the sound. For example: it is hard to beat an upholstered couch or chair for sound absorption. If an object like this is placed right at the spot on the wall where first reflections occur, a remarkable improvement in sound can result.

If these conventional wall taming approaches are inconvenient, you can consider professional products. RPG Diffuser Systems, for example, offers their Acoustic Tools For Home Theater line. This line contains several panels that can be used to attenuate wall reflections. Armstrong also offers a line of decorator styled acoustical wall panels designed for easy installation. Their SoundSoak panels are available in many woven fabric colors and are constructed from 79% recycled materials.

The remainder of the surfaces in the room contribute to the reverberatory ambiance. Amateur acousticians often test the reverberatory tendencies of a room by standing near the listener's position and clapping their hands together loudly. The resultant echo can tell you a lot about reflected sound signature of the room. If the clap produces a distinct echo, you probably have some surfaces that need to be toned down. Generally, the main offender is the rear wall. If it is flat and open you should consider some treatments. As in the other surfaces, this does not always necessitate professional products. Anything to reduce reflections is appropriate. Bookcases, pictures, drapes, etc. can absorb and diffuse the sound energy.

The last issue to consider is "standing waves". Standing waves occur when lower frequency sounds bounce off opposing walls and combine to produce unnatural sounding peaks or dips in the bass response. First of all, unless your room has large areas of opposing surfaces, this may not be a problem. However, if your room is stark, standing waves could occur. Absorption is the technique of choice here and one of the best ways to soak up standing waves is with furniture. Plushly upholstered seating does a great job. Several manufacturers also offer high tech professional approaches. Acoustic Sciences Corporation offers their Tube Traps to reduce standing waves. These devices are tall free standing cylinders that are designed to absorb specific sound frequencies. Half sections are available to hang on the wall and might be the best option where large expanses of hard surfaces exist. These tubes are available in a wide variety of shapes and sizes.

Feel free to experiment.

Now that you have an understanding of the techniques involved, tuning your home theater room for spectacular sound should be straight forward, right? Well maybe not. We asked Jay Trieber, the president of Home Theater Concepts, a home theater design firm based in Norwood, MA for his thoughts.

"Yes, we use all the standard acoustic treatments," Jay explains, "but the problem is that home theater rooms can be very complex acoustically. Each room is unique and techniques that work well in one room, may not work in others. What we recommend is that you work on known problem areas, such as hardwood floors, first. If further treatment is necessary, take it a step at time and experiment. The proof is what you hear, let your ears be the final judge."