Speaker Basics
Lets get right down to business with room acoustics.

Picture, in your mind, a peaceful pond. Now you toss a rock into the middle of the pond. The stone disrupts the pond's peacefulness, generating a series of 360 degree waves or ripples that radiate from the center outward, eventually dissipating.
Now lets do the same experiment in a smaller body of water, say your bath tub. Go through the same routine and watch what happens. The same ripples are generated, only this time the waves hit the walls of your tub and reverse direction. The result is a confused blur of direct and reflected waves that have very little resemblance to the picture perfect pond scenario.
Just like a stone energizing a pond of water, sound uses waves to energize the air. Ideal circumstances will cause sound waves to move in all directions like the water in the pond. But most home theaters resemble the bathtub, with acoustically reflective material causing the sound to change direction. The reflected waves will cause echoes and reverberation. In your home theater you will hear both direct radiated and reflected waves from your speakers. The key to a great sound is balancing the two waves, forming a perfect relationship between your speakers and your room.

Speaker Basics

Transducer - a device which converts one form of energy into another.

Your speaker is a transducer. A speaker cabinet may have one or more transducers, or drivers. These transducers, or drivers, convert electrical signals into acoustic energy. There are only 3 true speaker categories: Monopole dynamic, Dipole Planer, and Dipole dynamic.

Everyone knows the monopole dynamic. It is the most common speaker used as woofer, mid, and tweet. A dynamic driver uses a cone or dome diaphragm attached to a voice coil to move air. The "back wave" of the speaker movement is contained with in the speaker cabinet. As a result, these front firing designs restrict mid and high frequencies to a maximum of 180 degrees. Years past these designs used resonant cabinets and sluggish transducers, but today's technologies have revolutionized this design.
In order to accommodate every listener, the sub-sat 3 piece systems were introduced. This design is perfect for the person who can't fit large tower speakers in the room. It consists of two small satellite speakers and a subwoofer to give the low frequency bass. Some companies use a horn for the mids and highs. Klipsch is famous for these designs. A horn uses a small diaphragm attached to a large megaphone shaped horn. This design is extremely efficient, offering low distortion.

Now onto a completely different animal. In an attempt to overcome the limitations of box enclosures and moving coil transducers, Dipole Planers were introduced. Composed of a lighter than air plastic film or foil as a diaphragm suspended between wooden or metal frame. These designs offer superior phase and transient response, and completely eliminate cabinet coloration. Unfortunately, there are some disadvantages. Because the sound produced at the front of the driver is always 180 degrees out of phase with the rear, they are prone to dipolar cancellation. This is where two antiphase signals neutralize each other, creating a null at all frequencies from the side axis. In other words, don't expect deep, accurate bass without a subwoofer. Also, if you do not acoustically treat the rear wall, you will be forced to place the speakers several feet forward in order to accommodate the back wave. Finally, the large diaphragm size needed for full frequency reproduction cause horizontal dispersion to narrow as frequency increases. But still many hardcore enthusiasts favor these designs. The most popular manufacturers are Martin-Logen and Magnepan.

Is there one single design that can combine the tonal neutrality of monopole dynamics with the spatial supremacy of dipole planers? Bipole Dynamic speakers use conventional technologies, but include a driver on the rear baffle of the cabinet, as well as the front baffle. As with planers, they require plenty of breathing room behind it, but since the front and rear drivers are not out of phase, there is no need to worry about phase cancellation. But since these designs require more drivers, crossovers, and fancy cabinet designs, they do get a bit pricey.

Whenever possible, stay with they same manufacturer for all speakers; mains, center, and rears - audible discontinuities will likely occur. Since every driver, crossover, and cabinet is unique, each brand reproduces musical overtones, or "timbres" with a distinctive sonic signature. Most companies build their products with similar designs to ensure uniformity throughout the line. Although this is sometimes referred to as timbre matching, keep in mind that that term is a trademark of Lucasfilm and is a distinct facet of home THX certificate program. This is where all speakers will sound the same and there will not be a noticeable difference from left to right pans.

If you are shopping for a center channel speaker to match you current front mains, it is best to bring one of yours to the store or bring home several center speakers to audition them. Use the center speaker as a left or right in a stereo pair. Keep the heights the same and play your favorite orchestral or pink noise from a test disc. Sit in the "money seat". Is the tonal balance the same?

Size Is Over-Rated

Gigantic loudspeakers will obviously overpower a closet sized room. By the same token, a pair of tiny bookshelf models will not fill a warehouse-sized room. Aside from obvious mismatches, however it would be a mistake to declare that speaker size is proportional to room size. The majority of loudspeakers will perform properly in most room sizes.

Room Acoustics

Although most consumers obsess over every piece of equipment, few pay much attention to the listening environment. Unfortunately for them, the room has about the same amount of effect as the equipment does. Here's some tips:

• Stand in the middle of the proposed room and start clapping your hands. Is the sound muffled, or do you hear echoes? A dead room absorbs the sound waves before they can change course, while a live room reflects the waves, which cause interference with the speakers. You need to find a medium. Upon clapping, you should hear a quick but unmistakable decay, as the sound fades into ambience.

• Remember that each room is composed of three parallel surfaces; floor and ceiling, front and back, and the 2 sides. To prevent the reverberation, at least one of each pair should contain absorptive material. Sound like big bucks ? Not necessarily. Sometimes it's just a matter of layout, such as, adding a bookcase to one wall.

As for absorption, don't over due it. Too much absorption will rob your music of life and sparkle. Remember you need to combine the right amount of both absorption and reflection to get the perfect balance. Experiment.

Placement

Unfortunately, there is no one universal rule for speaker placement, you'll have to experiment. But that's half the fun. You'll have to rough in the speakers and then fine tune them. Start by deciding which wall will serve as a backdrop for your speakers (one of the room's shorter walls). Measure the length of this wall and divide by three and place small pieces of tape at the one-third and two-third spots. Place the stereo speakers on these marks. That is the "rule of the thirds". Try moving the speakers closer together and farther apart. You should hear differences in tonal balances. Placing the speakers closer to room boundaries, walls and corners, tends to add bass at the expense of midrange. Conversely, free-space tends to make for cleaner, crisper sound, with less bass. Once you've found the perfect placement, it's time to experiment with toe-in. treat them individually, like focusing a pair of binoculars. Use a female vocal CD, angle both speakers inward toward the listening seat, and slowly toe out. Listen for the voice to float between the pair of speakers. No more than 15 degrees of toe-in should be necessary.

Front Channel

So far we've been discussing stereo speakers. Now we will get into centers, surrounds, and subs. This will make the layout a little more complex. First, remember to keep the size of the video screen or TV in proportion to some degree. In other words, small pictures and big sound doesn't mix well. Select a laserdisc such as Lethal Weapon or Speed. As the action progresses from far left to far right, does the sound follow it, or do they appear to move off screen in staggered leaps? It won't be perfect unless you watching a very large screen since soundtracks are mixed for movie screens. Bring your speakers closer together until the sound and picture coincides with each other. To prevent reflections, move the speakers outward in front of the TV. Try to keep 2 -3 feet of space around each enclosure. Don't be surprised if you CD audio suffers from their current placement. It will happen.

Due to their precisely defined role, center speaker placement is rather limited. Since the center is responsible for most on-screen action and all of the dialogue - it is the most critical speaker in a surround sound environment. It should be even axis with the front mains. Perhaps atop a TV set, or above or below a projection screen. In a perfect environment, it would reside behind the perforated screen. If it is above or below the screen, it is a good idea to use shims to angle the speaker toward the listeners.
When comparing center channels, pay particular attention to off axis dispersion. To test for this, simply move from side to side while listening to the speaker during on screen dialogue sessions. If the sound degrades as you move around, find a new center.

Surround Sound Staging

The smallest and least obtrusive speaker, yet they do perform a critical role in your surround system and are often the most difficult to place. The idea is to not be able to pin-point there location by listening. The last thing you want to create is a pair of surrounds with a sweet spot in the listening position. To avoid this, Lucasfilm promotes dipole speakers, with matching out of phase speakers on either side. Traditionally regulated to the rear of the room, the advent of bi-directional surrounds have brought the side walls into play. For most instances, I prefer bipole or dipole models straddling the listener, about 6 feet off the floor. Front firing models work best on the sides and facing each other. But start with the speakers behind you and slowly move around to the side. Find the best location by using tripods and audition many locations. If the sofa is on the back wall and you are using dipoles, mount them on either corner or to the side walls, with the nulls directed toward the seating position.

Subwoofers

Most consumers mistakenly believe that a good sub is measured by how low it will go. With 20 Hz being the best. Unless your room is large enough to support these low frequencies, you'll end up with standing waves. Standing waves, which result from two parallel boundaries in a room, are actually audible at 200 Hz. In order to cleanly reproduce a given frequency, the largest dimension of your room must equal one-half of the wavelength generated by that frequency. Using the formula Wavelength = Speed of Sound/Frequency, a 20 Hz signal would generate a 55.5 foot sound wave (1,100 feet per second/20 cycles per second) and require a 27.5 foot long room. How low can you go ? By substituting the longest room dimension for wavelength and manipulating the formula to solve for frequency ( Frequency = Speed of Sound/Wavelength) you can calculate the point at which your bass will start to boom. For instance, don't expect to cleanly reproduce frequencies below 25 Hz (1,100feet per second/44feet) in a typical 16 X 25 foot room. It would be wasteful and sonically detrimental.

Should the sub go in a corner ? In addition to maximizing bass depth and power, corner placement allows the sub to excite all room resonaces equally- but that doesn't mean you shouldn't try other locations. Use Indiana Jones & the Last Crusade and fast forward to the tank scene.

Time to Buy

This is it. It's time to buy. Arm yourselves with time and your favorite test discs. Remember that a little knowledge is no substitution for rigorous experimentation.

Most of this information was gathered from Audio Video Interiors magazine. This magazine is available at your local newsstand and is a must for any enthusiasts.