Setting up a hi-fi system would be a simple matter if we only had to choose equipment that functions well technically. There's no great trick in maintaining a well-balanced audio signal up to the speaker itself, and today the best speakers come very close to producing an accurate acoustic replica of that signal. Once the sound is radiated into a listening room, however, all bets are off. A speaker is only one part of an acoustical system; the other is the room itself, and it can have a profound effect on the sound you hear.

A poor speaker -- however you might choose to define that -- will probably sound terrible wherever you use it, but a good speaker can be made to sound almost as bad unless a great deal of care is taken to match it to its surroundings. The size and shape of the room, its decoration and furnishings, the position of the audio equipment and the listening positions, and your own taste and listening practices all affect the overall performance of your system.

The first consideration is the size of the listening room. The larger the volume of air a speaker must excite, the more acoustic output you will require from it to achieve the sound levels you want. In any environment, sounds attenuate as you move farther away from their source, but in smaller rooms that tends to be offset by reinforcement from wall reflections. The larger the space is, the farther the sound has to travel both to reach the reflecting surfaces and to get to your ears, which means it has to be louder to start.

The "liveness" or reflectiveness of the room contributes to output requirements as well. Heavy curtains, deep-pile carpets, and overstuffed furniture all absorb sound, particularly at high frequencies. A very dead (absorptive) room will require more acoustic output than a live one with hard, non-absorptive surfaces that reflect sounds strongly.

In addition, the way you like to listen to music -- loud or soft -- and the music itself, can contribute to your acoustic power requirements: high volume levels generally need more power, and the high average levels of rock music place greater demands on a system than other sorts of material.

Producing sufficient acoustic output for a particular room requires the proper combination of amplifier power and speaker sensitivity, and this should be taken into consideration when you buy your equipment. Amplifier watts are relatively cheap these days, but simply buying a huge power plant is useless if your speakers can't handle it. Most speakers have a power-handling capacity specifications, but unfortunately these don't mean a lot as manufacturers don't measure or state them consistently. Mostly you have to rely on the advice of a dealer and on your own ears.

The alternative to more watts is to buy speakers that produce a lot of output with relatively low amplifier power. This sensitivity is usually stated as so many "dB SPL/w/m". This arcane-looking formula means the number of decibels of sound pressure level the speaker puts out for an input of 1 watt, measured at a distance of 1 meter. This is, in fact, about the closest speakers get to a standard measurement, so you can safely forget the specifics. Just remember that the low 80s are pretty insensitive, anything around 90 will do even better in quite large rooms, and anything approaching 100 is a killer. But remember too that this is a measure of quantity not quality, and the speaker's designer might well have had to make serious sonic compromises to get the sensitivity up.

Deficiencies either in amp power or speaker sensitivity, or both, usually lead to overdriving the system -- trying to get a higher sound level than the equipment is able to put out -- and this can cause severe distortion or even damage. In fact, although it seems to contradict sense, using an amplifier that is too small for a given set of circumstances may well be more damaging than too big an amp.

After size and furnishings, one of the most important aspects of a listening room is its shape. In any room, sound reflects off the walls, ceiling, and floor. If the distance between two opposite parallel surfaces is a simple fraction of the wavelength of a particular frequency, notes of that frequency will bounce back and forth in perfect phase -- an effect called a standing wave. At some point in the room, this note will be reinforced substantially; at others it will cancel out almost entirely. An ideal listening room would have no parallel surfaces -- an unusual situation, to say the least -- so that such waves would not establish themselves. The worst kind of room is a perfect cube.

Almost all rooms are susceptible to some standing waves at low frequencies, but their effects can be minimized by careful positioning of both the speakers and the listening seat. Moving either of these even a few inches is sometimes enough to cure -- or create -- an intolerable sound. The only way to find out what works best is by experimentation, but taking some time to deal with such problems is well worth the effort.

In particularly difficult cases, the use of a subwoofer -- a separate speaker that handles only very low frequencies -- may be necessary. The range of places you can put the main speakers and still get proper imaging may be fairly limited, and some of these positions may result in standing waves that can't be tamed. Positioning of the bass speakers is much less critical from an imaging point of view, so a subwoofer can be located with only standing waves in mind. The best arrangement is often a pair of subwoofers in acoustically dissimilar positions.

If you have only one sub, however, Dolby Laboratories suggests a neat trick for positioning it. Put the subwoofer in your listening chair, then play something with lots of bass through the system. Walk around the room and note where the bass sounds best; if you place the subwoofer there and yourself in your chair, you should get the same bass performance. When you do this, however, you may want to make sure your cohabitors are out, or they are likely to think you've lost your marbles.

Bass response of all speakers is also affected by their proximity to nearby reflective surfaces. The closer a speaker is to a wall or the floor, the more prominent will be its low-frequency output. Some speakers are designed to take advantage of this effect, but most are not. Those that are not should be located at least a foot from back and side walls -- the distances should be different -- and several inches off the floor. Many manufacturers offer stands to raise the speakers, which also places them closer to ear height.

Such positioning considerations affect the sound you achieve at high frequencies too. A speaker's overall spectral balance is a combination of several factors, none of which can be ignored. Not only do we hear sounds directly from the speaker -- the on-axis response -- but we also hear near-field reflections from the walls immediately adjacent to the speaker as well as the longer-term reverberant field of the listening room itself.

All of these sounds combine in a sort of acoustic "soup" to determine the speaker's sonic character in a particular environment, and each element of the overall response is influenced by a different aspect of speaker performance. In any live acoustic situation, therefore, a speaker's off-axis response has a significant effect on its perceived tonal balance.

In a very dead room, the on-axis response is usually the dominant factor, at least for someone sitting in the so-called "stereo seat" or "sweet spot" directly in front of both speakers. Typically, however, the on-axis signal combines with the near-field reflections to produce an average, composite response.

The walls and other surfaces close to the speakers tend to reflect what is being radiated obliquely at, say, 60 to 75 degrees off-axis. The extent to which these reflections will affect sound balance is determined by the proximity of the surfaces, their reflectivity, and the speaker's ability to radiate sound off-axis with a spectral balance that approximates the on-axis response -- that is, its dispersion.

The sonic balance in a room's reverberant field, and the sound listeners are likely to hear when sitting somewhere other than the sweet spot, is largely influenced by a speaker's response from about 30 to 45 degrees off-axis. The importance of this output increases the farther from the speakers one sits, and it is accentuated by a live room. But in all cases, it mixes with the on-axis and near-field signals to create an overall balance. Many manufacturers provide instructions on how to take advantage of these effects, and it is wise to check the owner's manual for any speaker you are considering buying to see if any special positioning requirements are specified.

With all that in mind, it's important to make sure that this composite acoustic energy be as similar as possible from the various speakers. For that to be so, they must be in as close to identical acoustic environments as possible. Placing one speaker near a wall or in a corner and the other out in the open, for instance, will almost inevitably make them sound very different. Even placing one upright and the other on its side can have obvious sonic effects, and so should be avoided.

Finally, you should be careful about the location of the primary listening position -- the sweet spot -- with regard to the speakers. The speakers should be far enough apart to provide an adequate stereo spread, and the listening seat should be the same distance from both speakers, in order to facilitate a stable center image.

Our ears and brains are extraordinarily sensitive to differences in arrival times of sounds: even the tiniest variations will make the sound seem to come from the direction of the earlier arrival, so it's vital that the speakers be an equal distance from the primary listening seat. One way to make sure the distances are right is to pin a piece of string to the center of your chair and stretch it to one speaker and then to the other to make sure they are the same distance away. If yours is a surround sound system with a center channel, use the string trick for all three front speakers; you will probably find that you need to push the center speaker back or move the main speakers forward. Do it; the results can be dramatic.

The ideal setup for two-channel stereo is in the form of an equilateral triangle with the speakers in two of the corners and the listening chair in the third. A center channel should be halfway between the main speakers (and back a bit). This is not always possible so some experimentation may be necessary.