Loudspeakers come in several types, mostly delineated by their frequency range. A full-frequency-response loudspeaker is one that covers the full audio band, from approximately 20 to 20,000 Hz. These are always large, usually expensive, floor-standing designs, neither practical nor priced within reach of most consumers. But they can offer incredibly realistic, rich sound.
The majority of loudspeakers, capable of responding from around 45-20,000 Hz, fall in this category. Prices range from a few hundred dollars to thousands of dollars a pair. Remember, just because a speaker can cover this range does not mean it will sound good, or that it will have good quality bass. Frequency response is but one tiny piece of the sonic puzzle.
One aspect of the frequency-response spec that can offer a useful information about a speaker's performance is its variation from "flat," which is stated as a "+/- x dB." The tighter the variance, the more flat, or accurate, a speaker's response. Typical variances range from +/- .5 dB to +/- 3 dB, with the lower figure usually bounding the frequency extremes. That is, a speaker whose published frequency response is 50-25 kHz, +/- 3 dB, will be -3 dB below "flat" at 50 Hz and 25 kHz. This doesn't mean that information below 50 Hz will not be heard, only that the drop-off after that point may be steep.
Most restricted-frequency-response speakers are smaller than the floor-standing full-frequency-response products, and are often known as "bookshelf" speakers. You can usually get stands to mount bookshelf speakers in the optimal room position (actual bookshelves are rarely the ideal place to put them).
The subwoofer's job is to produce the very low tones of your audio system's dynamic range. A true subwoofer will be able to reproduce a 20 Hz test tone, which will be felt more than it will be heard (making it great for home-theater applications), but 30 Hz will do very well in most listening environments and for most music. Most subwoofers today are "powered," meaning that they contain a built-in amplifier and a crossover network that lets you adjust the sub's upper frequency response so as to more effectively blend in with your main speakers.
While a full 20 Hz sound wave is 40 feet long, don't worry if your room is smaller. The wave will fit, though it will literally bounce off the walls. The problem in smaller rooms is bass pressurization, where the waves literally pile up and create an ugly, booming sound. The solution is to turn the bass volume down to the point where the room doesn't become too "excited."
Subwoofer performance is greatly affected by placement. Some locations in a room will cancel the bass entirely. So where you place the subwoofer and where you sit are critical to getting the most from it. Subwoofer set-up hint: put the subwoofer in your chair and play a test CD with a 20 Hz test tone. Walk around the room and you'll hear places where there's no bass, and other locations where there's great bass (usually in a corner). That's where you should place your subwoofer.
While most music is broadcast on two channels (for a stereo effect), home theater (DVDs and laser discs) uses five-channel audio. A home theater speaker set consists of two front speakers (sometimes the same ones you'd normally use for stereo music), two rear or "surround" speakers, a center channel (used mainly for dialogue), and sometimes a subwoofer.
A pair of high-quality loudspeakers is expensive enough. Affording five of them is prohibitive for most consumers. So, for most of us, a five-channel (plus subwoofer) home-theater speaker system will necessitate a compromise. Buying a high-quality stereo pair and adding lower-cost center and surround speakers tends to result in disjointed and uneven performance. Five identical speakers (left, center, right, and a pair of surrounds) of somewhat lower quality will, in fact, work better for home theater than a "mix and match" set.
If you have a high-quality stereo pair that you want to keep and you're turning your stereo into a musical-surround/home-theater system, consider buying the center channel and surrounds from the same manufacturer as your original pair. While this will not guarantee sonic compatibility, it will be a good starting point. Make sure, however, that the pair, and especially the center channel, are magnetically shielded so as not to interfere with the image on your television. Serious audiophiles will likely want two separate systems: one for music (two-channel or six) and one for home theater. The main impetus for the separation is the television or video screen; having anything between the main speakers tends to interfere with the speakers' soundstaging, or imaging.
When multi-channel surround-sound music becomes more commonplace, as it will with the introduction of DVD-Audio (your favorite CDs are being remixed for 5.1 channels as you read this), perhaps the "convergence" of audio and video will become more of a reality. Surround mixes of major-label artists are already available on DTS (Digital Theater Sound) compact discs, which can be decoded using any DTS-equipped A/V receiver. Still, diehards will tend to keep two-channel and multi-channel entertainment systems separate.
Most reasonably priced home-theater speaker systems consist of five small, limited-frequency-response speakers (with bass down to around 60 Hz) and a powered subwoofer. For film surround sound, bipolar speakers (which radiate sound equally in two directions) or dipoles are best for the rear channels, but regular "direct radiators" will work well, too.
These small speakers work for home theater because the "bass management" systems in most modern A/V receivers permit you to route the bass from all five channels to the subwoofer, which also reproduces the separate LFE (low frequency effects) track added to Dolby Digital and DTS soundtracks (the LFE track is the ".1" in 5.1-channel surround sound).
Unlike music, which has a basis in reality, film sound, except for the music track, is usually arbitrary and artificial (created "after the fact") and therefore tonal accuracy, while important, is somewhat less critical when choosing low-priced home-theater speaker systems. More important to good home theater sound are dynamics, the ability to play loud without compression and/or distortion, and clarity. The latter is particularly important for the center-channel speaker, which reproduces most of the dialogue. If you can't understand what the characters are saying, it's hard to enjoy the movie!
Things to Consider
The price range on loudspeakers is incredibly wide. There are bookshelf-sized speakers costing a few hundred dollars and others the same size that'll set you back thousands--and are worth every dollar and then some.
In the $300-and-under range, expect to get a solidly built bookshelf-sized speaker with optimal performance from 60 Hz (mid-bass) and up. For stronger and deeper bass (into the 40-Hz region) while keeping a desirable balance across the audio spectrum, you're probably going to be looking at $300-$600 a pair. Speakers that offer deep bass in lower price ranges are almost certainly short-changing other important parts of the frequency spectrum (like the crucial midrange) and, we feel, simply won't be worth your time and money.
Frequency Range and Balance
A speaker's frequency range is a measurement of how wide a selection of sounds it can reproduce. Can it reproduce the lowest bass and the highest highs? Here's one of the areas where specs can help you out. Keep in mind, though, that very deep, high-quality bass is usually expensive, and if you're on a budget and not after high-powered home-theater performance, you might be more satisfied with good midrange performance since that's where most of the music is anyway. Extended high-frequency response is somewhat easier to achieve at reasonable cost, but watch out for "peaky," edgy, over-enhanced highs, which can readily induce fatigue.
Bass and treble extension are worth having, but usually not at the expense of frequency balance--that is, the speaker's overall tonal neutrality. If the speaker overemphasizes bass frequencies the sound will be booming and muddy, the actual notes difficult to distinguish. If it accentuates high frequencies it may sound bright and harsh. Conversely, if a speaker de-emphasizes a frequency band, like the upper midrange, it will sound dry and lifeless. If it short-changes the midbass, it will likely sound thin and "sucked out."
A great test of a loudspeaker's neutrality is the human voice; does it sound like a "whole" person, or do you hear distinct "chesty" and "tizzy" components? Also, make sure the speaker adequately expresses the difference between soft passages and loud ones. Keep in mind that most rock music is seriously compressed (the soft passages are, in effect, boosted to sound nearly as loud as the loud ones), so other kinds of music like classical and jazz tend to be better for testing a speaker's dynamic range.
Another important consideration is stereo imaging or "soundstaging." We try to evaluate whether a speaker pair can create an accurate soundstage with adequate width, depth and height.
To appreciate a speaker's soundstaging ability, we find it's important to sit directly between the speakers and listen to a simply produced live or "acoustic" recording, rather than a multi-tracked, artificial studio production. The human ear responds well to spatially correct "cues" in the form of subtle reflections from surfaces in the room where music is recorded. When these reflections are faithfully recorded and played back, the result can be a stunningly real sonic "portrait" of a musical event.
Another critical factor in choosing your speakers is how loud you like to listen and how a given speaker performs at that level. If you live in a dorm room or an apartment with thin walls, you may be playing your system at low levels much of the time. Some speakers sound surprisingly "alive" at low volume while others need to be played loud to get going. Some speakers start sounding hard, grainy and compressed at higher volumes. A well-designed speaker will sound good at both low and high volumes.
When you're evaluating a speaker, you should also keep an ear cocked for its rhythmic certitude and pacing. Does it "swing"? Does it sound appropriately snappy and fast, keeping pace with the music, or does it sound sluggish and constipated no matter what material you give it? These are qualities you can't measure, though some of what is measurable--sensitivity, for instance--will have definite effects on a speaker's pacing.
Your taste in music
Your preference in speakers will be strongly affected by your taste in music. Assuming you're on a budget, if you listen to a lot of rock, you'll probably be willing to sacrifice some tonal accuracy for more bass. And you'll probably want a speaker that will play loud without compression, distortion and "graininess," or a sense of artificial coarseness in the high frequencies. If, on the other hand, you listen to a great deal of acoustic music--classical, folk, or jazz--you might be willing to give up the bottom couple octaves of bass to get smooth, accurate, or liquid-sounding midrange frequencies.
Your Listening Space
The room in which you put your speakers and where in the room you place them will have a profound effect on the sound you get. Where you sit, of course, is also a factor. The best sound can usually be heard from a point directly between the two main speakers. Rooms with reflective surfaces such as windows, bare walls, and wood or linoleum floors will produce hard, "confused" sound no matter how much you spend on speakers. Fortunately, carpeting, drapes or a well-placed tapestry can absorb, reflect, and diffuse sound.
Moving a speaker just a few inches can greatly affect the sound--especially for a speaker with strong low-frequency response. It can mean the difference between no bass and too much bass, or just the right amount. If you just want to get sound, and if your speakers are small enough, you can place them on a shelf, but if you'd like to create and experience a "soundstage" you'll need to move the speakers into the room, placing smaller ones on specially designed stands at least six feet apart. This is often not practical in dorm rooms or in small apartments.
Most loudspeakers consist of two cones, or drivers (a woofer and a tweeter), a crossover network, and a cabinet. The cones are the actual producers of sound--thin funnel-shaped pieces of material (usually plastic) that vibrate. Each cone produces the frequencies best suited to its size--the woofer produces bass while the tweeter emits high frequencies. The midrange falls somewhere between the two and is determined by the crossover network, which divides the frequencies between the cones.
A well-designed crossover network consists of a low-pass filter (LPF), which keeps the highs from reaching the woofer, and a high-pass filter, which keeps the lows from seeping into the tweeter. The point at which the LPF rolls off the high frequencies and the HPF rolls off the lows is referred to as the crossover point. A common crossover point for a two-way system is around 2,300 Hertz (or cycles per second), but different designs vary the crossover frequency for different effects.
In a three-way design, a dedicated midrange speaker augments the work of the woofer and tweeter. There are advantages to such a design, but getting three drivers to work as one can be difficult and expensive; be wary of three-way designs costing less than a few hundred dollars each.
The cabinet is a critical component in the loudspeaker's design, and has a major effect on its sound. A cabinet should be rigid, well braced and internally damped to avoid sound-coloring resonances. Plastic cabinets may work fine for computer speakers, but not for serious audio components. Cabinets should usually be made of wood, or, more commonly, medium density fiberboard (MDF).
The cabinet design is typically one of two types: a sealed enclosure, or a "bass-reflex" type with an opening, or "port," used to provide a longer/deeper resonance cavity for the low frequencies. When a woofer moves, it pushes air in two directions, forward and back. Unless dealt with properly, the out-of-phase sound from the back wave will cancel the front wave, resulting in poor bass. A reflex design's port gives the back wave a place to go. A properly designed system causes the two waves to add instead of canceling, resulting in reinforced, rather than diminished, bass. An alternative and very popular bass reinforcement methodology is called "acoustic suspension." This design uses a sealed box wherein the trapped air acts like a spring. The advantage of this approach is that a smaller box can thereby produce very deep bass; the disadvantage is lower efficiency, resulting in the need for a more powerful amplifier.
What the numbers can and can't tell you
When shopping for loudspeakers you'll usually find numerical specifications along with a written description. Be aware: Measurements and numbers tell only part of the picture, and any one measurement can be more misleading than none at all.
For instance, you might get a frequency-response specification such as "40 Hz-22 kHz," but unless the spec also tells you the variance within however many decibels (+/- 3 dB, for example), the numbers are essentially useless. The speaker may well reproduce 20 Hz, but that tone could be 20 dB "down" from (or below) "flat" response, meaning you won't really hear 20 Hz from that speaker. Additionally, how a speaker is measured can affect the response. Was it measured in a room or in a reflectionless anechoic chamber? Where was the microphone placed to measure the response? Essentially, a frequency-response measurement by itself, unaccompanied by a critical review, is pretty much useless!
The specifications will also usually offer an "impedance" measurement in ohms, which refers to the resistance an amplifier will encounter when trying to drive a given speaker. Today, most loudspeakers are rated at 8 ohms, but this is another essentially meaningless specification since, in reality, the impedance of a loudspeaker varies with its frequency. Without seeing the impedance "curve" you cannot know whether the speaker presents an amplifier with an easy or a difficult load. In any case, today's modern solid-state amplifiers can effectively drive most properly designed loudspeakers. Still, for reasons too complex to delve into here, look for loudspeakers with a "nominal" 8-ohm impedance, though most amplifiers will easily handle a 6-ohm load.
Two more important specs are voltage sensitivity and power handling. Voltage sensitivity tells you how loud a speaker will play for a given voltage. This gives you some idea as to how big an amplifier you'll need to drive the speakers. This measurement is expressed as a certain number of decibels (dB) per 2.83 V input. For example: "88 dB/2.83V." Unless you're using a monster amplifier, you probably want speakers with an efficiency of at least 86 dB, though 88 dB or higher is preferable.
Power handling tells you how much power in Watts the speakers can take without damage. If a speaker is rated at "100 Watts maximum," don't worry too much if you choose or own a 200-Watt-per-channel amplifier. Chances are you'll never put that much power into the speakers. In fact, what usually damages a loudspeaker is using too small an amplifier and driving it to "clipping" (distortion) levels. The loud-level high harmonics in the distortion is what does the damage.