Speakers, Step by Step
Great picture, but where's the sound? Oh, yeah - better throw in some speakers, too . . . .
Okay, that might be an exaggeration. But if the humble analog speaker isn't a mere afterthought in our digital-everything, 24-bit century, many home-entertainment shoppers still seem to give it less attention than it deserves. The fact is, video without sound is just wallpaper, and it's the speakers, not the electronics, that have by far the biggest impact on how your A/V system will sound.
|The drawing shows a typical stereo subwoofer/satellite setup. At right we see two tower speakers, one of which is a "power tower" with a side-mounted powered woofer. (Illustration by Chris Lensch)|
What Speakers Do
The purpose of a speaker is to convert an electrical audio signal from a receiver or amplifier into sound. It does this by vibrating the air around it, creating sound waves that eventually reach a listener's ears. What vibrates the air is one or more diaphragms, which are usually cone- or dome-shaped but sometimes flat. In most cases the diaphragm itself is moved by a coil of wire - the voice coil - that's wound around (or near) a fixed magnet, creating a type of electromagnetic motor.
The combination of a diaphragm and whatever moves it (typically a voice-coil/ magnet assembly) is called a driver. Almost always, a cone diaphragm is used for low- and mid-frequency drivers, called woofers and midranges, and a smaller cone or dome is used for high-frequency drivers, called tweeters. Of course, determining the precise composition and arrangement of the voice coil, magnet, and diaphragm - along with the design of the box or cabinet that contains them - are where speaker engineers earn the big bucks.
It would be ideal to use just one driver to reproduce the entire audible range, but that isn't practical. The very qualities that make a woofer good at reproducing low (bass) frequencies make it lousy at high (treble) frequencies, and vice versa for tweeters. So designers combine the different types of drivers in two-way speakers (woofer and tweeter) or three-way speakers (woofer, midrange driver, and tweeter), as well as other combinations you'll see occasionally. In any case, the number of "ways" refers to distinct frequency ranges, not necessarily to the number of drivers.
An electrical filter called the crossover network, almost always built into the speaker, divides up the signal from your amplifier or receiver and sends signals from the appropriate frequency range to each driver. The circuit gets its name because it determines where the audio signal "crosses over" from one driver to the other, and the characteristics of its design has a strong influence on the speaker's sound quality. On the other hand, it's not something that you can directly evaluate when you're shopping, any more than you can judge speakers by counting how many drivers or crossover points they have.
Bass and Treble Basics
It takes far more energy to produce low-frequency sounds than to produce high-frequency sounds of the same apparent loudness. That's why the tuba is so much bigger than the piccolo and why the tuba player appears to be working so much harder than the piccolo player - he is. All else being equal, a speaker that can powerfully reproduce deep bass requires a bigger driver, and a bigger box to hold it, than one that can only reproduce sounds starting in the midrange (roughly, the pitch of an average male voice and above).
However, designers keep on trying to "cheat" the laws of physics and get honest bass out of smaller boxes. What makes it hard is that every time a woofer cone moves in or out, it moves as much air in back as in front. But the back wave it creates tends to cancel out some of the front wave at low frequencies, weakening the total sound energy. If, however, the woofer is mounted so that the back wave is blocked from running around the driver and canceling part of the front wave, more of the energy that goes into the cone's forward motion can reach our ears.
That's why speaker designers came up with the sealed, acoustic-suspension enclosure, where the woofer's back side is enclosed in a cabinet that leaves no way for the air behind the cone to escape. As a result, the trapped back wave not only can't interfere with the front wave, but at low frequencies it acts like an acoustic spring, helping the cone respond more rapidly to changes in the audio signal than would otherwise be possible.
Another common variation is the ported (or vented or bass-reflex) enclosure, in which the back wave is allowed to exit in a controlled way via a tube running from the back of the woofer to an opening on the front or back of the cabinet. The diameter and shape of the port opening and the length of the tube are all meticulously calculated so that the back wave will reinforce the front wave rather than weaken it, with minimal side effects (like "port noise"). There are many more variations - multiple small ports, slots instead of a round or oval port, and even a passive radiator, where the opening is sealed by a speaker diaphragm that lacks a driving voice coil (to a sound wave this "looks" like a port, but there's no opening, which can have practical advantages). The underlying principle remains the same: take the back-wave energy and add it to the front wave rather than subtracting it, making the speaker's bass section more efficient in using the amplifier power feeding it.
On the treble end the problems are different. It may take less energy to reproduce high-frequency signals, but designing a good tweeter still requires real engineering art. It needs to deliver smooth, even sound over three octaves or so, it has to handle a wide dynamic range from soft to loud with unchanging sound quality, and it has to spread its output evenly across the listening area.
Speakers are available in so many shapes and sizes because they have to accommodate so many different situations. The simplest and most common model is the "bookshelf" speaker, nearly always a compact, two-way arrangement of woofer and tweeter in a small sealed or ported enclosure that can fit on a shelf. Despite the name, however, most bookshelf speakers will sound distinctly better if you take them off the shelf and move them away from the wall. Mounting small speakers on stands a few feet out into the room usually delivers smoother bass and clearer, more accurate sound overall.
There are exceptions, though. A few bookshelf speakers have "boundary" switches designed to adjust their response, with one setting for placement next to a wall and the other for "free space" locations. Nonetheless, even in these cases, placement away from the wall is usually preferable.
There's a huge variety of bookshelf-type speakers, at prices ranging from little more than the cost of a few DVDs to enough to cover a nice Kia. The better examples - even the more affordable ones - can produce remarkably good sound even though most can't handle deep bass or play really loud without distorting.
Another popular small-speaker approach is the "architectural," or in-wall speaker. Most are essentially bookshelf designs without the box: the drivers and crossover are built onto a plate designed to be fastened over a hole cut into a typical sheetrock or plaster wall. The allure is obvious: good sound without sacrificing a single square inch of floor space - indeed, with near invisibility, since most in-walls have grille covers than can be painted over.
|Mission's FS-2AV system uses NXT technology for the flat-panel satellites.|
On-wall speakers split the difference between conventional and in-wall designs with shallow cabinets that can be hung on a wall like a picture frame, without any cutting or other carpentry. A number of impressive on-walls have recently become available, giving system builders an attractive option. However, most of them have the same deep-bass and dynamic limits as in-walls (some are even a bit worse). And they're not much more flexible in terms of placement - though at least you can experiment without cutting into your walls.
A stereo music system consisting of a modest receiver, a CD player, and a pair of excellent bookshelf speakers can deliver high fidelity sound with surprisingly wide dynamics. That said, there are many situations that simply call for larger or more specialized speakers, or both. You might have a bigger room or recordings with a lot of deep-bass content, or maybe you just prefer loud playback or want multichannel home theater sound.
Sweet and Low
Unless your listening is strictly casual, using music as background, or your taste runs only to string quartets or folk guitar and voice, you probably won't be satisfied with the limited bass (no lower than 50 to 70 Hz) and modest dynamic range you'll get from most small speakers. Sooner or later, you are going to lust for more - deeper, more powerful bass, realistic impact on transients, and more flat-out loudness.
The most popular, space-efficient, and affordable way to get them is to add a subwoofer, a specialized speaker engineered to produce only deep bass. The trick is to integrate a subwoofer's low-bass output with the midbass and higher frequencies from two (or more) "satellite" speakers. This works because human beings are not very good at pinpointing the origin of bass sounds much below 100 Hz or so. A subwoofer can be placed almost anywhere you like, though most subs will perform best in a corner, because that's where room acoustics tend to reinforce their output. When you place the satellites, you need to take into account the room layout as well as where they can give the best imaging, or a sense that a singer or instrument is coming from a specific, stable position in the "soundstage." With a bit of care and patience in tweaking the setup, the trio will usually blend seamlessly.
Most subwoofers are powered (or "active") designs with built-in amplifiers for their large drivers. Typically, a powered sub can be connected to an audio system in two ways. In the first, an A/V receiver or preamp/processor sends a bass-only (already filtered), line-level (unamplified) signal from its subwoofer output. The second option is useful if your stereo receiver or amplifier lacks a dedicated subwoofer output or you prefer to use the crossover facilities built into the sub. Connect the receiver or amp's left/right speaker outputs to the sub's speaker-level inputs and the left/right satellite speakers to the sub's corresponding speaker-level outputs.
Either way, there's a potential advantage beyond more/deeper bass. Adding a subwoofer releases the satellites from the burden of trying to reproduce deep bass. For multichannel DVD movie soundtracks and other Dolby Digital or DTS sources, your receiver/processor crossover will strip the deep bass from the signals sent to the satellite speakers. The payoff is increased dynamic range: better clarity and "punch" on sound effects and loud music passages, and higher distortion-free volume levels.
All else being equal, bigger subs will play lower, louder than smaller ones. However, there are specially designed ultra-compact subwoofers that combine "long-throw" 10- or 8-inch drivers, extra-high-power "smart" amplifiers, and active equalization to match conventional (and far larger) subs with 12- and even 15-inch drivers. Nevertheless, getting honest deep bass at movie theater or concert volumes usually requires a bigger subwoofer.
Don't go by size alone, but do be sure that any sub you consider delivers solid bass a full octave or so deeper than your satellite speakers. For example, with typical bookshelf speakers whose bass limit is around 50 Hz, I'd want a subwoofer that can play down to at least 30 Hz. Otherwise, why bother?
Of course, a subwoofer is only one way to get real deep bass. Big, full-range speakers are another, time-honored one. Most of today's big boys are "tower" designs that combine deep-bass abilities with slim-profile cabinets, often by integrating relatively shallow, side-firing 10- or 12-inch woofers. The slimness can be more than just a matter of style: a narrow cabinet promotes clearer sound and better imaging by reducing edge diffraction.
Using full-range speakers has obvious advantages over a subwoofer/satellite setup, including simpler room layout and wiring and less trial and error with placement and level/crossover settings. But this cuts both ways: with full-range speakers you can't fine-tune the balance between the deep bass and the higher frequencies the way you can with a standalone subwoofer and separate satellites, which means you're stuck with whatever balance the speaker designer built in - plus whatever problems your room acoustics pose.
|Martin-Logan's Odyssey electrostatic speaker.|
Front and Center
Everything we've touched on so far applies equally to plain, two-channel stereo systems and to the front left/right speakers and subwoofers in multichannel surround sound systems. A home theater system, or even a surround sound music system, typically adds three or four more speakers beyond the stereo pair and the sub (or towers that include subs).
The center speaker - located midway between the left and right front speakers, usually on top of the TV - reproduces the center channel found in multichannel soundtracks or generated from two-channel sources by processing like Dolby Pro Logic. In movie soundtracks the center channel carries the most important sonic element, namely the dialogue, along with some music and effects.
Dedicated center-channel speakers usually have a horizontally oriented cabinet that can sit comfortably atop a big-screen television. Designers try to match a center speaker's tonal character to its left/right front mates when they're part of the same package or line - that's important because it helps create a believably seamless soundstage in the front of the room.
Many such speakers incorporate two small woofers or midrange drivers, one at either end with a tweeter between them. This lets the designer use a lower-profile cabinet than would be possible with a single large woofer while letting the speaker play far enough down through the low midrange and upper bass to reproduce male voices without strain. Because the treble rolls off as you move to either side, however, such designs usually sound best for listeners seated directly in front of the TV. Various workarounds have been conceived, but the most effective - and often most expensive - is a three-way design with a vertically stacked midrange driver and tweeter flanked by two woofers.
Sound All Around
Surround speakers (often called "rear" speakers, though they're only sometimes placed literally behind the listeners) reproduce the other main channels of 5.1- and 6.1-channel programs - that is, everything but the ".1," or LFE (low-frequency-effects) channel, which usually goes to the subwoofer along with deep bass from the other channels. Movie sound designers use the surround channels for ambient cues that suggest the acoustics of the onscreen setting as well as for discrete sound effects, like overflying helicopters, and musical elements, including both reverberation and discrete instruments or voices.
Most home theater systems have a single pair of surround-channel speakers located toward the sides or rear of the listening area. However, with the introduction of 6.1-channel Dolby Digital EX and DTS-ES soundtracks employing a "back surround" channel for sounds centered between the L/R surrounds, and more and more home gear equipped to decode them, arrays of three or four surround speakers, with the back surround(s) placed on the rear wall, are becoming more common.
Most speakers deployed for surround use are identical to those found in the front left/right positions - usually small bookshelf types - and there's a lot to be said for the tonal uniformity this provides. However, specialized surround speakers of two basic sorts, dipoles and bipoles, deserve attention.
Dipole speakers use paired drivers, or sets of drivers, firing in opposite directions but wired out of phase, meaning that one driver or set pushes out while the other pulls in (and yes, if you've been following closely, at the lowest frequencies there is cancellation). The result is a speaker that distributes its sound mostly forward and back, but very little to either side. When placed high on the side walls, a dipole spreads sound throughout the listening area (mostly toward the front and back of the room), with relatively little sound aimed directly at the listeners, promoting a sense of spaciousness and making it harder to pinpoint the speaker.
Bipole speakers look similar but the paired drivers (or sets) are wired in phase, pushing and pulling together. A bipole's sound is usually spread wider and more evenly than that of a conventional "monopole" or direct-radiating speaker, but otherwise not very different. The conventional wisdom (reinforced by listening tests published in Sound & Vision) is that dipole surround speakers excel for movie sound and acoustic music (like classical), where ambience is the primary goal, and bipole or direct-radiating surrounds for multichannel music, where discrete instruments or voices are more likely to appear in a surround channel. But there are plenty of dissenting opinions, and plenty of speakers work well at both tasks.
Most folks assembling surround systems naturally think of one or another sub/sat variation because satellite speakers tend to be décor friendly, take up very little space, and are reasonably priced. But there are some reasons why you might want to go with full-range speakers. For instance, a lot of people already own a fine pair of stereo tower speakers, and they'd rather build their systems around those speakers than start from scratch. There's no reason not to use such full-range speakers for the left/ right front positions, where they can reproduce those channels as well as any deep bass redirected from other channels where smaller speakers are used. Just keep in mind that you'll still need to have a good tonal match among the main channels, and especially across the front.
Then there's the audiophile or purist argument for using full-range speakers all around. The new DVD-Audio and Super Audio CD (SACD) multichannel music formats offer a growing library of high-resolution recordings. But neither format mandates any standard way to handle deep bass, and very few disc players, receivers, or processors provide any bass management for the multichannel analog output from these discs. So when you play a DVD-Audio disc or an SACD, each channel is reproduced in full by its assigned speaker - which means that the deep bass isn't redirected to a subwoofer or another speaker that can handle it.
The result: if your system consists of five small satellite speakers and a subwoofer, and if the recording contains deep bass in the five main channels rather than mixing it all to the "subwoofer" channel (this varies from disc to disc), the music will sound thin. You can avoid this problem by using full-range speakers for every channel, but that's not a very practical solution for most of us. Another option is to invest a few hundred dollars in an outboard bass-management component, available from Outlaw Audio and M&K Sound.
Now that you have a better idea of what you're looking at (and listening to), you can begin to sort out the hundreds of speaker brands, types, and models you're sure to encounter. That process can be daunting, but patient research and careful listening will yield the information you need to make the best choice for your system, your room, and your budget.