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Figure 11.1

The traditional design for speakers uses one pair for stereo reproduction.

An alternative to the two-speaker design that's gained popularity in computer circles in the late 1990s uses three speakers for stereo sound reproduction. The human ear is good at localizing high-pitched (or treble) sounds, but not so good at locating the source of lower (bass) tones. The reproduction of bass tones, however, is the source of the increased size of higher-quality speakers. These tones require large speaker elements, known as woofers or subwoofers. If most of the bass tone reproduction is shifted into a third speaker, located between the two stereo speakers (which are now referred to as satellite speakers), those satellite speakers can be greatly reduced in size. A typical three-speaker system uses satellite speakers that are only about 2-4 inches on a side, and a subwoofer roughly 12-inchx8-inchx8-inch in size. Figure 11.2 depicts such an arrangement.

Which type of speaker should you get? There's no easy answer to this question. Both two- and three-speaker designs can achieve good sound reproduction; neither type is uniformly better than the other. The bulk of the higher-quality speaker sets marketed for computers in 2000, however, use three-speaker designs. The advantage of the three-speaker design is that it's less demanding of desk space. The satellite speakers are small and unimposing. The three-speaker system's subwoofer is quite bulky, however, so if under-desk space is limited, you might want to favor a two-speaker design. Another drawback to the three-speaker design is that it involves more in the way of cabling. A typical two-speaker design uses just two audio cables, one leading from one speaker to the computer and another leading from the first speaker to the second.

Part iii

A three-speaker system, on the other hand, requires one additional cable to handle the sub-woofer. These speakers also sometimes use a fourth cable for a volume control.

Figure 11.2

A three-speaker system places the bulkiest part of the speaker out of the way—typically under your desk.

Figure 11.2

A three-speaker system places the bulkiest part of the speaker out of the way—typically under your desk.

Speaker Connectors and Options

Speakers for stereo systems generally use bare wire contacts or RCA plugs, as shown to the right in Figure 11.3. To simplify cabling and because of limited space on the backs of sound cards, however, computer speakers use 1/8-inch stereo jacks, as shown to the left in Figure 11.3. If you want to use stereo speakers that use the "wrong" type of connector for computers, adapters are commonly available from stereo shops and many computer stores.

Some USB-interfaced speakers have begun to appear on the market. These devices don't use the computer's sound card; instead, they use a combination of the computer's CPU and circuitry in the speakers themselves to produce sound. To use USB speakers in Linux, you must use a 2.3.x or later kernel, or a back-port of the USB drivers from these kernels to a 2.2.x kernel. Activate the USB Audio support option in the kernel configuration menu. I recommend against using USB speakers because they're so new. Conventional audio cards are much better supported in Linux.

Figure 11.3

RCA plugs (right) have long been used for connections between stereo components, whereas 1/8-inch jacks (left) are common on portable stereos and computer audio equipment.

Figure 11.3

RCA plugs (right) have long been used for connections between stereo components, whereas 1/8-inch jacks (left) are common on portable stereos and computer audio equipment.

Computer speakers must normally be amplified, which means that they include the capability to increase the volume of a signal. Many sound cards can't output a strong enough signal to drive an unamplified speaker. Along with the amplifier, most speakers include a knob or slider to let you control the volume of the sound. This control often resides on one speaker, but with three-speaker designs it's often on a cable that leads to the subwoofer. If your speakers lack a separate volume control, you can use a mixer application in Linux to control the volume. If you prefer something more tactile, you can purchase a separate volume control, as shown in Figure 11.4, which you insert between the speaker and the sound card. The knob in the middle of the short cable shown in the figure allows you to reduce the strength of the signal traversing the wire, and thus reduce the volume of the sound coming out of the speaker.

To power the amplifier, speakers require a power source. This is normally ordinary AC house current. Some speakers, though, provide an option to use batteries. Unless you're using the speakers with a portable computer, I recommend you use house current, because speakers can consume batteries quickly. Most battery-powered speakers either come with an AC adapter or include a jack for one. You can purchase a separate AC adapter at an electronics store.

Part III

Figure 11.4

Add-on volume controls can be convenient if your speakers lack their own volume adjustment knobs.

Figure 11.4

Add-on volume controls can be convenient if your speakers lack their own volume adjustment knobs.

Some speakers include controls that let you adjust the response of the speakers to different tones. For instance, there might be a switch that boosts the speakers' response to bass tones, or bass and treble adjustment dials. These controls can help you fine-tune a speaker for the best performance in a given room.

Shielded and Unshielded Speakers

One unusual characteristic of the area near a computer is the large number of objects that are sensitive to magnetic fields. Objects that can be damaged or malfunction in the presence of magnetic fields include

• Removable disks Magnetic fields can erase data stored on floppy disks, Zip disks, LS-120 disks, and many other removable disk formats. Fortunately, neither magneto-optical disks nor CDs are affected by magnetic fields.

• Backup tapes Backup tapes rely on the same recording technologies as do floppy disks, and so are just as sensitive to magnetic fields.

• Monitors Traditional cathode-ray tube (CRT) monitors work by firing electrons from the rear of the picture tube to the front, where they strike phosphors that glow. Because electrons are charged particles, they're sensitive to magnetic fields. If you bring a magnet too close to a monitor, the image on the screen will become distorted. LCD monitors, such as those used in laptop computers, are not sensitive to magnetic fields.

Chapter 11

Unfortunately, speakers work by the application of magnetic fields. The current from the sound 11 card, amplified by the speaker or an external amplifier, causes changes in a magnetic field inside the speaker. These changes, in turn, cause vibrations in the membrane of the speaker, PP

thus producing sound. A speaker can therefore potentially erase magnetic media and cause a T A

monitor's display to go haywire. OI

Fortunately, speaker manufacturers are cognizant of these problems, and so they produce t shielded speakers for use with computers. These speakers use layers of metal to block their magnetic fields, so they have little or no impact on nearby magnetically sensitive devices. Nonetheless, I recommend exercising some caution, even with magnetically shielded speakers. Don't place removable disks or tapes directly on or next to the speaker. If the speaker's shielding is inadequate and the speaker rests too close to the monitor, you'll see the difference, but you won't permanently damage the monitor, so you can adjust speaker/monitor distance as you see fit.

Most speaker manufacturers produce unshielded speakers in addition to their shielded models. These unshielded speakers can be electrically compatible with computer equipment, and they often cost less than their shielded counterparts. They're usually marketed for use with small or portable stereos. If at all possible, I recommend against using such unshielded speakers with computers, however, because of the great danger posed to your removable media. If you choose to use unshielded speakers with your computer, exercise extreme caution in where you store your magnetic media. Also, keep the location of the speakers relative to your monitor and the computer itself in mind. Removable media must be inserted into, and removed from, your computer to be of any use, so placing unshielded speakers too close to a computer can be a recipe for disaster.

How close is too close? For the sort of small speakers you're likely to use with a computer, 6-12 inches is probably adequate clearance, in my experience, but 24 inches doesn't represent an excessive safety margin. Keep in mind that the damage from a speaker's magnetic field can accumulate over time. Even if you've tested a floppy disk at 6 inches for a few minutes and found no damage, an exposure of days or weeks might destroy your data. Also, the higher the capacity of the media, the more sensitive it's likely to be to damage from magnetic fields. A floppy disk might not be damaged by a given exposure, for instance, but a Zip disk might become unreadable from the same exposure.

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