The Planar-Magnetic Transducer
The next popular driver technology is the planar-magnetic transducer, also known as a ribbon driver. Although the terms “ribbon” and “planar magnetic” are often used interchangeably, a true ribbon driver is actually a sub-class of the planar-magnetic driver. Let’s look at a true ribbon first.
Instead of using a cone attached to a voice coil suspended in a magnetic field, a ribbon driver uses a strip of material (usually aluminum) as a diaphragm suspended between the north-south poles of two magnets (FIG. 4). The ribbon is often pleated for additional strength. The audio signal travels through the electrically conductive rib bon, creating a magnetic field around the ribbon that interacts with the permanent magnetic field. This causes the ribbon to move back and forth, creating sound. In effect, the ribbon functions as both the voice coil and the diaphragm. The ribbon can be thought of as the voice coil stretched out over the ribbon’s length.
FIG. 4 In a true ribbon driver, the audio signal flows through the
ribbon (left). In a planar-magnetic driver, a conductor is bonded to
the back of a diaphragm (right).
In all other planar-magnetic transducers, a flat or slightly curved diaphragm is driven by an electromagnetic conductor. This conductor, which is bonded to the back of the diaphragm, is analogous to a dynamic driver’s voice coil, here stretched out in straight-line segments. In most designs, the diaphragm is a sheet of plastic with the electrical conductors bonded to its surface. The flat metal conductor provides the driving force, but occupies only a portion of the diaphragm area. Such drivers are also called quasi-ribbon transducers ( FIG. 4, right diagram).
A planar driver is a true ribbon only if the diaphragm is conductive and the audio signal flows directly through the diaphragm, rather than through conductors bonded to a diaphragm, as in quasi-ribbon drivers. (Despite this semantic distinction, I’ll use the term “ribbon” throughout the rest of this section, with the understanding that it covers both true ribbons and quasi-ribbon drivers.)
Ribbon drivers like the one in FIG. 4 (left) are called line-source transducers because they produce sound over a line rather than from a point, as does a dynamic loudspeaker.
The main technical advantage of a ribbon over a moving-coil driver is the rib bon’s vastly lower mass. Instead of using a heavy cone, voice coil, and voice-coil former to move air, the only thing moving in a ribbon is a very thin strip of aluminum. A ribbon tweeter may have one quarter the mass and 10 times the radiating area of a dome tweeter’s diaphragm. Low mass is a high design goal: the diaphragm can respond more quickly to transient signals. In addition, a low-mass diaphragm will stop moving immediately after the input signal has ceased. The ribbon starts and stops faster than a dynamic driver, allowing it to more faithfully reproduce transient musical information.
The ribbon driver is usually mounted in a flat, open-air panel that radiates sound to the rear as well as to the front. A loudspeaker that radiates sound to the front and rear is called a dipole, which means “two poles.” FIG. 5 shows the radiation pat terns of a point-source loudspeaker (left) and a dipolar loudspeaker (right).
FIG. 5 Point-source loudspeakers (left) produce sound in one direction
(unidirectional radiation pattern); dipole loudspeakers (right) produce
sound equally to the front and rear (dipolar radiation pattern).
Another great advantage of ribbons is the lack of a box or cabinet. As we’ll see in the sub-section of this section on loudspeaker enclosures, the enclosure can greatly degrade a loudspeaker’s performance. Not having to compensate for an enclosure makes it easier for a ribbon loudspeaker to achieve stunning clarity and lifelike musical timbres. FIG. 6 is a popular full-range dipolar quasi-ribbon loudspeaker.
Ribbon loudspeakers are characterized by a remarkable ability to produce extremely clean and quick transients—such as those of plucked acoustic guitar strings or percussion instruments. The sound seems to start and stop suddenly, just as one hears from live instruments. Ribbons sound vivid and immediate without being etched or excessively bright. In addition, the sound has an openness, clarity, and transparency often unmatched by dynamic drivers. Finally, the ribbon’s dipolar nature produces a huge sense of space, air, and soundstage depth (provided this spatial information was captured in the recording. Some argue, however, that this sense of depth is artificially produced by ribbon loudspeakers, rather than being a reproduction of the actual recording.
FIG. 6 A full-range planar-magnetic loudspeaker. (e.g., Magnepan)
Despite their often stunning sound quality, ribbon drivers have several disadvantages. The first is low sensitivity; it takes lots of amplifier power to drive them. Second, ribbons inherently have a very low impedance, often a fraction of an ohm. Most ribbon drivers therefore have an impedance-matching transformer in the crossover to present a higher impedance to the power amplifier. Design of the transformer is there fore crucial to prevent it from degrading sound quality.
Finally, some loudspeakers use a combination of dynamic and ribbon transducers to take advantage of both technologies. These so-called hybrid loudspeakers typically use a dynamic woofer in an enclosure to reproduce bass and a ribbon midrange/tweeter. The hybrid technique brings the advantages of ribbon drivers to a lower price level (ribbon woofers are big and expensive), and exploits the advantages of each technology while avoiding the drawbacks.
The Electrostatic Driver
Like the ribbon transducer, an electrostatic driver uses a thin membrane to make air move. But that’s where the similarities end. While both dynamic and ribbon loudspeakers are electromagnetic transducers—they operate by electrically induced magnetic interaction—the electrostatic loudspeaker operates on the completely different principle of electrostatic interaction.
In the electrostatic driver, a thin moveable membrane—sometimes made of transparent Mylar—is stretched between two static elements called stators ( FIG. 7). The membrane is charged to a very high voltage with respect to the stators. The audio signal is applied to the stators, which create electrostatic fields around them that vary in response to the audio signal. The varying electrostatic fields generated around the stators interact with the membrane’s fixed electrostatic field, pushing and puffing the membrane to produce sound. One stator pulls the membrane, the other pushes it. This illustration also shows a dynamic woofer as part of a hybrid dynamic/electrostatic system.
Electrostatic panels are of even lighter weight than planar-magnetic transducers. Unlike the ribbon driver, in which the diaphragm carries the audio signal current, the electrostatic diaphragm need not carry the audio signal. The diaphragm can there fore be very thin, often less than 0.001”. Such a low mass allows the diaphragm to start and stop very quickly, improving transient response. And because the electrostatic panel is driven uniformly over its entire area, the panel is less prone to breakup. Both the electromagnetic planar loudspeaker (a ribbon) and the electrostatic planar loud speaker enjoy the benefits of limited dispersion, which means less reflected sound arriving at the listening position. Like ribbon loudspeakers, electrostatic loudspeakers also have no enclosure to degrade the sound. Electrostatic loudspeakers also inherently have a dipolar radiation pattern. Because the diaphragm is mounted in an open panel, the electrostatic driver produces as much sound to the rear as to the front. Finally, the electrostatic loudspeaker’s huge surface area confers an advantage in reproducing the correct size of instrumental images.
In the debit column, electrostatic loudspeakers must be plugged into an AC outlet to generate the polarizing voltage. Because the electrostatic is naturally a dipolar radiator, room placement is more crucial to achieving good sound. The electrostatic loudspeaker needs to be placed well out into the room and away from the rear wall to realize a fully developed soundstage. Electrostatics also tend to be insensitive, requiring large power amplifiers. Nor will they play as loudly as dynamic loudspeakers; electrostatics aren’t noted for their dynamic impact, power, or deep bass. Instead, electrostatics excel in transparency, delicacy, transient response, resolution of detail, stunning imaging, and overall musical coherence.
FIG. 7 In an electrostatic loudspeaker, the moving diaphragm is a thin sheet of tightly stretched Mylar suspended between two elements called stators. This illustration shows an electrostatic panel mated to a dynamic woofer to form an electrostatic hybrid loudspeaker.
Electrostatic loudspeakers can be augmented with separate dynamic woofers or a subwoofer to extend the low-frequency response and provide some dynamic impact. Other electrostatics achieve the same result in a more convenient package: dynamic woofers in enclosures mated to the electrostatic panels. Some of these designs achieve the best qualities of both the dynamic driver and electrostatic panel. (A hybrid/dynamic electrostatic loudspeaker is illustrated in FIG. 8.)
One great benefit of full-range ribbons and full-range electrostatics is the absence of a crossover; the diaphragm is driven by the entire audio signal. This pre vents any discontinuities in the sound as different frequencies are reproduced by different drivers. In addition, removing the resistors, capacitors, and inductors found in crossovers greatly increases the full-range planar’s transparency and harmonic accuracy.
FIG. 8 An electrostatic/dynamic hybrid loudspeaker mates an electrostatic panel (top element) with a conventional dynamic woofer. (from Martin Logan)
The Dipolar Radiation Patterns of Ribbons and Electrostatics
Because planar loudspeakers (ribbons and electrostatics) are mounted in an open frame rather than an enclosed box, they radiate sound equally from the front and back. As we saw earlier, the term dipolar describes this radiation pattern. This is contrasted with point-source loudspeakers, whose drivers are mounted on the front of a box. Point-source loudspeakers are usually associated with dynamic drivers, but any type of driver In an enclosed cabinet qualifies as a point-source loudspeaker.
The dipolar radiation patterns of ribbons and electrostatics make using them very different from point-source loudspeakers. Because they launch just as much energy to the rear as they do to the front, positioning a dipole is more crucial, particularly their distance from the rear wall. Dipoles need a significant space behind them to work well. In addition, the rear wall’s acoustic properties have a much greater influence over the sound. The wall behind a dipole loudspeaker should be fairly live but with some diffusion, which can be achieved with furniture or bookcases.
Horn Loudspeakers
One of the first loudspeaker technologies, the horn, has enjoyed a resurgence in popularity in the last 20 years. The horn loudspeaker employs a small dynamic driver mounted in the throat (narrow end) of a horn structure, which more efficiently couples the driver’s diaphragm to the air ( FIG. 9). The horn loudspeaker operates on the same principle as a megaphone, producing increased volume. Any type of driver can be horn-loaded, but it is uncommon to find true horn-loading in woofers because of the enormous size of the horn required. The lower the frequency at which the horn is designed to operate, the larger the horn must be. Consequently, most horn-loaded loudspeakers are augmented with a conventionally loaded woofer to reproduce bass.
Horn-loading a driver confers many performance benefits. First, horns have extraordinarily high sensitivity, and can be driven to very high volume with just a few watts of power. It’s not unusual for a horn-loaded system to have a sensitivity of 100dB or more. This brings us to the second benefit of horns: they can be driven by very small power amplifiers, often those with just 10 Wpc of output power. Third, the excursion (back and forth motion) of the cone or dome diaphragm can be an order of magnitude less than in a direct-radiating driver. This allows the driver to operate in the linear range of its motion at all times, greatly reducing distortion. The electrical and magnetic forces involved in moving a horn-loaded diaphragm back and forth are about one-tenth of those of a direct-radiating driver. As a result of these attributes, horn- loaded systems have state-of-the-art dynamics and lifelike recreation of music’s transient signals. The sharp attack of a snare drum, for example, is reproduced with a stunning sense of realism. This quality, known as “jump factor,” gives horn-loaded loud speakers a lifelike presentation unmatched by any other loudspeaker technology.
Now the bad news. Horn-loading often introduces large tonal colorations. Try this experiment: read the previous sentence, and then read it again, this time with your hands cupped around your mouth to make a horn. For many listeners, the horn-loaded system’s colorations (particularly through the midrange) are a deal-breaker, outweighing all of the horn’s advantages.
There are, however, a few horn-loaded systems that do not sound colored in the slightest. These systems are very large, exotic, and extremely expensive, the result of the extraordinary manufacturing techniques required. The horn-loaded system in FIG. 9 is an example. In this five-way system, three of the horns are machined out of solid blocks of aircraft-grade aluminum; the top horn, which handles lower-midrange frequencies, is constructed from thick aluminum reinforced with 56 hand-welded ribs.
Loudspeaker Enclosures
The enclosure in which a set of drivers is mounted has an enormous effect on the loudspeaker’s reproduced sound quality. In fact, the enclosure is as important as the drive units themselves. Designers have many choices in enclosure design, all of which affect the reproduced sound.
In addition to the very different enclosure types described in this section, any speaker cabinet will vibrate slightly and change the sound. The ideal enclosure would produce no sound of its own, and not interfere with the sound produced by the drivers. Inevitably, however, some of the energy produced by the drive units puts the enclosure into motion. This enclosure vibration turns the speaker cabinet into a sound source of its own, which colors the music.
One of the factors that makes today’s high-end loudspeakers so much better than mass-produced products is the extreme lengths to which high-end manufacturers go to prevent the enclosure from vibrating. Mass-market manufacturers generally skimp on the enclosure because, to the uninformed consumer, it adds very little to the product’s perceived value.
A casual acquaintance once tried to impress me with how great his brother’s hi-fi system was by describing how water in a glass placed on top of the loudspeaker would splash out when the system played loudly. This person held the mistaken impression that the ability to make the water fly from the glass was an indicator of the system’s power and quality.
Ironically, his description told me immediately that this loudspeaker system was of poor quality. That’s because low-quality loudspeaker systems have thin, vibration-prone cabinets that color the sound. The more a loudspeaker cabinet vibrates, the worse it is. Any speaker system that will splash water out of a glass must sound dreadful. We’re about to see why.
When excited by the sound from the driver (primarily the woofer), the enclosure resonates at its natural resonant frequencies. Some of the woofer’s back-and-forth motion is imparted to the cabinet. This causes the enclosure panels to move back and forth, producing sound. The enclosure thus becomes an acoustic source: we hear music not only from the drivers, but also from the enclosure.
Enclosure vibrations produce sound over a narrow band of frequencies centered on the panel’s resonant frequency. The loudspeaker has greater acoustical output at that frequency. Consequently, cabinet resonances change the timbres of instruments and voices. With a recording of double-bass or piano, you can easily hear cabinet resonances as changes in timbre at certain notes.
Enclosure resonances not only color the sound spectrally (changing instrumental timbre), they smear the time relationships in music. The enclosure stores energy and releases that energy slowly over time. When the next note is sounded, the cabinet is still producing energy from the previous note. Loudspeakers with severe cabinet resonances produce smeared, blurred bass instead of a taut, quick, clean, and articulate foundation for the music. Enclosure vibration also affects the midrange by reducing clarity and smearing transient information.
FIG. 10Internal cabinet bracing reduces cabinet resonances.
The acoustic output of a vibrating surface such as a loudspeaker enclosure panel is a function of the excursion of the panel and the panel’s surface area. Because loudspeaker cabinet panels are relatively large, it doesn’t take much back-and-forth motion to produce sound. A large panel excited enough that you can feel it vibrating when you play music will color the sound of that loudspeaker. That’s why smaller loudspeakers often sound better than similarly priced but larger loudspeakers; it’s much harder (and more expensive) to keep a large cabinet from vibrating.
Cabinet vibration is why the same loudspeaker sounds different when placed on different stands, or with different coupling materials between the speaker and stand. Cones, isolation feet, and other such accessories all cause the speaker cabinet to have a different resonant signature.
A simple way of judging an enclosure’s inertness is the “knuckle-rap” test. Simply knock on the enclosure and listen to the resulting sound. An enclosure relatively free of resonances will produce a dull thud; a poorly damped enclosure will generate a hollow ringing tone.
Designers reduce enclosure resonance by constructing cabinets from thick, vibration-resistant material. Generally, the thicker the material, the better. Most loud speakers use 3/4” Medium Density Fiberboard (IMDF). MDF 1” thick is better; some manufacturers use 3/4” on the side panels and top, and 1” or 2” MDF for the front baffle, which is more prone to vibration. Exotic materials and construction techniques are also used to combat cabinet resonances. Some manufacturers go to extreme lengths to keep their cabinets inert, including advanced materials research and extraordinary construction techniques.
Cross-braces inside the enclosure reduce the area of unsupported panels and make the cabinet more rigid. Some braces are called “figure-8” braces because their four large holes form a figure-8 pattern. These braces are strategically placed for maxi mum effectiveness. (Heavy-duty internal cabinet bracing is shown in FIG. 10.)
Most quality loudspeakers are equipped with threaded inserts on the bottom panel to accept spikes. These spikes better couple the speaker to the floor and reduce enclosure vibration.
Similarly, small loudspeakers should be mounted on solidly made stands for best performance. In fact, the stand’s quality can greatly affect the reproduced sound. Flimsy, lightweight stands should be avoided in favor of rigid models. The stand should include spikes on its bottom plate to better couple the stand and loudspeaker to the floor. Some loudspeaker stands can be filled with sand or lead shot for mass loading, making them more inert and less prone to vibration. A great loudspeaker on a poor stand will suffer significantly degraded performance. Plan to spend several hundred dollars on stands. When comparing a floorstanding loudspeaker to one requiring stand- mounting, include the cost of the stands in your budgeting.
The interface between loudspeaker and stand also deserves attention. Spikes, cones, and other isolation devices (see Section 11) can allow the loudspeaker to per form at its best. An effective yet inexpensive interface is a sticky, gum-like material called Bostik Blue-Tak, available at hardware stores. Simply place a small ball of Blue Tak at each corner of the speaker stand.
Enclosure Shapes
The enclosure can also degrade a loudspeaker’s performance by creating diffraction from the cabinet edges, grille frame, and even the drivers’ mounting bolts. Diffraction is a re-radiation of energy when the sound encounters a discontinuity in the cabinet, such as at the enclosure edge. Diffracted energy combines with the direct sound to produce ripples (i.e., colorations) in the frequency response. Rounded baffles, recessed drivers and mounting bolts, and low-profile grille frames all help to reduce diffraction.
Some loudspeaker enclosures are tilted back to align the drivers in time. This tilt aligns the acoustic centers of all the drivers so that their outputs arrive at the listen er at the same time ( FIG. 11).
FIG. 11 An angled baffle helps to time-align the loudspeaker’s individual
drive units at the listening position.
Air-Suspension and Bass-Reflex Enclosures
The air suspension enclosure (technically called an infinite baffle) simply seals the enclosure around the driver rear to prevent the two waves from meeting. The air inside the sealed enclosure acts as a spring, compressing when the woofer moves in and creating some resistance to woofer motion.
I mentioned earlier that a woofer in a box produces just as much sound inside the box as outside. The bass-reflex enclosure exploits this fact, channeling some of this bass energy out of the cabinet and into the listening room. You can instantly spot a reflex-loaded loudspeaker by the hole, or vent, on the front or back of the enclosure. Reflex-loaded loudspeakers are also called ported designs.
Reflex loading extends the low-frequency cutoff point by venting some of the energy inside the enclosure to the outside. That is, a reflex-loaded woofer will go lower in the bass than one in a sealed enclosure, other factors being equal.
Reflex loading has three main advantages. First, it increases a loudspeaker’s maximum acoustic output level—it will play louder. Second, it can make a loudspeaker more sensitive—it needs less amplifier power to achieve the same volume. Third, it can lower a loudspeaker’s cutoff frequency—the bass goes deeper. Note that these benefits are not available simultaneously; the acoustic gain provided by reflex loading can be used either to increase a loudspeaker’s sensitivity or to extend its cutoff frequency, but not both.
On the down side, a woofer in a vented enclosure will tend to keep moving after the drive signal has stopped. This difference in transient response can be manifested as sluggish sound in, for example, a kickdrum. A sealed enclosure has better transient response and better bass definition, but at the cost of lower sensitivity and less deep bass extension. Nonetheless, many very-high-quality loudspeakers have been based on ported designs.
Passive Radiator
A variation on the reflex system is the passive radiator, also called an auxiliary bass radiator, or ABR. This is usually a flat diaphragm with no voice coil or magnet structure that cannot produce sound on its own. Instead, the diaphragm covers what would have been the port in the reflex system, and moves in response to varying air pressure inside the cabinet caused by the woofer’s motion.
Powered and Servo-Driven Woofers
Loudspeaker designers are increasingly choosing to include a power amplifier within the loudspeaker to drive the woofer cone. In such designs, the external power amplifier is relieved of the burden of driving the woofer, and the loudspeaker designer has more control over how the woofer behaves. For example, equalization can be applied to extend the bass response.
Some designers don’t try to juggle these laws of physics to produce the most musically satisfying compromise in bass performance; instead, they take brute-force control of woofer movement with the servo-driven woofer. A servo-woofer system consists of a woofer with an accelerometer attached to the voice coil, and a dedicated woofer power amplifier. An accelerometer is a device that converts motion into an electrical signal. The accelerometer sends a signal back to the woofer amplifier, telling the woofer amplifier how the woofer cone is moving. The woofer amplifier compares the drive signal to the cone’s motion; any difference is a form of distortion. The woofer amplifier can then change the signal driving the woofer so that the woofer cone behaves optimally.
Crossovers
A loudspeaker crossover is an electronic circuit inside the loudspeaker that separates the frequency spectrum into different ranges and sends each frequency range to the appropriate drive unit: bass to the woofer, midband frequencies to the midrange, and treble to the tweeter (in a three-way loudspeaker). FIG. 12 illustrates this process.
Ill.9-12: A loudspeaker’s crossover splits up the frequency spectrum,
sending bass to the woofer, midrange frequencies to the midrange driver
and treble to the tweeter.
A crossover is made up of capacitors, inductors, and resistors. These elements selectively filter the full-bandwidth signal driving the loudspeaker, creating the appropriate filter characteristics for the particular drivers used in the loudspeaker. A crossover is usually mounted on the loudspeaker’s inside rear panel.
A crossover is described by its cutoff frequency and slope. The cutoff frequency is the frequency at which the transition from one drive unit to the next occurs— between the woofer and midrange, for example. The crossover’s slope refers to the rolloff’s steepness. A slope’s steepness describes how rapidly the response is attenuated above or below the cutoff frequency. For example, a first-order crossover has a slope of 6dB/octave, meaning that the signal to the drive unit is halved (a reduction of 6dB) one octave above the cutoff frequency. If the woofer crossover circuit produces a cut off frequency of 1 kHz, the signal will be rolled off (attenuated, or reduced in level) by 6dB one octave higher, at 2kHz. In other words, the woofer will receive energy at 2kHz, but that energy will be reduced in level by 6dB. A first-order filter producing a 6dB/octave slope is the most gentle rolloff used.
FIG. 13 Crossovers are specified by their slope, or how steeply they
attenuate frequencies above or below their cutoff frequency.
The next—steeper filter is the second-order crossover; which produces a roll-off of 12dB/octave. Using the preceding example, a woofer crossed over 1 KHz would still receive energy at 2 kHz, but that energy would be reduced by 12dB (one quarter the amplitude) at 2kHz. A third-order crossover has a slope of 18dB/octave, and a fourth-order crossover produces the very steep slope of 24dB/octave. Using the previous example, the fourth-order filter would still pass 2 kHz to the woofer, but the amplitude would be down by 24dB (1/16th the amplitude). FIG. 13 compares crossover slopes.
Typical crossover points for a two-way loudspeaker are between I kHz and 2.5kHz. A three-way system may have crossover frequencies of 800Hz and 3kHz. The woofer reproduces frequencies up to 800Hz, the midrange driver handles the band between 800Hz and 3kHz, and the tweeter reproduces frequencies above 3kHz.
Digital Loudspeakers
The capacitors, inductors, and resistors that make up a loudspeaker crossover are far from perfect. Not only do they exhibit variations in value that affect performance, these components split up the frequency spectrum in a relatively crude way. Moreover, a significant amount of amplifier power is wasted in the crossover.
The widespread availability of digital-audio sources provides an opportunity to remove traditional crossovers from the signal path. If we can divide the frequency spectrum digitally—that is, by performing mathematical computations on the digital representation of the audio signal—we can create just about any crossover characteristics we want, with none of the problems of capacitors, inductors, and resistors.
These “digital loudspeakers” accept a digital input signal and implement the crossover in the digital domain with digital signal-processing (DSP) chips. Instead of subjecting the high-powered audio signal to resistors, capacitors, and inductors, DSP crossovers separate the frequency spectrum by performing mathematical processing on the digital audio data. DSP crossovers can be programmed to have perfect time behavior, as well as use any slope and frequency the designer wants without regard for component limitations or tolerances, and employ equalization to the individual drive units.
FIG. 14 is a side view of a combination pictorial/block diagram of a digital loudspeaker. The speaker accepts a digital input signal from a CD transport, DVD Audio player, or other digital source. DSP chips inside the speaker split the frequency spectrum into bass, midrange, and treble. These chips can also equalize and delay signals to produce nearly perfect acoustic behavior at the drive units’ output. Each of the three digital signals (bass, midrange, treble) is then converted into an analog signal with its own digital-to-analog converter (DAC). Each DAC’s output feeds a power amplifier specially designed to power the particular drive unit used in the digital loudspeaker. This aspect of a digital loudspeaker confers a large advantage: The power amplifiers are designed to drive a known load. The power amplifiers amplify an analog signal, just like a conventional power amp, and then drive conventional cone loudspeakers.
FIG. 14 Digital loudspeakers employ a digital-to-analog converter and power amplifier for each driver.
In addition to employing virtually perfect crossovers, digital loudspeakers also simplify your system. You need only a digital source and a pair of loudspeakers for a complete music system. This eliminates large power amplifiers and garden-hose speaker cable on your floor. Moreover, the digital nature of the system provides for remote control of the loudspeaker. You can adjust the tonal balance, correct for different listening heights, and even delay one loudspeaker if your room doesn’t permit symmetrical speaker placement. The digital speaker will also have a volume readout, which makes setting the correct playback level for different music more convenient. Another advantage of a loudspeaker under DSP control is the ability to protect the drivers from damage; the DSP knows the driver’s limitations and can impose a maximum excursion limit. When considering the price of digital loudspeakers, keep in mind that you don’t need to buy an outboard digital-to-analog converter or power amplifiers.
Digital loudspeakers were pioneered by Meridian Audio, which has been producing them since 1993. Meridian’s first models accepted the 44.1kHz, 16-bit data-stream from a CD transport, but newer models have been upgraded to accept high-resolution digital audio with sampling rates up to 192kHz and word lengths up to 24 bits.
Subwoofers
A subwoofer is a loudspeaker that produces low frequencies that augment and extend the bass output of a full-range loudspeaker system. The term “subwoofer” is grossly mis used to describe any low-frequency driver system enclosed in a separate cabinet. But “subwoofer” actually means “below the woofer,” and should be reserved for those products that extend bass response to below 20Hz. A low-frequency driver in an enclosure extending to 40Hz and used with small satellite speakers is more properly called a woofer.
You’ll also see full-range speakers with a built-in “subwoofer” powered by its own amplifier. Most of these products actually employ woofers that are simply driven by an integral power amplifier. Such a design relieves your main amplifier of the bur den of driving the woofer, but requires that the loudspeakers be plugged into an AC outlet.
Subwoofers come in two varieties: passive and active. A passive subwoofer is just a woofer or woofers in an enclosure that must be driven by an external amplifier. An active subwoofer combines a subwoofer with a line-level crossover and power amp in one cabinet. Such a subwoofer has line-level inputs (which are fed from the preamplifier), line-level outputs (which drive the power amp), and a volume control for the subwoofer level. In some active subwoofers, an integral crossover separates the bass from the signal driving the main loudspeakers at line-level, which is much less harmful to the signal than speaker-level filtering. The line-level output is filtered to roll off low- frequency energy to the main loudspeakers. This crossover frequency is often adjustable on subwoofers to allow you to select the frequency that provides the best integration with the main loudspeakers (more on this later).
A subwoofer used as part of a home-theater loudspeaker system is often connected in a completely different manner. When used for home-theater, the subwoofer is driven by the SUBWOOFER OUTPUT jack of your A/V receiver or A/V controller. In this case, the receiver or controller performs the crossover function, sending only bass to the subwoofer. Some subwoofers have separate inputs for home-theater connection (the input is often labeled “Processor”) and conventional stereo reproduction (the input is often labeled “Line”). (Section 10 includes a more complete description of these connection choices.)
Adding an actively powered subwoofer to your system can greatly increase its dynamic range, bass extension, midrange clarity and ability to play louder without strain. The additional amplifier power and low-frequency driver allow the system to reproduce musical peaks at higher levels. Moreover, removing low frequencies from the signal driving the main loudspeakers lets the main loudspeakers play louder because they don’t have to reproduce low frequencies. The midrange often becomes clearer because the woofer cone isn’t furiously moving back and forth trying to reproduce low bass.
A surprising additional benefit of adding a subwoofer is an increased sense of space and soundstage size, even when playing music with little low-frequency content. I’ve heard a demonstration of an unaccompanied singer in a large hail with a sub- woofer turned on and turned off. One would expect that a solo voice would not be affected by the presence of a subwoofer, but with the sub, the hall size appeared to increase. The reason is that low-frequencies contain subtle spatial cues (in recordings made in large rooms) about the size of the acoustic space.
Unfortunately, many subwoofers degrade a playback system’s musical performance. Either the subwoofer is poorly engineered (many are), set up incorrectly, or, as is increasingly common, designed for reproducing explosions in a home-theater system, not resolving musical subtleties. Another problem is getting the subwoofer to integrate with the main speakers so that you hear a seamless and coherent musical whole.
As instruments traverse the crossover region, you should hear no discontinuity in the sound. Ascending and descending bass lines, for example, should flow past the crossover point with no perceptible change in timbre or dynamics.
All of these problems are exacerbated by most people’s tendency to set sub woofer levels way too high. The reasoning is that if you pay good money for some thing, you want to hear what it does. But if you’re aware of the subwoofer’s presence, either its level is set too high, it isn’t positioned correctly, or the subwoofer has been poorly designed. The highest compliment one can pay a subwoofer is that its contributions can’t be heard directly. It should blend seamlessly into the musical fabric, not call attention to itself.
For home theater, a subwoofer is essential to the movie-watching experience. A good subwoofer adds an excitement and visceral thrill to some movies. If you have full-range left and right speakers that have adequate bass response for music, drive the subwoofer with the receiver or controller’s subwoofer output signal. In the receiver or controller’s set-up menu, select LARGE for the left and right speakers. With this arrangement, the subwoofer won’t be engaged when playing stereo music through the left and right loudspeakers.
Section 10 includes some special set-up techniques for connecting a subwoofer and getting it to sound good in your system.