A Guide To Better Bass (TAS 197)

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A Guide To Better Bass (TAS 197)

Accurately reproducing music’s low frequencies is the single biggest challenge facing the audiophile. The laws of physics make it difficult to achieve a smooth, extended, and articulate bottom end in our listening rooms. Consequently, many of us live with less than great bass performance.

 But wonderful-sounding bass is immensely rewarding musically. The bottom end forms the tonal foundation of some types of music, and in others bass is the source of music’s rhythmic drive, propulsion, and energy. The visceral, whole-body experience of a great drummer and bass guitarist locking into a groove—the kick drum’s transient and the attack of the bass guitar strings combining synergistically—is one of music’s supreme pleasures (at least for me).

In this “Guide to Better Bass,” we’ll look at how you can improve the bottom end of your existing audio system, explore different options if you’re just starting out or upgrading, and consider some general principles for getting great bass.

Let’s start with some fundamentals about bass reproduction. Despite what I just wrote about the importance of bass, it’s possible to put together an extremely involving music system based on smaller speakers that don’t reproduce bass below about 50Hz. This is particularly true for listeners whose tastes lean toward chamber and smaller-scale works in classical music, singer/songwriters in pop, and acoustic jazz. Listeners with those musical inclinations are better off with smaller speakers with limited bass response than with full-range speakers of a similar price that may be compromised throughout the entire sonic spectrum.

Second, bass quality is vastly more important than bass quantity. A leaner presentation without much extension is preferable to lots of bass if that bass is thick, colored, and sluggish. If the bass isn’t well reproduced, we’d rather not hear it at all. The poor bass performance becomes a constant annoyance and a reminder that we’re listening to a reproduction. This is why a superbly engineered mini-monitor can be more musically involving than a large floorstanding speaker.

Third, accurate bass reproduction is expensive. The lower the frequency accurately reproduced, the more expensive bass becomes. Note the word “accurate” in both sentences; you can buy a $500 loudspeaker that has output below 40Hz, but it’s unlikely that the bass it produces will be accurate. Realistic reproduction of the bottom octave (16Hz–32Hz) requires large woofers, which in turn requires a large cabinet. The larger the cabinet the more prone it is to vibration that will color the sound. Enclosure vibration colors the music tonally and destroys music’s dynamic structure. The solution is to build heroic enclosures that don’t vibrate, but such enclosures are extremely dense, heavy, and expensive.

Fourth, a system’s bass presentation affects such seemingly unrelated aspects of the sound as midrange clarity and soundstaging. Thickness in the midbass reduces the midrange’s transparency. A cleaner midbass not only makes the midrange sound more open, it also lets you hear more clearly into the extremely low frequencies. Moreover, extending a system’s bottom end has the odd effect of increasing soundstage depth and our overall sense of the recorded acoustic, even on music with no low-frequency energy. I’ve heard an unaccompanied voice in a large hall reproduced by a pair of mini-monitors with and without a subwoofer. Adding the subwoofer revealed the full extent of the hall’s size, as well as presented the vocalist as a more tangible image within the acoustic.

With those concepts in mind, let’s see how we improve a system’s bass performance.

Match the Speaker to the Room

The deeper the loudspeaker’s bass extension and the more bass output it produces, the larger the room needed to realize great bass performance. Lots of very low bass will overload a small room, making it almost impossible to get smooth response. This fundamental fact is played out countless times at hi-fi shows as exhibitors fight to get a large full-range loudspeaker to work in a hotel room. If you choose too much speaker for your room, you’ll wage an uphill battle in getting good-sounding bass.

Loudspeaker Placement

Correctly positioning your loudspeakers is the single most important thing you can do to achieve better bass. The topic is beyond the scope of this article, but you can get an idea of its importance from the accompanying sidebar “The Physics of Bass.” For specific loudspeaker-placement techniques, download the free booklet “Robert Harley’s System Set-Up Secrets” at avguide.com/hifibooks. The booklet is an excerpt from The Complete Guide to High-End Audio (Third Edition).

Adding A Subwoofer to Your System

There are two reasons to consider a subwoofer. The first is if you like the sound of your main speakers and just want more bass extension, power, and impact. The second is if want a full-range sound but don’t want the intrusion of large, floorstanding speakers in your living room.

Both cases sound simple in theory, but in practice, getting a subwoofer to blend with your main speakers is quite a challenge. Although you’ll undoubtedly get more bass, you might not achieve a sound that is seamless and coherent from bottom to top. That is, you might be aware that there’s this big cone chugging away, seemingly disconnected from the rest of the music.

It’s possible, however, to avoid this nightmare scenario with a little knowledge. First, choose a subwoofer designed for musical accuracy, not home-theater fireworks. Some subwoofers exist to produce the highest possible sound-pressure-level at the lowest possible frequency for playing back explosions in film soundtracks. Others are crafted by musically sensitive designers with high-end sensibilities. Be sure which kind you’re buying.

Second, choose an appropriate subwoofer for your main speakers and your room. If you have a 5.5" two-way mini-monitor in a small room, a sub with an 8" driver is more likely to blend with your mini-monitors than a model with a 12" cone. Moreover, the smaller subwoofer is less likely to overload your small room. The smaller the room and the greater the subwoofer’s output, the greater the odds against achieving a musical result.

Third, use proper loudspeaker placement techniques (referenced earlier) so that the sub produces a smooth response. One of the huge advantages enjoyed by a subwoofer/satellite system is the ability to position the satellites for best imaging without worrying about the bass response, and then to put the subwoofer where it best integrates with your room.

Fourth, spend some time tweaking the subwoofer’s controls so that it blends seamlessly with your main speakers. On the one hand, getting two different products (the main speakers and sub), designed by two different designers, to work together in harmony is asking a lot. On the other hand, you have much more control over a subwoofer than you do over the bass output in a full-range system. Take advantage of the subwoofer’s volume, phase, crossover frequency, and other adjustments to perfectly dial it into your system. Generally, the lower the crossover frequency between the sub and main speakers the better; the main loudspeaker’s bass is probably of higher quality than the subwoofer’s, and a low crossover frequency moves any crossover discontinuity lower in frequency, where it will be less audible. In addition, a low crossover frequency ensures that you won’t be able to locate the sound source of the low bass. A subwoofer reproducing frequencies above 100Hz can be “localized”—i.e., the location of the source of the bass can be detected—which is musically distracting. Too low a crossover frequency will, however, burden small loudspeakers with excessive bass and reduce the system’s power handling and maximum listening level.

Another variable in subwoofer crossovers is the slope. Most use second-order (12dB/octave) or higher filters. Ideally, the crossover frequency and slope would be tailored to the particular loudspeaker used with the subwoofer. But because the subwoofer manufacturer doesn’t know which loudspeakers will be used with the subwoofer, these parameters are compromised for good performance with a variety of loudspeakers.

Some advanced subwoofers have an automatic equalization system built into them that removes the worst room-induced peaks and dips. For example, the superb JL Audio line incorporates the company’s Automatic Room Optimization (ARO) technology. You simply connect the supplied microphone to the subwoofer, press a button, and ARO measures the subwoofer’s response in the room and custom-tailors an equalizer to fill in the dips and attenuate the peaks.

A subwoofer’s phase control allows you to time-align the subwoofer’s wavefront with that of the main speakers. Here’s a simple trick for perfectly setting this adjustment. (This technique assumes that the phase control is a continuously variable knob, not just a simple “0/180°” switch.) Drive the system with a pure tone at exactly the crossover frequency between the woofer and main speakers. (Many test CDs include a full range of test tones.) Driving the system with a pure tone at the crossover frequency causes the main loudspeakers and the subwoofer to reproduce the same signal. Now invert the polarity of the main loudspeakers relative to the subwoofer by reversing the red and black leads going to both loudspeakers. Sit in the listening chair and have an assistant slowly vary the phase control until you hear the least bass. Return the loudspeaker leads to their former (correct) polarity. The phase control is now set optimally. Here’s why: When the main loudspeakers’ and subwoofer’s wavefronts are 180° out of phase with each other, the greatest cancellation (the least sound heard) will occur. That’s because as the subwoofer’s cone moves outward, the main speakers’ cones are moving in, canceling each other. When the loudspeaker leads are returned to the correct position (removing the 180° phase shift), the subwoofer and loudspeaker outputs are maximally in-phase. Any time lag between the main speakers and subwoofer has been eliminated. This technique works because it’s much easier to hear the point of maximum cancellation than the point of maximum reinforcement.

Ported vs. Sealed Enclosures

Most loudspeakers today use either a sealed enclosure or a ported enclosure. Which type you choose will greatly affect the character of the bass the speaker produces. In a sealed enclosure, also called acoustic-suspension loading in some designs, the air inside the cabinet acts as a spring behind the woofer, compressing when the woofer moves in. In a ported enclosure, also called bass-reflex loading, the woofer’s rear wave is channeled outside the cabinet by a port or duct.

This simple difference results in very different technical and subjective bass performances. All sealed speakers have a low-frequency roll-off slope of 12dB per octave; this means that, one octave below the system’s resonant frequency, the output will be reduced by 12dB. This roll-off is relatively gradual, meaning you’ll still hear some lower bass below the specified cut-off frequency. Reflex enclosures have a much steeper roll-off of 24dB per octave. That is, one octave below the system’s resonant frequency, the amplitude is reduced by a whopping 24dB.

So why would a loudspeaker designer choose reflex loading if the bass rolls off much more steeply? Because reflex loading confers several 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 cut-off 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 cut-off frequency, but not both.

To summarize, the reflex-loaded system maintains flat bass response down to a lower frequency, but then the bass output drops off more quickly than it does in a sealed system. You can see this phenomenon in Fig.1, a comparison of the frequency responses of sealed and reflex loading.

The common way of specifying a speaker’s low-frequency extension is to cite the frequency at which its response is attenuated by 3dB (“-3dB at 28Hz” for example). This method unfairly favors reflex loading because it doesn’t take into account the very steep roll-off below the -3dB cut-off frequency. The ideal method of specifying a loudspeaker’s bass extension is to cite the frequency in which its response is rolled off by 3dB as well as the frequency at which its response is rolled off by 10dB. A loudspeaker’s -10dB point is a more reliable indicator of a loudspeaker’s subjective bass fullness and extension because it takes into account not only the low-frequency cut-off point, but also the steepness of the roll-off.

There’s one more technical difference between sealed and ported enclosures you should know about before we talk about the sonic characteristics of the two types—transient performance. A woofer in a sealed enclosure, when subject to a transient signal such a kick drum, will tend to stop moving immediately after the transient. Conversely, a woofer in a vented enclosure will tend to keep moving after the drive signal has stopped, as shown in Fig.2. The speaker with the sealed enclosure has more accurate dynamic performance.

These technical differences can add up to very different bass presentations. The following observations are generalities rather than cast-in-stone facts that apply to all examples. First, the bass from sealed enclosures tends to be tighter, leaner, and more precise. Pitch definition is superior, as is the sense of articulation of each bass note. The sealed loudspeaker’s higher cut-off frequency and more gradual roll-off provide a more satisfying feeling of bass fullness than the reflex system’s lower cut-off frequency and steeper roll-off. Very low bass, such as organ pedal tones, tends to produce a feeling of pressurization of the air in the room when reproduced by sealed systems that have truly deep extension. Reflex systems, by contrast, have more weight, warmth, and fullness. They can subjectively sound like they have more bass and deeper extension when you’re listening to instruments with energy in the midbass rather than the extremely low bass. Kick drum tends to be weightier, but less crisp and dynamic.

These impressions are by no means definitive; poorly designed sealed systems can sound thick, colored, and lacking in articulation and dynamic agility. Moreover, they are gross generalizations that are less applicable at the upper end of the price spectrum. The best bass I’ve ever heard in every aspect of performance—extension, dynamics, precision, articulation, and tonality—was from a ported system (Wilson Alexandria X-2 Series 2). But it takes extraordinary design talent to deliver the benefits of a particular woofer loading while eliminating the shortcomings. In most entry-level and mid-priced loudspeakers, the characteristics of sealed and reflex-loaded designs I’ve described are applicable.

Loudspeakers with Powered Woofers

Some loudspeakers include integral power amplifiers to drive the woofers. Such speakers provide a range of bass-tuning options not possible with conventional passive loudspeakers. A speaker with a powered woofer can be equalized to extend its cut-off frequency, allowing a relatively small-footprint speaker to deliver response to 20Hz. You can also adjust the bass level to best match your room. The ability to adjust each speaker’s bass output independently is a huge benefit, particularly in asymmetrical rooms. For example, if one speaker is in a corner and one speaker has no sidewall next to it, you can dial down the bass level from the corner-located speaker to compensate for its greater room gain.

Another advantage is that your system becomes bi-amped, freeing your main amplifier from the burden of driving the woofers. The loudspeaker’s integral power amplifier can be designed precisely to drive the known load of the woofer; similarly, the woofer is driven by a known amplifier. The woofer and amplifier can be designed as a system, optimizing performance.

Two examples of speakers with powered woofers are the YG Acoustics Kipod Studio and the Vandersteen Model 5A. The Kipod Studio is a two-piece system whose lower section serves as the woofer as well as a stand for the Kipod mini-monitor. The woofer section is fed by a line-level signal, which in turn feeds an integral 200W amplifier that drives the woofer. While recently setting up the Kipod Studio (review forthcoming) in my room with YG Acoustics’ Dick Diamond, it struck me that powered woofers make a lot of sense. The ability to set the bass balance with the turn of a knob is quite compelling.

The Vandersteen Model 5A has an integral amplifier that drives a massive 12" push-pull subwoofer. The signal feeding the 7" woofer, midrange, and tweeter is high-pass-filtered at line level before your main power amplifier, with a complementary boost before the subwoofer amplifier. Richard Vandersteen also devised for the Model 5A an ingenious system for achieving flat bass response in any room. The speaker’s rear panel contains a row of eleven tiny adjustment screws, each of which controls the amplitude of one of eleven frequencies between 20Hz and 120Hz. By concentrating this eleven-band equalizer over the area where bass problems arise, room-induced peaks and dips can be flattened. The equalizer cannot be set by ear. Rather, Vandersteen developed a custom calibration device and test signal that allows the dealer to perfectly dial-in the equalizer settings. Another adjustment sets the system “Q” for your particular room and preference (see sidebar).

A variation on the powered-woofer concept is 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. For example, the inertia in a woofer would cause it to continue moving after a bass-drum whack. The woofer’s servo-amplifier would see the cone moving and instantly stop the cone motion. In fact, if you try to gently press in the cone of a servo-driven woofer with the amplifier connected without playing music, you’ll find that the cone doesn’t move. Because there’s no drive signal, the servo-amplifier knows the cone shouldn’t be moving and thus locks it in place.

Adding Acoustic Absorbers

It’s an unfortunate fact of life that carpets, drapes, couches, and chairs absorb mid and high frequencies, but do nothing to low frequencies. Consequently, midrange and treble frequencies decay much more quickly than bass frequencies. The bass tends to hang in the room, muddying the music’s tonality and smearing its dynamic structure. The bottom end becomes an unintelligible roar underneath the music. Moreover, this bass and upper-bass coloration tends to obscure and thicken the sound of the midrange. It’s astonishing how cleaning up the bass confers a huge increase in midrange transparency, openness, and timbral purity.

Although proper loudspeaker placement goes a long way toward achieving smooth bass response, placement alone can’t completely eliminate room-induced bass colorations. The next step is to add commercially available bass absorbers. When low frequencies strike bass absorbers, some of the bass energy is converted into a minute amount of heat rather than being reflected back into the room. A classic use of a bass absorber is a pair of 16" Full-Round Tube Traps from Acoustic Sciences Corporation in the corners behind the loudspeakers. This placement is ideal, preventing bass from being reflected from the rear wall back into the room where it would have combined with the loudspeaker’s direct sound. Other types of bass traps are available, but don’t be swayed by the promise of bass absorption in a tiny, inconspicuous package; long wavelengths are simply unaffected by small structures. If you’ve got bass problems that can’t be solved by loudspeaker placement or any of the other techniques described, it’s time to bring out the Big Guns of acoustic treatments.

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SIDEBAR: THE PHYSICS OF BASS

Although a 20Hz sinewave and a 20kHz sinewave traveling in air are essentially the same phenomenon (compressions and rarefactions above and below normal atmospheric pressure), the two frequencies behave very differently. Simply put, bass is essentially omnidirectional and high frequencies tend to beam. The higher the frequency, the more directional the sound. That’s because low frequencies have very long wavelengths that bend around objects that are a small fraction of their wavelength. In fact, objects of the size found in a living room, including loudspeaker enclosures, are essentially invisible to the long wavelengths of low frequencies. To give you an idea of just how great the disparity in wavelengths is within the range of human hearing, a 20Hz sinewave in air has a wavelength of 56.5 feet and a 20kHz sinewave has a wavelength of about half an inch. (Wavelength equals velocity [the speed of sound] divided by the frequency, or =v/f.)

If you had an aerial view of your loudspeaker and could see the speaker’s dispersion pattern, you’d see very low frequencies bending around the enclosure and reflecting off the rear and sidewall, midrange frequencies dispersed in a hemisphere in front of the speaker, and very high treble acting almost like a beam of light emanating from the tweeter. This is why speaker toe-in has such a huge effect on the treble balance you hear at the listening position but no effect on the bass response. Conversely, it’s why loudspeaker placement in relation to the room boundaries affects bass balance but not treble balance.

Bass from the loudspeaker, radiating omnidirectionally, is reflected from the room’s rear wall and sidewalls. This reflected energy combines with the direct wave from the woofer essentially in-phase, combining constructively to increase the bass amplitude. This is the common phenomenon known as “room gain”—a bass boost added by the room. The direct and reflected waves aren’t precisely phase-aligned, which causes some frequencies to be boosted more than others. In the upper bass, the direct and reflected waves can be out of phase, combining destructively to create a dip in the frequency response. This phenomenon colors the bass, adding an unnatural emphasis or de-emphasis to certain registers of certain instruments, particularly basses. Because the frequency at which this reinforcement or cancellation occurs is dependent on the distance between the speaker and the room boundaries, we can use good loudspeaker placement techniques to mitigate the deleterious effects of this phenomenon.

You can see the effects of “room gain” in Fig. 3, a loudspeaker’s anechoic frequency response (its response in a reflection-free room) and its response in a normal room. Not only is the low bass boosted because of room gain, but we now have peaks and dips in the response caused by the constructive and destructive combination of direct and reflected energy, along with room-resonance modes. Figure 4 is the same speaker in the same room, but at different distances from the rear and sidewalls. In this poor placement, caused by the speaker being the same distance from the rear wall as from the sidewall, the side-wall and rear-wall cancellations and reinforcements occur at the same frequency. If we move this speaker in the room so that the distances to the rear and side walls are staggered (by about a third), the response is much smoother (Fig. 5). Take a look at the vertical scale of Figs. 3–5; we’re talking about colorations of a whopping 15dB. This is why a loudspeaker designer once told me “I have 100% control over the sound of my speaker above 300Hz, 50% control from 150Hz to 300Hz, and 20% control below 150Hz.”

Proper loudspeaker placement—positioning the speakers different distances from the rear and side walls—can greatly reduce these colorations.

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Bass Tweaks

So far, we’ve talked about bass problems on a macro-level: room-induced peaks and dips of 15dB, excessive bass caused by lack of low-frequency absorption, poor subwoofer integration, and the significant differences in bass performance between sealed and ported loudspeakers.

But there’s another, more subtle, approach to getting better bass that works on the micro-level, relatively speaking. This approach includes system matching and tuning, and the careful use of just the right accessories. Objectively, these techniques and products have a miniscule effect on the signal compared to the effects of room-induced peaks and dips, but they are significant nonetheless. A fundamental tenet of high-end audio holds that there’s not a linear relationship between the magnitude of a difference and the musical effect of that difference—that is, a “small” improvement can have a profound influence on musical perception. This is why a tweak can be audible and significant even in the face of room-induced peaks and dips of 15dB.

Particularly effective accessories that can tighten up the bass are feet, cones, and isolation devices, particularly under tubed equipment. I’ve heard isolation feet that make the bass sound tighter, weightier, and more articulate tonally and dynamically. A good equipment rack can have a similar effect. The combination of more weight and power with greater precision is particularly rewarding.

The right choice of cable and interconnects can also push a system that last little bit into perfect bass balance. I recommend trying cables under consideration in your own system before buying. Similarly, the right AC conditioner can seemingly add low-end extension, authority, and a more realistic rendering of bass textures.

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SIDEBAR: SYSTEM “Q”

Just as a struck bell produces a certain pitch, a woofer in an enclosure will naturally resonate at some frequency. The nature of that resonance is an important characteristic of the loudspeaker, and one that greatly influences its sound. The term Q, for “quality factor,” is a unit-less number that expresses how a woofer resonates in an enclosure.

Specifically, a loudspeaker’s Q equals the resonant peak’s center frequency divided by the peak’s bandwidth. A woofer that “rings” (resonates) over a very narrow frequency band is said to have a higher Q than a woofer that resonates less severely over a wider band of frequencies. The steeper the resonance, the higher the Q.

The woofer has its own resonant Q, which is modified by the enclosure’s Q. These resonances combine and interact to reach the system Q, which usually falls between 0.7 and 1.5, as shown in Fig. 6. A Q of less than 1 is considered overdamped, while a Q of more than 1 is underdamped. You’ll sometimes hear a loudspeaker described as having subjectively “underdamped bass,” which means the bass is full and warm but lacks tightness. Technically, these terms refer to the system’s anechoic response (the speaker’s response in a reflection-free room), specifically whether the response is up or down at the resonant frequency. A “critically damped” system having a Q of 0.5 provides perfect transient response, with no detectable overhang. That is, the woofer stops moving the instant the drive signal stops. The higher the Q, the more the woofer rings.

Subjectively, an underdamped alignment has lots of bass but lacks tightness, has poor pitch definition, and tends to produce “one-note” bass. An overdamped alignment produces a very tight, clean, but decidedly lean bass response. An overdamped loudspeaker has less bass, but that bass is of higher quality than the bass from an underdamped system. Overdamped speakers tend to satisfy intellectually by resolving more detail in the bass, but often lack the bass weight and power that viscerally involves your whole body in the music. Most loudspeaker designers aim for a Q of about 0.7 to reach a compromise between extended bass response (down only 3dB at resonance) and good transient response (very slight overhang). Some designers maintain that a Q of 0.5 is ideal, and that a higher Q produces bass of poorer quality.

Mass-market loudspeakers are virtually always underdamped (high Q) so that the unwary will be impressed by the loudspeaker’s “big” bottom end. An example of absurdly high Q is the “boom truck” that produces a big bass impact but fails to resolve pitch, dynamic nuances, or any semblance of musical detail. That boom you hear is the woofer resonating in its enclosure at a specific frequency—the antithesis of what we want in a high-end loudspeaker.

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DSP Room Correction

Finally, you can attack bass problems with the massive firepower of modern digital signal processing technology. A DSP room-correction system analyzes, with great precision, the exact frequency of peaks and dips along with their magnitudes and bandwidths. It then creates a custom equalization curve that is the inverse of the frequency response in your room at the listening position. The DSP essentially “distorts” the signal, in the digital domain, in a way that results in flat response when modified by the room’s distortion.

One drawback of DSP room correction is the need to digitize analog signals. If LP is an important source for you, converting the output of a phonostage to digital might seem anathema. In addition, many DSP room-correction devices have built-in digital-to-analog converters which don’t allow you to use your DAC of choice.

It’s important to note that DSP room correction isn’t a magic bullet that will perfectly fix even the most severe problems. If you elect to add DSP correction to your system, the better you can make the system perform without correction, the better. Keep in mind that if you have a suckout of 15dB at 80Hz, the DSP correction system will modify the signal driving your loudspeakers by adding 15dB boost at 80Hz, greatly taxing your woofers.

Nonetheless, DSP room correction undoubtedly results in dramatically cleaner bass. The bloat, thickness, and weight are replaced by a much leaner, faster, and more agile sound. Interestingly, removing the midbass bloat makes the very lowest frequencies much more audible, probably because the lowest frequencies are no longer masked by excessive midbass energy. In addition, cleaning up the midbass allows the midrange to sound more open, clean, and transparent. DSP room correction also results in a more sharply defined and focused soundstage. That’s because it makes the response from the left and right speakers at the listening position identical. When the left and right speakers have slightly different frequency responses at the listening position, instrumental images can shift slightly as a function of the register in which they are playing.

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SIDEBAR: DESCRIBING BASS PERFORMANCE

Perhaps the most prevalent bass problem is lack of pitch definition or articulation. These two terms describe the ability to hear bass as individual notes, each having an attack, a decay, and a specific pitch. You should hear the texture of the bass, whether it’s the sonorous resonance of a bowed doublebass or the unique character of a Fender Precision. Low frequencies contain a surprising amount of detail when reproduced correctly. When the bass is reproduced without pitch definition and articulation, the low end degenerates into a dull roar underlying the music. You hear low-frequency content, but it isn’t musically related to what’s going on above it. In music in which the bass plays an important rhythmic role—rock, electric blues, and some jazz—the bass guitar and kick drum seem to lag behind the rest of the music, putting a drag on the rhythm. Moreover, the kick drum is buried in the bass guitar’s sound, obscuring its musical contribution. These conditions are made worse by the common mid-fi affliction of too much bass.

Excessive bass is a constant reminder that you’re listening to reproduced music. On the other hand, if you hear too little bass, the presentation is thin, lean, threadbare, or overdamped. An overly lean presentation robs music of its rhythm and drive. Thin bass makes a doublebass sound like a cello, a cello like a viola. The rhythmically satisfying weight and impact of bass drum are reduced to shadows of themselves. However, overly lean bass is preferable to boomy bass.

Two terms related to the quantity of bass are extension or depth. Extension is how deep the bass goes—not the bass and upper bass, but the very bottom end of the audible spectrum. This is the realm of kick drum and pipe organ. All but the very best systems roll off these lowermost frequencies. Fortunately, deep extension isn’t a prerequisite to high-quality music reproduction. If the system has good bass down to about 30Hz, you don’t feel that much is missing. Pipe-organ enthusiasts, however, will want deeper extension and are willing to pay for it.

Much of music’s dynamic power—the ability to convey wide differences between loud and soft—is contained in the bass. A system or component that has excellent bass dynamics will provide a sense of sudden impact and explosive power. Bass drum will jump out; the dynamic envelope of acoustic or electric bass will be accurately conveyed, allowing the music full rhythmic expression. We call these components punchy, and use the terms impact and slam to describe good bass dynamics. A related aspect is speed, though, as applied to bass, “speed” is a misnomer. Low frequencies inherently have slower attacks than higher frequencies, making the term technically incorrect. But the musical difference between “slow” and “fast” bass is profound. A product with fast, tight, punchy bass produces much greater rhythmic precision. Although reproducing the sudden attack of a bass drum is vital, equally important is a system’s ability to reproduce a fast decay; i.e., how a note ends. The bass note shouldn’t continue after a drum whack has stopped. Many loudspeakers store energy in their mechanical structures and radiate that energy slightly after the note itself. When this happens, the bass has overhang, a condition that makes kick drum, for example, sound bloated and slow. Music in which the drummer used double bass drums is particularly revealing of bass overhang. If the two drums merge into a single sound, overhang is probably to blame. You should hear the attack and decay of each drum as distinct entities.

 Portions of this article excerpted and adapted from The Complete Guide to High-End Audio (Third Edition). Copyright ©1994–2009 by Robert Harley. hifibooks.com