Excellent description of bass , thank you. I often find it difficult to describe in words what I consider good bass response but I think your information about "critically damped" at a specific system Q factor of 0.7, wich is a good balance of bass quantity vs bass quality, is very descriptive and useful. Sort of like Goldilocks' porridge, just right.
I understand that a sub's woofer driver has its own resonant Q, which is modified by the sub enclosure’s resonant Q. These resonances combine and interact to reach the system Q. A Q of less than 1 is considered overdamped, while a Q of more than 1 is underdamped. Most sub 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.
I'm not sure of the exact Q factor of each of the Audio Kinesis Debra's 4 subs I use in my system, or if the Q factor of each sub differs from that of all 4 operating in concert. I expect the dba system Q factor would be somewhere in or very near the 0.5-0.7 range since I consider the bass performance to be very well balanced between bass quantity and quality.
I think it’s possible to put together an extremely involving music system based on smaller speakers that don’t reproduce bass below about 50Hz. But I know I enjoy music and ht tremendously more with 6x2ft planar-magnetic panels and a 4-sub dba system that seamlessly integrates the high quality midrange/treble/imaging of the panels with the high quality bass of the dba that accurately extends the bass response down to 20 Hz. I've discovered that experiencing even just most of the bottom octave (16-32 Hz) on music and ht is immensely enjoyable mainly because I perceive both as more realistic. I'm not a Bass-Head but I admit I enjoy the bass weight and power that viscerally involves your whole body in the music or ht. But I also believe in setting the crossover frequency as low as possible so the subs only engage when required for accuracy and not for an artificial general system bass boost.
I've also discovered that bass quality is vastly more important than bass quantity. A leaner presentation without much extension is preferable to me than lots of bass if that bass is thick, colored, and sluggish. If the bass isn’t well reproduced, I think most would agree 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.
I've learned that realistic reproduction of the majority of the bottom octave (16Hz–32Hz) doesn't require large woofers in large enclosures, 4 subs with 10" woofers in relatively small enclosures are equally capable.
It's also true that a system’s bass presentation affects such seemingly unrelated aspects of the sound as midrange clarity and sound staging. Thickness in the mid bass reduces the midrange’s transparency. A cleaner mid bass 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 minimal low-frequency energy.
I believe there are two main 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 you want a full-range sound but don’t want the intrusion of large, floor-standing speakers in your living room, although subs with floor-standing speakers can also definitely improve overall system bass response.
Both cases sound simple in theory, but in practice, getting subs 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 big cones chugging away, seemingly disconnected from the rest of the music. But the 4 10" cones, chugging away at a more moderate pace and even with all in mono mode, of a dba's subs seem exceptionally well integrated with the rest of the music in my system.
However, I'd suggest choosing subs designed for musical accuracy, not home-theater fireworks. Some subs 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 prefer and are selecting.
Proper placement of the subs is very important in providing optimum bass smoothness and detail. One of the huge advantages of sub/satellite systems is their ability to position the satellites for best imaging without worrying about the bass response, and then to locate the subs for best integration and bass response with the room and satellites. This includes treating full-range floor standers as satellites, too.
It's critical to spend some time adjusting the subs’s or dba's amp controls so that it blends seamlessly with your main speakers. On the one hand, getting two different products (the main speakers and subs), designed by two different designers, to work together in harmony is asking a lot. On the other hand, you have much more control over subs or a dba system than you do over the bass output in a full-range system.
Take advantage of the subs’s or dba amp's volume, phase, crossover frequency, and other adjustments to perfectly dial it into your system. Generally, the lower the crossover frequency between the subs and main speakers the better; the main loudspeaker’s bass is often of higher quality than the sub’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.
Subs 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 sub 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 loudspeakers used with the subs. But because the sub manufacturers don’t know which loudspeakers will be used with the subs, these parameters are compromised for good performance with a variety of loudspeakers.
A sub’s or dba amp'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 subs 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 subs to reproduce the same signal. Now invert the polarity of the main loudspeakers relative to the subs 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 subs’s wavefronts are 180° out of phase with each other, the greatest cancellation (the least sound heard) will occur. That’s because as the subs’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 subs and loudspeaker outputs are maximally in-phase. Any time lag between the main speakers and subs has been eliminated. This technique works because it’s much easier to hear the point of maximum cancellation than the point of maximum reinforcement.
Most subs use either a sealed enclosure or a ported enclosure. Which type you choose will affect the character of the bass the sub 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. Bass-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.
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 to know about- transient performance. A woofer in a sealed enclosure, when subject to a transient signal such as a kick drum, will tend to stop moving immediately after the transient. Conversely, a woofer in a vented enclosure could tend to keep moving after the drive signal has stopped. The speaker with the sealed enclosure generally has more accurate dynamic performance.
The AK Debra dba system has removable port plugs that allow the choice of configuring them as either sealed/acoustic suspension or ported/bass-reflex subs. I've tried them in both configurations in my room/system and determined that I perceived both as equally dynamic on transients but preferred the ported configuration since the bass extension seemed superior.
I have the crossover frequency on the sub amp set at 45 Hz, the volume set slightly under halfway and the phase control set in-phase at "0".
My main Magnepan 2.7QRs are operated as full-range and I utilize no bass room treatments and no room correction, other than configuring my mains as 'Large' and my center and surrounds as 'Small' for ht.
Sorry, I tend to err on the side of TMI on my posts,
Tim
