QSC KSub powered subwoofer

A subwoofer is completely superfluous and irrelevant to reproducing guitar sound. A guitar in standard tuning doesn't create any musical tones lower than about 82 Hz, which falls within the low-end frequency response of virtually all powered speakers created for quality music reproduction. The next octave, down to about 41 Hz for a standard four-string bass guitar, is much more difficult to reproduce. The half-octave or so lower than that - the domain of keyboards and bass guitars with extra strings - is even more difficult to cover. So when I added a QSC KSub to my pair of K10s, it was with the intent of making my system better suited for playing bass guitar and - more often - listening to music.

Reproducing low bass frequencies isn't easy. There are laws of physics that must be obeyed, no matter how much cost accountants and charlatans might wish otherwise. Simply put, a speaker must move more air mass to reproduce lower frequencies. Moving more air takes a larger moving surface and more power. This is why subwoofers are large, heavy and expensive.

The KSub is a bandpass subwoofer, which means that the back of the speaker faces into a sealed enclosure while the front fires into a tuned, ported enclosure. It's the port that actually couples to the room; the speaker itself drives only the interior of the enclosure.

I can't describe all the nuances of bandpass subwoofer design and performance. I simply haven't done enough research to make educated comments on the subject. What I can tell you is that a bandpass sub, true to its name, passes a narrow band of frequencies in the low-bass range and that the energy drops off very rapidly (at a rate of 24 dB/octave) outside of the pass band. The designer can't arbitrarily widen the pass band. The laws of physics are very strict when it comes to trying to get something for nothing.

The KSub's pass band covers a little over an octave, from the high 40 Hz range to a little over 100 Hz.

Wait a minute! If a four-string bass guitar goes down to 41 Hz but the KSub starts rolling off in the high 40 Hz range, aren't we missing the lowest several notes on the bass guitar. Yup, you bet. The question is: how much will we notice what's missing? Our auditory system is very good at filling in missing information. There's a well-known phenomenon by which we perceive a missing fundamental frequency.

There are very few pure tones in music. A musical note has a fundamental frequency plus a series of overtones at (or very nearly at in the case of stringed instruments) integer multiples of the fundamental. It's the relative amount of each overtone, and how the amount of the fundamental and each overtone changes over time, that gives a musical instrument much of what we perceive as its characteristic tone or "timbre".

What happens when we play a bass instrument through a speaker that can't reproduce the fundamental frequency? How do we not only perceive that the low note is still present, but also that it's a lower frequency than our speaker can reproduce? Some of the overtones of that low note do get reproduced by the speaker. Our ears pick up the overtone frequencies. Then our auditory system does something very clever: it recognizes that the overtone series is related and causes us to perceive the fundamental frequency even if it's not actually reproduced by the speaker system.

As you might imagine, this auditory phenomenon can be exploited to good advantage. If you have a speaker system that can't reproduce low frequencies very well (or in some cases, at all) you can boost the overtones of the lowest frequencies and cause the listeners' auditory system to perceive that the fundamental frequency is not only present, but emphasized. This is the principle upon which bass exciters are implemented. Of course Mother Nature - stern bitch that she is - never lets us get something for nothing. We pay for the perception of enhanced bass with mud and clutter in upper bass range.

OK, enough background... How does the KSub sound?

Bandpass subwoofers tend to have a bad reputation. Their design involves an engineering compromise. The designer trades expensive speakers in large, heavy cabinets for smaller speakers in less heavy cabinets. That compromise is paid for with a far more complex and difficult design that's more sensitive to normal variations in speaker manufacturing, has more variation in loudness at different frequencies, and is more prone to distortion when pushed hard.

To add to the problem, some bandpass subwoofers are not well designed. I very briefly owned a Roland KCW-1 subwoofer which is, IMO, a great example of a bad design. The KCW-1 was extremely peaky. Not only that, but it actually wheezed when driven at any reasonable volume. The wheezing was almost certainly due to undersized ports.

By way of contrast, the KSub doesn't suck. In fact, it sounds pretty good when paired with my K10s. You're probably wondering: why the guarded praise? It's only because bass is very difficult to evaluate in a small untreated room in which small changes in one's listening position can be more important to perceived low-frequency balance than the speaker's actual frequency response.

That said, the addition of the KSub has improved my music listening experience in this room. The thunder of the drums in Bear McCreary's soundtrack to Battlestar Galactica's third season (best known for its stunning reinterpretation of "All Along The Watchtower") becomes a visceral experience with the addition of the KSub. Phil Lesh's bass speaks with its full voice on tracks like "Dark Star" from "Live Dead" and "Cowboy Movie" from David Crosby's "If I Could Only Remember My Name". Even more refined tracks from John Abercrombie's many jazz recordings benefit from the added low-end presence added by the KSub. Live recordings of The Mermen (available on archive.org) could easily convince you that you're standing at center stage.

As far as playing bass guitar through the system, the KSub makes a significant improvement. The K10s alone sound OK for bass guitar, but the small drivers limit the amount of usable headroom. With the KSub present and the K10s switched to their "EXT SUB" setting (which frees them from having to reproduce their lowest octave), the system has plenty of headroom to handle multiple instruments - including bass guitar - at performance volumes.