The Science of Subwoofing Page 5

A few years ago I reviewed the ProAc Tablette (Vol.7 No.4). When J. Gordon Holt, the champion of tonal accuracy, first heard them he was violently put off by their tonal imbalance. "Violins sound like children's toy violins, celli are emasculated," and on and on he went. My review was less than a rave, yet he castigated me as "having gone off the deep end" for my failure to trash them. Yet in my system, side by side with my reference speakers at the time—the helium-driven Hill Plasmatronics—the Tablettes were superior in defining the underlying bass lines and highlighting the pulse or heartbeat of the music.

Thus, through the Tablettes the range below about 1kHz, while not as "accurate," was more "tuneful." Shortly thereafter, I sold the Plasmatronics. Not because the Tablettes became my new reference (I also value accuracy), but because they whetted my appetite for a speaker that could reproduce the lower mids and bass octaves with transparency and detail.

Much later, the Celestion SL600s arrived. Of course their imaging was superb, and although there was no deep bass, the tonal balance was a bit too lean for my tastes, and the highs, while extended, lacked air and transparency, what really captured my imagination was the speaker's ability to focus in on bass detail and clearly resolve bass information. I was almost willing to forgive all of the '600's shortcomings for the privilege of clearly hearing what I could not hear before.

The issue can now be appreciated and restated as one of quality vs quantity. The Lilliputians would argue that the finesse areas of bass reproduction are best served by minimizing the speaker. The Brobdingnagians, on the other hand, would argue that finesse is not enough, that power and extension are paramount for maximizing realism. The issue of bass quality has had a difficult gestation period in the US, even in the hands of veteran audio reviewers. Not too long ago Anthony H. Cordesman snubbed the Lilliputians with his "bass is bass" motto, finding them guilty of stealing the bass. More recently, a well-known reviewer on the comeback trail—OK, Peter Aczel—preferred the bass of the Carver Amazing speaker to that of the Apogee Scintilla and Celestion SL600. Speak about a blatant preference for heavy and featureless bass!

Philosophically, it comes down to a personal decision about what factors matter most in enhancing the illusion of life in reproduced music. We talk about the stereo soundstage as a three-dimensional entity; a complete spatial construct with width, height, and depth perspectives. We try to imagine within that space instrumental outlines with palpable breadth and focus. In reality, that soundstage is a phantasm populated by invisible musicians. For the physical trappings of the listening room to virtually dissolve does require a zen-like mental state. To be teleported to another hall and another time is not merely science fiction, but the domain of a superbly crafted state of mind.

But before Scotty can beam you up, certain attributes of live music must be present in the listening room to coax you into the transporter room. Which factors are important to you? Which ones elicit goosebumps? Because loudspeakers often offer either bass quantity or bass quality, you will have to decide which of these attributes matters most to you in fleshing out that illusory soundstage. If quantity is your ticket, go for it. If it's quality, then seek it out. This is as it should be. The illusion of life is what it must be all about. If you accept that, then it becomes clear that, ultimately, the decision of what is important is a highly personal one; what works for me might not work for you. But at least let's be wise enough not to be judgmental about our choices. In my case, I have realized that both quality and quantity are necessary if that soundstage in my small room is to come alive for me. The desire for detailed, well-defined bass with power and extension propelled me on my search for a subwoofer.

The choice of satellite speakers obviously affects the choice of subwoofer and crossover. For satellites to use for my forthcoming subwoofer reviews, I settled on the Celestion SL600 and Magneplanar MG-2.5/R. These off-the-shelf speakers are well-represented in audiophile circles, so my experiences with them will be immediately meaningful to a large number of readers. They are also typical representatives of their respective design classes: minimonitors and small planars. My findings may then be generalized and used (with caution) for similar main speakers.

Driven full-range, both of my satellites were capable of excellent imaging and resolution of low-level detail. Both had problems, however, which would preclude them from long-term residency in my system. Subjectively, the MG-2.5 proved a musically enjoyable speaker with a transparent and spacious soundstage. I agree with JA that it does offer the audiophile on a budget a slice of what high-end sound reproduction is all about. But I found the MG-2.5 to be significantly colored. The upper bass and lower mids are elevated about 4dB compared with the upper mids and treble. The emphasis extends from about 200 to 600Hz. To put it another way, the range above 1kHz is shelved down 4–6dB compared with the lower mids. The net effect of these tonal balance deviations is a recessed and distant upper range and a thickening of textures in the lower mids. In contrast with the MG-2.5, the tonal balance of the SL600 begins to look like a reference standard. Its in-room response is extremely tight from 200Hz to 20kHz. There is a modest suckout of about 4dB in the octave between 100 and 200Hz, which results in a "lean" sort of balance. And, as I mentioned earlier, despite the frequency extension of the SL600's tweeter, the subjective impression was of a closed-in, lifeless, and slightly opaque upper treble.

The Celestion's in-room bass half-power response was at a frequency of 50Hz. The SL600 woofer can thermally sink quite a lot of power, able to suck dry the 150Wpc Boulder 500 amplifier into clipping without any apparent damage. Yet the power the woofer sinks is not going into acoustic output, but merely heating the voice-coil. It is obvious that, on heavy bass transients, the SL600s are unable to "rise" to the occasion because the woofer is excursion-limited. The resulting compression robs wide-range orchestral music of its full dynamic range and power. The SL600s, therefore, proved an excellent candidate for a subwoofer; and because of their facility in the finesse areas of bass reproduction, they turned out to be a very critical tool in assessing the impact of the subwoofer on mid- and upper-bass quality.

While the MG-2.5 measures slightly deeper in the bass with a bass half-power frequency of about 40Hz, it too is quite obviously excursion-limited in the bass, descending convincingly into the deep bass only on small-signal drive. There's a 50Hz high-Q peak that on first impression gives the illusion of bass power, but after extended listening a more accurate picture is possible: The bass is slightly loose and wimpy when negotiating large-scale bass transients. Here, a subwoofer would hopefully yield better bass dynamics and control.

With a clearer picture in mind of the bass-performance capabilities of our satellites, an important issue to resolve is the optimum crossover point. It is common practice to use 100Hz because it is generally a good compromise between the needs of the satellite and those of the subwoofer. Placement of the subwoofer is less critical at 100Hz compared with a higher crossover point, and the subwoofer response need not be very smooth above 100Hz.

The benefits to the satellite are still substantial at 100Hz—providing that the high-pass (HP) filter roll-in is sufficiently steep to exclude most of the deep bass energy. The perceived benefits involve reductions in intermodulation and harmonic distortions due to a reduction in excursion demands on the suspension of the satellite's woofer. The overall improvement should be along the lines of cleaner and more effortless reproduction of the middle octaves. The bass alignment designed into most small speakers features a modest bump in the deep bass, or—to put it in equivalent terms—a Q slightly greater than 1. This is done in order to achieve a sense of a fuller and, if you will, a more "natural" tonal balance, and if it isn't overdone the loss in bass tightness is not particularly objectionable. The resultant peak in the frequency response is usually around 60Hz. Now, with a crossover point of 100Hz, this peak is a mere half-octave away, and unless the satellite's HP filter is quite steep it will not be adequately attenuated. The peak will then be superimposed on the response of the subwoofer to yield a boomy bass quality.

In general, then, at 100Hz a steep-sloped filter of at least 18dB/octave is very desirable. In the course of this evaluation I experimented with crossover points in the 100–250Hz range. My findings clearly show that a higher crossover point around 250Hz provides further gains in the area of dynamic range expansion—even when an 18dB/octave HP filter is used. The overall sound of the system becomes more powerful and effortless at the higher crossover points. However, there is a price to pay: It becomes much more difficult to preserve the quality of the satellite's upper bass. For example, at 250Hz, I could not retain the SL600's original level of upper bass resolution and had to settle on 100Hz as a compromise between dynamics and definition.

Of course, operating at a crossover frequency as high as 250Hz presumes that the subwoofer is comfortable in the woofer range. Some subwoofers can be pushed high enough in frequency, while others are only comfortable as subwoofers.

Based on my findings, minimonitors first and foremost cry out for a good woofer that can be crossed over in the upper bass without loss of quality. There's a lot of energy in the 100–250Hz range. Minimonitors would benefit more from help in this range, compared to a fix below 100Hz. A linear woofer of sufficient excursion, properly integrated into the system and operating over the 40–250Hz range, should prove to be much more impressive in terms of bass dynamics and portrayal of orchestral power than a true subwoofer. Fortunately or unfortunately, depending on your viewpoint, many commercial subwoofers are only so in name, incapable as they are of decent bass extension below about 40Hz. So if, by chance, you're stuck with one of these sub-par "subwoofers," don't despair. The more burning question is whether it can perform smoothly and with speed up through the lower mids.

It stands to reason then that, in addition to steep slopes, the crossover should also provide ready means to adjust the crossover point, at least over the 100–250Hz range. These considerations pretty much rule out the use of a passive crossover. You might as well resign yourself to a biamped system with the extra expense of a second amp and active crossover. And, as you will see next, a good crossover isn't cheap either.

Warning! You are now entering the twilight zone of speaker design. By controlling the crossover slopes and frequency and the relative positioning of the speakers, you're in effect playing speaker designer. You're in charge of integrating the overall system. For many people this could turn out literally to be mission impossible. Many people are initially excited by the prospect of bass extension, cleaner mids, and enhanced dynamics, the sorts of benefits subwoofers are ideally supposed to provide. After the initial period of excitement, usually brought on by the novelty of more bass, there follows a long period of frustration when it is realized that the sound quality of the whole is less than that of the parts. Imaging, speed, focus and/or resolution of bass detail usually suffer in the process.

You then realize that the reasons for which you bought the minimonitors in the first place are seriously compromised. In the end, the subwoofer is put up for sale. It really is that difficult to successfully integrate a subwoofer or woofer into a system. I'm not suggesting that it's impossible; but you should carefully research the various possibilities and alternatives before you jump in with your checkbook.

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