Back when I first got started in hi-fi, the “hard part” of designing a loudspeaker was deep bass response: It was the “limiting factor” in choosing whether to buy or to build your own speaker system.
Perhaps because radio (even FM) and virtually all recordings—other than a very limited few on quarter-inch prerecorded tape—were in mono, issues of imaging, soundstaging, phase coherence, and just about everything else other than distortion and frequency response were either unknown or not considered. Back then, a “flat” frequency response of 50Hz to 15,000Hz +/-3dB was the loudspeaker standard to shoot for, even in recording studios.
Part of the logic for this standard was due to the near-total domination of the studio-monitor market by the RCA LC-1a and the Altec A7 loudspeakers (for East Coast and West Coast studios, respectively), both of which just happened to claim a frequency response of 50Hz–15kHz +/-3dB. (Not coincidentally, these same frequency-response limitations were also applied to Ampex professional tape recorders and the dominant disc-cutting systems of the day from Westrex and Grampian-Gotham.)
Even if this had not been the case, getting good deep bass with the technology then available was much more difficult and costly than it is today, more than sixty years later. At that time, if you wanted deep bass, you had to choose among horns, “infinite baffles,” or “tuned ports” (bass-reflex)—and all three had their limitations.
Because the size of horns depends on the wavelength of the lowest signal to be transmitted, and because the wavelength of a 20Hz tone is 54 feet (16.46 meters) at sea level, horn loudspeakers designed to produce deep bass tend to be huge. Differing design approaches to length and effective “mouth” size, as well as “folding” and corner-loading (placing the speaker in the corner of a room and using the walls to extend the effective length and mouth of the horn), help reduce size requirements, but since the wavelength of even a 60Hz tone (about the bass capability of a good, modern, direct-radiating “bookshelf” speaker) is 18 feet (5.49 meters), it should be obvious that, even if you want limited deep bass from a horn, it is going to take a pretty big box.
Infinite baffles aren’t really infinite, but they could be. The whole purpose of an infinite-baffle enclosure is to keep the pressure wave generated by the back of the woofer (always 180 degrees out of phase with the front wave) from meeting the pressure wave from the front of the woofer and cancelling it out. In theory, if you were to mount your woofer on a board (a “baffle”) large enough so that the distance from the driver to the edge of the board (in every direction) was greater than half the wavelength of the lowest desired bass frequency, that would be “infinite” enough, and cancellation can be avoided. Keeping in mind, though, that even a 60Hz wavelength is 18 feet long, you see, once again, that it’s going to take a mighty big baffle board to get even minimally good extension.
One way that people successfully overcame this size issue was to mount their woofers on a wall, so that the front wave went into one room and the back wave into another. The same thing could be done by mounting the woofer in a fireplace, so that the front wave radiated into the listening room and the back was vented out through the chimney. Or, the whole size issue could be avoided by just replacing the baffle board with a closed box. That way, you kept the back wave from ever meeting the front one, and the problem was solved.
Infinite baffle boxes still tended to be large, however. If I remember correctly, the box for the Bozak B310, a speaker that, even in the early 1950s, made real 24Hz bass, was 16 cubic feet, and even the minimum box size for the single woofer Bozak B302 was 4½ cubic feet. However, if the box was too small, internal pressure buildup could restrict the bass.
The bass-reflex (or tuned-port) enclosure is one way to get around both the internal pressure problem and the additional problem of getting loud deep bass from an infinite baffle enclosure. (Because only some of the air—the front wave—is being moved by the driver, getting powerful deep bass from an infinite baffle requires a lot of woofers. The Bozak B-310 used four of them, making it both bulky and expensive.)
What the bass-reflex enclosure does is start with a closed box. A hole (or holes) of an appropriate size and position is cut in it so that the volume of air inside the box becomes a Helmholtz resonator. This “tuning” creates a pressure wave at a chosen frequency that comes out the hole (the port) in phase with the front wave of the woofer(s), thereby reinforcing the bass between the natural roll-off frequency of the driver(s) and the tuned frequency of the port. This method works pretty well, and, if the port is properly designed (remember that designers didn’t have Thiele-Small formulae back then), both enhances and extends the bass of a moderate-sized speaker/enclosure system. One problem, though, is that at bass frequencies below the tuning of the port, with both the driver and the port in sharp roll-off, bass response drops like a rock. With its single, driver-controlled roll-off rate, the infinite baffle enclosure could generally go deeper, though at reduced output levels.
There were all kinds of variants on ported enclosures, including “distributed ports” (lots of little holes, instead of one big one), ducted ports (many of which offered variable tuning), multiple ports tuned for different frequencies, resistive ports, passive radiators (in which the port is plugged with an undriven cone that is moved by the changing internal pressures inside the enclosure), and so on. Transmission-line designs were also tried. They combined characteristics of horns and tuned ports to achieve the infinite baffle’s goal of eliminating the speaker’s back wave—albeit expensively and complexly (and, ideally, with less bulk).
Nonetheless, the fact remained that if you wanted deep bass at anything like realistic volumes, you were going to have to buy or build a big speaker, and your big speaker—whether because of the complexity of its enclosure or the cost or number of its drivers—was going to be expensive.
The first real breakthrough in bass design came in the early 1950s when Edgar Villchur—adopting, for the first time, a “if you can’t lick ’em, join ’em” philosophy toward low frequencies—developed the “acoustic-suspension” woofer. Thoroughly aware of the problem of pressure buildup in a small sealed-box enclosure, he decided not to resist it, but to take advantage of it. Instead of equipping the woofer in his AR-1 (Acoustic Research Model 1) speaker with an accordion-like spring surround and spider (to pull its cone back to its starting position, as “normal” speakers did and do), he designed an essentially limp cone-suspension system, coupled it to a more than usually massive cone to keep the fundamental resonant frequency low, and used the air compression/decompression effects of very long cone travel in a very small (only about 1.7 cubic foot) box to provide the spring force necessary to control cone movement.
The results were remarkable. For the first time ever, a very small speaker system was able to provide very deep (below 30Hz) bass. Although recognized as a remarkable achievement in most audio circles, the new system was about 10dB less efficient (only one-tenth as loud) as a typical direct-radiating system, and required ten times as much power to bring it to the same volume. When you consider that, at only 60 watts, the McIntosh MC-60 was thought to be a huge amplifier at the time, and that 20-watt amps were far more typical for the average home system, it’s easy to see why the AR-1 had only limited appeal when it first came out.
The passage of time brought solid-state electronics and much more powerful amplifiers. By the 60s, there were plenty of power amps available to drive Villchur’s AR-3 (AR’s then-current model), and the acoustic-suspension systems from Acoustic Research, KLH, and others became a force in the marketplace.
Although many people made improvements in existing technologies, the next giant step in bass reproduction came from Bob Carver, who had already been a major force in creating a new generation of ultra-high-power amplifiers. Carver figured that even a smallish bass driver could produce very loud, very deep bass if it had a long enough “throw” (available range of total forward and backward cone excursion), if it were equalized sufficiently, and if it were driven by enough power. He had already created his “Class M” and “Tracking Downconverter Power Supply” amplifiers, which were capable of developing very high outputs from small, cool-running devices. In the mid-1990s, he came out with what was to become the “Sunfire” woofer—an 11½-inch cube powered by a built-in amplifier of, if I remember correctly, 2800 watts. Carver claimed users could get massive, deep, loud, high-quality bass (20Hz at the near-head-crushing volume level of 110dB) from a very affordably priced unit that could be used either singly or as a stereo pair with almost any main speaker system.
With the subsequent advent of Class D (switching) amplifiers, which have made huge amounts of power available really cheaply, and with equalized, long-throw subwoofers more commonplace, deep (or at least reasonably deep) bass, once possible only from very large and very expensive speaker systems, can now be had in very compact “outboard” units, such as the Andrew Jones-designed Pioneer SW-8MK2, which sells for well under $200.
Changing Influences, Changing Tastes, Changing Times
When I was a kid, the thing that “got” me—that dragged me, irresistibly, into our hobby and made me a life-long Hi-Fi Crazy—was bass. When I was just twelve years old, and had never heard better or deeper bass than that produced by a table radio, all of a sudden I was exposed to the bone-shaking 32Hz glories of George Wright and E. Power Biggs playing pipe organs at realistic levels on a single (remember that in those days, mono was all we had) Bozak B-310, powered by a McIntosh MC-60—and I was hooked forever.
I suspect—judging by the “boomers” who occasionally drive by shaking the earth, and by the sound of hip-hop leaking from the earphones of all those kids (I don’t even want to think about how deafeningly those must be playing)—that bass is still what catches youngsters. As I’ve gotten older, however, my tastes have changed and grown a little more refined. Although abyssal bass still turns me on, I’m now more attracted to imaging and soundstaging—the utterly amazing ways that a truly fine system can not only transport me to the performance venue, but also allow me, with my eyes closed, to “see” the performers in action.
Although some of this magical ambient effect certainly depends on the bass, most of what is now my biggest thrill is the province not of the woofer(s), but of the tweeter, midrange, and (possibly) mid/bass drivers. And, to finally get to the point of this article, that leads us all to a new way of buying, evaluating, and setting up speakers that’s very different than what it once was, not all that many years ago.
In the old days, if you wanted to buy or build a full-range speaker system—however “full-range” was defined—you would need the drivers for producing bass, midrange, and treble in one box. Except for bookshelf systems, which generally weren’t expected to produce anything much below 70 or 80Hz, that box would (as I’ve explained) be both large and expensive, with the majority of the size and cost going toward producing low bass.
With 15kHz as even a great speaker’s upper limit, not a whole lot of effort (at least compared with went into making low frequencies) was devoted to producing advanced and sophisticated mid- and high-frequency drivers. The tweeters used by Bozak, for example, were 2" paper cones, although Lansing (now JBL) did have its 075 “ring radiator,” which—though a new approach—I thought was prettier than it was good. Until at least the 1960s, midrange drivers also tended to be just re-purposed 4" to 8" cones. This allowed the bulk of the money and R&D to go into the bass, and that’s exactly where it went.
What started to change all this was the advent of stereo. With the possibility of three-dimensional imaging and soundstaging, more and more attention went into improving the reproduction of mid and upper frequencies; in addition, and for the first time, designers started considering things like “phase coherence” and (enclosure) “edge diffraction.”
In short, we started to get really good “mid-top” performance, with less attention paid to the bass component of the sound. This “mid-top” emphasis meant that spatial and ambient details could be reproduced better than ever before—while box size and cost could be reduced. With the introduction of CDs in 1982, and the consequent increase in dynamic range and recordable frequencies, mid-top response became especially important.
A New Approach to Full-Range Speakers
A couple of decades ago when the first separate subwoofers began to find a solid market, people started adding them to the new, more sophisticated “mid-top” units, and a new kind of modular full-range speaker system was born.
The first commercial implementation of this new paradigm that I remember came from 3D Acoustics, which for a very moderate price offered two “satellite” mid-tops and a single (sub)woofer to support them. The sound was great and 3D Acoustics sold lots of speakers. Another similar product was the Wilson WATT—essentially a mid-top, though it did offer limited bass response—which grew into the famous WATT/Puppy system through the addition of a pair of outboard woofers to the original pair of WATTs.
Thousands of systems like the 3D Acoustics “satellite/sub” and the WATT/Puppy have been sold to and enjoyed by audiophiles all around the world, and something like them is what I would suggest, as a different approach for maximizing your sound the next time you decide you need new speakers.
Here are just seven of many reasons why “mid-tops” and separate woofers may be right for you:
- Truly excellent mid-top speakers require smaller boxes, fewer drivers, and less complex crossover than full-range systems because they don’t need to produce anything but the most limited bass. With deep bass no longer an issue, manufacturers can concentrate their money and effort on the other drivers, giving you better mid- and upper-frequency performance and better imaging and soundstaging at a lower price.
- Smaller boxes may have a higher wife-acceptance factor. They also tend to image better because of the lower potential for edge diffraction.
- Because your mid-tops will be good-sounding speakers in their own right, you may, if you wish, budget your complete speaker purchase over time, possibly buying a better set of mid-tops to begin with and adding your woofer(s) later.
- When you do add your woofer(s) you can add just one, and, if you’ve chosen well, have a true full-range system of higher quality and better performance than a system with built-in woofers—and possibly for less money.
- If you choose, you can add another woofer at any time to augment the one you already have.
- Having your bass and mid-top speakers separate from each other means that both can be positioned for optimum performance (e.g. mid-tops properly spaced in the middle of the room and woofers in the corners or elsewhere), thus achieving better overall sonic performance.
- Finally, if you decide that you’re not satisfied with your speakers and want to make a change, you can replace the mid-tops and keep the woofers, or the other way around. Either way, since you’re only changing part of your speaker system, it’s cheaper and easier to upgrade, and you’re less likely to suffer a big financial loss doing so.
There are all kinds of advantages to a sub/satellite setup, and no disadvantages that I’ve yet discovered. Try it; you’ll like it!