Audio Physic Rhea powered subwoofer
"Then I guess headphones can't reproduce any frequency longer than the length of the ear canal, which, of course, is absurd." I responded. "That would mean headphones can't reproduce midrange—not to mention bass. And we all know they can."
The problem with small rooms isn't that they can't do deep bass, it's that below the frequency of the smallest standing wave (a wave/room interaction that causes a sonic "pileup" or reinforcement at certain frequencies in certain locations within a room), the room becomes pressurized. This results in low-frequency gain ("room gain"), which rises in amplitude as the frequency falls.
That's why many designers roll off a speaker's deep-bass response at a controlled rate. When it's measured in an anechoic chamber, the rolloff occurs; but in a "real world" setting, the in-room response may actually measure reasonably flat, depending on where you place the speakers, the room's dimensions, how "stiff" the walls are, etc. The lower the speaker is designed to go, the more difficult it is to get it right—especially in a small room.
That's why many audiophiles, especially those with "tiny" rooms, are willing to forgo the bottom octave or two rather than deal with the serious, sometimes ineradicable colorations deep bass can create.
There really isn't much musical information in the bottom few octaves anyway, many audiophiles argue, so why open up that can of deep-bass worms and risk polluting the rest of the presentation? In the less rigorous world of home theater, subwoofers are considered mandatory, both because sonic standards are lower (there's little that's "real" on a soundtrack), and because there's much more low-bass information, in the forms of explosions and other "deep impact" sound effects.
The Audio Physic Rhea powered subwoofer (originally called the Terra; the name was changed due to a trademark conflict) was designed to deal with the problems of deep bass in real-world rooms. Intended to be placed in a corner or against a wall, the Rhea is big, heavy, and not particularly attractive—unless, as I do, you like a 1950s "retro" kind of Klipschorn look. But despite its fine veneer finish, with two big long-throw, stiff paper-cone, 10?" woofers in front, no grillecloth, and heatsink and controls on the side, the Rhea is not going to be a fashion statement in any living-room setting I know of. At almost 30" tall and 18" square, the 80-lb Rhea doesn't exactly blend into the woodwork!
Speaking of blending, anyone who's ever tried to seamlessly wed a subwoofer to a system using a complex crossover network knows how difficult, if not impossible, this can be. Passing a signal through a frequency-dividing network almost always causes sonic deterioration, and then there's the extra cabling and connectors involved in such a setup. Getting the system to gel can be a daunting task that leads many audiophiles to conclude that it's not worth trading air, transparency, and coherency to get that bottom octave.
To get around those problems, the powered Rhea derives its signal from the system power amplifier's output taps. The sub's 600 ohm input impedance is designed to ensure that the connection has no sonic effect on the signals reaching the L/R speakers. I heard no differences with the Rhea tapped in—electrically, at least, the Rhea is as good as not there. This arrangement also means that you lose the benefits derived from limiting bass to the main speakers. In other words, the Rhea is best used with robust speakers designed to "see" a full-range signal.
Designer Berndt Theiss told me his goal was to create a subwoofer that would "extend the bass response of the main speakers to the lowest perceivable frequency without compromising the coherency of the speakers when played alone." Of course, that's every subwoofer designer's goal! Or should be.
Theiss set out to build a speaker with a bandwidth of greater than two octaves because, he told me, "electrical or mechanical systems with less than two octaves [of bandwidth] will ring wildly if stimulated." Since the upper-frequency limit is variable (depending on the frequency response of the main speakers and the room layout), Theiss chose to extend the low-frequency cutoff point to 10Hz, rolling off 6dB/octave down to 5Hz in order to achieve his goal of more than two octaves. That's why Theiss designed the Rhea to be flat down to 10Hz into a large room—not because he thinks there's a world of musical sounds down there to reproduce. The built-in, fully balanced amplifier is rated at 300W, and of course is flat down to a few Hz. The Rhea's rated output is more than 110dB at 30Hz in a very large room (2400 cubic feet). In other words, the thing's designed to be a monster.
That kind of low-bass performance can be achieved with a very large driver in an even larger enclosure (not really practical), or through the use of "feed-forward" or "velocity feedback" circuits. Theiss claims that velocity feedback is the best choice, but rather than using a complex accelerometer circuit to measure cone movement, the Rhea uses a "bridge circuit" that compares the output voltage of the amplifier to the current through the loudspeaker. The loop maintains the proportionality of the velocity of the speaker to the input voltage.
According to Theiss, velocity-feedback circuits can suffer instability problems above their working range (in the 150-2000Hz region). While the ringing is suppressed by the high-frequency attenuation built into the lowpass crossover, he claims it can be audible, and contributes to an "electronic sound" that can reveal the subwoofer's physical location even if it's reproducing only omnidirectional frequencies. Theiss claims great attention was paid to ensuring loop stability well above the working range.