A new kind of woofer.

Most of us know how dynamic woofers work. A voice coil working within a magnetic field is excited by the AC voltage applied by an audio amplifier. The voice coil moves back and forth within the field at the input frequency, and attached to the outer rim of that coil is a cone-shaped diaphragm that is secured at its apex near the coil junction by a spider and at its outer edge by a flexible surround. As the cone moves back and forth it generates sound, due to its interaction with the air in front of it. There are variations on this arrangement, of course, but those are the basics.

So, what are the disadvantages of dynamically driven cone woofers? Well, other than the rather common ones of distortion-causing cone breakup and response-modifying resonances, and of course surround deterioration with age (at least with some designs), the biggest problem is that the cone does not couple effectively with the air mass in front of it below a certain frequency. A woofer deals with a big impedance mismatch with the air at really low frequencies. This means that the cone moves, but does not create much pressure change in the air to create sound, and much of the force from the voice coil is wasted.

One solution for this efficiency limitation would involve building cone-support assemblies with extreme excursion capabilities. Yet this remedy is not as viable as one would like. Compared to what it has to do at 100 Hz, a cone must move 4 times as far to play 50 Hz, 16 times as far to play 25 Hz, and 64 times as far to play 12.5Hz. It is easy to see why even fairly potent subwoofers begin to roll off at very low frequencies if decent volume levels are required.

Other options involve using cones of very large diameter, employing port or drone-cone tuning that makes more efficient use of active-driver cone area, and/or making use of equalization that flattens out the response as air-coupling efficiency falls off down really low. This last solution usually involves integrated amplifiers of considerable power, and it still runs into the problem of excursion limitations.

Now, physical limitations notwithstanding, for most of us, dynamic woofers of this kind work just fine. I have reviewed a lot of subwoofers for this magazine and the best ones are able to shake the rafters in just about any home-listening-room environment, and do so right down to 20 Hz or lower with minimal distortion. Yep, today's best subwoofers (and woofers) are very fine performing items, and they really do not take up all that much floor space.

Still, it never hurts to try to improve on a concept, and that is just what Bruce Thigpen, of Eminent Technology, has done, at least in some important ways, with his Rotary-Vane Woofer system. Bruce, who holds nine audio patents, has a number of remarkable product designs to his credit. About 35 years ago he worked on putting air bearings to use with audio gear, and the Infinity Air-Bearing Turntable, Colony AB-1 and Mapleknoll turntables, as well as his own Eminent Technology tone arms all evolved from that work. He also patented a turntable vacuum hold down for LP records. In addition, he also made the first full-range, push-pull planar magnetic speaker system (one of these was reviewed by me in issue 94), and the first flat-panel computer/multimedia speaker. Relating to the latter project, Bruce worked with Sonigistix on the design of Monsoon computer and home speaker systems. Finally, Eastern-Asia Technologies licenses Bruce's planar-magnetic loudspeaker technology and markets some of its planar products under the Philips brand name.

OK, so just what is involved with this new woofer design?

Outwardly, the rotary-vane woofer operates much like a fan, but with the ability of the blades to twist back and forth around their axes at audio frequencies. Instead of a cone connected to the voice coil at its apex, there is a set of linkages attached to the coil that are hooked to each of the blades in the assembly. (One prototype that I listened to has six such blades.) The linkages change the pitch of the blades in much the same way that variable-pitch aircraft propellers or helicopter blades are pitch controlled, although the pitch change is constant (depending on the frequency of the input signal) and involves rotation in both directions around the neutral point. Instead of a cone moving back and forth with an audio signal, you have the blades pivoting around each connecting link to the centered voice coil.

While this signal-controlled, back-and-forth twisting is taking place the blade array is rotated around the voice-coil axis by means of a motor that has its drive shaft running right through the center of the driver's magnet assembly. So, two movements are involved: the twisting action of each vane within the assembly responding to the audio input signals, plus the rapid rotation of all of them around the central axis, as powered by an electric motor.

But how do the rotating blades themselves increase bass-performance efficiency?

Stick your hand into the air, put your fingers together, and twist your hand back and forth as fast as you can. You do not feel much resistance and do not move much air. Now go for a drive, stick your hand out of a car window going sixty miles per hour, and again twist your hand back and forth. You can now feel air pressure and the stiffness of a large number of air molecules hitting your hand. The speed of the air passing over your hand amplifies the air-deflecting efficiency of the hand rotation.

In a similar manner, the rotary-vane woofer trades a stationary cone for a high-speed, spinning blade, enabling it to work against a large number of air molecules. The acoustic output of the device is not limited to motion of air particles in front of a cone or the maximum displacement of the cone. It can move additional air particles pulled into the vane from either side.

This coupling to the air is a function of how fast the blades are moving. Because of this, the impedance match with the air is continuously variable (depending upon rotation speed), enabling highly efficient low frequency output from a small-diameter transducer. The vane assembly can grab air in close proximity to the blades, and by its rotational motion, the high-velocity vanes sweep a relative large area.

According to Bruce, the impedance match with the air is proportional to the ratio of the desired output-frequency period, the blade-rotational period, the number of blades, and the size of the blades. He further indicates that this design only requires twice the pitch of the blades for each halving of frequency. The upshot is this is the first woofer that can have flat response all the way to DC in a room with air leaks.

Also according to Bruce, if we compared a 12-inch rotary-vane woofer with a cone woofer of the same diameter, referenced them to the same levels at 80 Hz, and then gave them the same voltage input at 40 Hz, the vane woofer would produce twice the low frequency output of the cone job. More impressively, and again according to him, with the input voltage again remaining constant, at 20 Hz the rotary-vane woofer would equal the output of four 12-inch woofers. At 10 Hz the vane woofer would be able to equal eight 12-inch woofers and at 5 Hz it would have the same output as sixteen 12-inch woofers.

OK, now there is no such thing as a free lunch and even Bruce admits that there are problems with the rotary-vane woofer.

First and foremost, the blades generate random (white) noise. As they spin faster still more noise is generated. Indeed, the speaker actually has a signal to noise ratio, due to the additional input from the vane assembly rotation. The motor itself can also generate noise. And of course while the driver is very efficient at converting electrical energy input from an amplifier, compared to cone versions, the motor itself is also drawing power from the wall outlet. So, no, we are not getting something from nothing. The laws of physics must still be obeyed.

Another problem is that the speaker does not tune to a box in the way that a cone does by Thiele-Small parameters. An infinite baffle provides the best performance. Consequently, you cannot mount this woofer in a small box, acoustic-suspension style, and expect it to work all that effectively. It requires a large area space behind it (attic, basement, or large closet, although in a closet it might be flat down to "only" 20 Hz, according to Bruce), and so it is more able to dovetail with built-in and custom home installations than it is with "buy it and set-it-up quick" systems. Yep, this is a high-end woofer, almost for sure, although it also has potential industrial and test-bed applications.

Is that it? Well, no. You can also make use of a small-scale version of this device in a car-audio system. Indeed, the first prototype I heard was installed in a car, with the trunk area serving as the enclosure. While the motor/blade assembly did make a small amount of noise, that noise was effectively masked by the road and engine noise generated by the movement of the automobile. The result was very good bass performance. Bruce has even made a car-audio prototype version that is only 4-inches in diameter!

I had a chance to listen to another prototype installed in the attic of Bruce's home (see illustrations). The unit was mounted to the attic rafters above the ceiling and aimed horizontally, and its output was directed via a short, enclosed and angled tunnel area to a largish opening in the listening room's ceiling. The area behind the stiff grill over the opening was covered by a blanket of fiberglass, which did a very effective job of masking the higher-order harmonics generated by the motor and vane rotation.

Sonically, the result was impressive. The distortion was very low and the output was often solid right down to below 20 Hz. Indeed, Bruce measured the room-compensated output of the woofer and it was +/- 3 dB from 4 Hz to 40 Hz, with a mild dip between 50 and 60 Hz, and then flat again out to 80 Hz. The dip is almost certainly room-dimension related, because measurements he has made with other prototypes outdoors were practically ruler flat. For the most part, with serious and customized high-end systems the sub need only get up to about 50 Hz, anyway, with the premium satellite speakers that would be involved with such systems easily being able to handle the response down to a 50-Hz crossover point.

With only a couple of watts amplifier input and about 60 watts being drawn out of the electrical service by the rotational motor, the attic-mounted 12-inch vane mechanism was able to rattle the walls quite decently at just about any frequency. With a 110-dB output at 30 Hz, the amp was only putting out about 8 watts and the rotational motor was pulling about 80 watts from the wall outlet, corresponding to 2 or 3 percent efficiency. (Typical acoustic-suspension woofers operate at about one percent efficient.)

The bass-range distortion levels this device produces are also notably low. For example, with a 20-Hz input at 100 dB in his listening room the second harmonic was only at 3% and the third harmonic was only 2%. At 40 Hz it was 3% and 5%, respectively, at the same output level, and at 80 Hz it was even lower. Remarkably, even at 10 Hz with a 100 dB output the second was 3.5% and the third 3.1%, and even at 110 dB at that same frequency, the second and third were each only 8%. Those 110 dB, 10-Hz results were achieved with a 30-watt input, by the way. Bruce tried to get a solid readout at 20 Hz at 110 dB out, but the room walls were resonating too much to get a decent reading.

Bruce has also generated as much as 117 dB at 30 Hz, with the rotational motor pulling about 120 watts and the amp generating about 32 watts, corresponding to about 6 percent efficiency. He also managed to get a door in that room to vibrate in sympathy with a powerful 2-Hz input! Response down this low and output levels that high are not exactly musically relevant, but it does show the potential of the woofer in customized, very high-end home installations.

While this subwoofer is no replacement for many of the more conventional units available today (particularly those in smaller boxes), it has some unique characteristics that will make it a viable choice for certain kinds of installation requirements as well as industrial applications. When will we see this device in production? Well, at this time there is no set date. However, once it does go on the market it is certainly something serious bass enthusiasts and custom installers might be interested in.

--HF