Pat, you've made a name for yourself designing transmission lines. Before we get to your latest take on it, recap briefly why you feel this is a superior approach to traditional vented or sealed bass alignments.
Sure! There are many misconceptions about how bass systems actually work. Here are the simple facts. The purpose of the speaker enclosure is to absorb the energy of the woofer's back wave. Convert it into heat and do so in a way that couples the front-firing driver output to the room's air most advantageously. Keep in mind that exactly half of a woofer's output is directed backwards into the enclosure. This is no small task then!
Why bother, you ask? Because the back wave is, of course, 180 degrees out of phase with the forward radiation. If allowed to escape, the two would cancel each other out.
A sealed box uses the enclosed air's simple elasticity to dissipate the back wave. Naturally there are losses in any elastic system - feel the springs on your car after a ride. You'll notice that they are warm. Some of the energy of compression and expansion is lost to heat. Precisely the same thing happens in a closed box. Part of the rear wave is converted to heat by frictional losses during the compression and rarefaction of the trapped air. Some is lost to the padding inside the box. Some to flexure of the cabinet walls. Whatever isn't absorbed reflects back out through the diaphragm itself.
The system has a native resonance that corresponds to the combined mass of driver + air. The elasticity of the air is constant with respect to frequency. It provides restorative force at the back of the woofer to help it couple with the air in the room.
In a vented enclosure -- and in addition to these activities -- the back wave must also move the air mass trapped inside the port to and fro. This increases the system's lossiness. It sets up a resonant frequency lower than the air in the box would attain without the port. At the expense of increased resonance, the port yields a lower rolloff F3 frequency (the frequency at which the system's output is down 3dB).
Below the resonant frequency, the woofer is not coupled to the enclosure. Via the port, it simply breathes in and out. At those low frequencies, the woofer bottoms out easily and barely interfaces with the room's air mass. Well-designed vented enclosures do work reasonably well. However, poorly conceived ones can sound horrible. Typically, if the designer goes for a low F3 above all else, the result will be a terribly ringing mess. This is why customers who blindly focus on F3 with the idea that they are comparing bass quality are really just asking to be hoodwinked.
How about transmission lines then?
A transmission line is an altogether different animal. It employs a complex and well-damped labyrinth that deals more effectively with the back wave. A long, twisted column of air is made to draw itself around numerous convolutions and past a huge surface area of absorptive material. In addition to elasticity -- and, to a lesser extent, resonance -- the TL works by damping and absorption. The goal is to produce deep bass without the ringing traditionally associated with vented alignments.
It sounds simple enough. In practice though, TLs are fussy to design and expensive to build. You won't see too many of them in today's consumer products. True, a poorly designed TL can sound just as bad as a poorly designed vented system. But, I'm blessed -- or bedeviled -- with a fascination for the workings of transmission lines. Unlike my colleagues who work for big corporations, I'm able to pursue the lengthy and often quirky development of TLs without the kneaded brow of some doting VP hovering over my shoulder. I can happily pursue their protracted development. My new factory is perfectly adapted to the rigors of making them.
With the release of the Swift, you're announcing a new bass-loading scheme dubbed BASS-IC. You claim 35Hz extension from a modest 5-inch woofer. How's that possible?
BASS-IC stands for Impedance Coupled Bass. After many years of hands-on experience with transmission lines, I've finally overcome their remaining weakness. Just as with conventionally vented designs, traditional TLs are not coupled to the driver below system resonance. Below F3, the woofer merely sucks air in and out of the port. It lacks resistive force applied against it. At those frequencies, the air in the enclosure does nothing to impede or control the motion of the woofer.
The reason for this behavior? Below system resonance, the enclosure's acoustic impedance or resistive force falls off rapidly. This withdraws support for the driver. It escapes the control imposed upon it by the resistive action of the trapped air that governs frequencies above F3. Though it moves freely, it hardly produces audible output. It flaps around and bangs its stops. Sometimes it manages to double the frequencies it's fed by the amplifier. And all that adolescent misbehavior stems from the fact that below F3, the driver has become actively dissociated from the cabinet it's attached to.
As simple as it may sound, the whole idea of BASS-IC is to stand this process on its head: Configure the TL such that below resonance, the acoustic impedance of the enclosure rises rather than falls.
That's it. You see, now the enclosure does couple to the driver below F3. It forces the woofer to interact with the air in the room at much lower frequencies than before. Relatively low excursions can produce prodigious output because they're once again amplified or supported by the cabinet itself.
I should add that this heretical notion also allows us to sidestep the dynamic restrictions of low diameter ports imposed on most of today's vented alignments. We keep the line cross section of our TL terminus quite high. As you listen to the Swift, the first thing that grabs you by the throat is its astoundingly dynamic bass. The audio-lexicon translation? Well, I don't know - open? Exciting? Visceral? Whatever - I call it fun.
The concept itself is so darn basic. I can't believe it eluded me over the last ten years! It's as obvious as the wheel once you see it. The idea was there all the time. The bastard was hiding in the background biding his time to pounce. Of course, as with all good and simple ideas, proper execution is often anything but straightforward. But I'm afraid that this is where my description of the inner working of BASS-IC must leave off. There isn't much sense in my telling the whole world exactly how to do it!
[Swift & new Osprey (ca. $3K/pr) to right]
Indeed. I won't ask you to disclose trade secrets.
BASS-IC makes its debut in our little Swift. Despite its dainty dimensions, this speaker really dishes it out. In early June of this year, we premiered it at the Stereophile Show in Manhattan. For a change from the usual customs, the lines of eager listeners that always form outside the big mega-system rooms also formed in front of our doors. Everyone wanted to hear the modestly priced Swift!
I'm not exaggerating when I tell you that the overall reaction was one of stark disbelief. During those few short days, we did sell quite a number. At shows and in the press, most of the attention always focusses on the expensive all-out gear. There's a certain prestige associated with making those kinds of products. Perversely perhaps, I've always had a soft spot for building great affordable stuff. That's why we're having so much fun with the Swift.
Doesn't subwoofer specialist Bag End run their woofers below resonant frequency as well? I seem to recall that part of their approach requires electronic EQ-ing to account for the drastic rolloff in output. Is their scheme at all similar to yours?
I could be wrong here but believe that Bag End uses a sealed box. At some frequency, it will resonate, of course. They boost output below that frequency with a DSP-controlled amplifier. That's indeed a good idea and works nicely if the amplifier is an integral part of the design. BASS-IC is a different concept though. It operates purely in the passive domain, as a function of how the cabinet is designed.
I looked at the retail differential between the Swift ($995) and the Kestrel ($1,495). Knowing the complex innards of the Kestrel from the cutaway images on your website, it's difficult to imagine how the Swift could be cheaper yet offer such phenomenal extension and elaborate cosmetics.
Well, last fall we relocated from San Diego to beautiful Northern New York for the purposes of setting up a new facility that would be specifically adapted to build our complex cabinets. We are pioneering a few new and frankly quite daring production methods. They allow us to accomplish easily what previously remained almost impossible. It's an exciting time for us. With our newest products, we are taking full advantage of our new capabilities.
And indeed, the Swift's innards are more complex than the Kestrel's. But believe it or not, our new methods make the Swift that much easier to build. The cabinet was designed from the start to go together quickly. This would free up man-hour labor costs that we'd rather spend on improved materials, finishes and parts. It's a win-win-win, very important in today's fiercely competitive market. We can offer our customers a better performing product that looks great and is made right here in the USA - and sell it at a wonderful price!
At the NY show, Ken Kessler suggested a clever little catch phrase for our newest creation: "Swift. It's a gift!" To music lovers on a budget, I think that's just about the right message!
Meadowlark Audio website