Siegfried Linkwitz Page 5
Well, just for kicks, I took the thing home, split this long column into two shorter stereo columns, and decided to see how it sounded in my living room. I was really surprised to hear that it had some of the qualities that impressed me so much with other dipole speakers, like the Quad electrostatics. Not that these PA speakers were anywhere close in tonal balance or transparency, but there was something fundamental about the character and quality that reminded me of these better dipoles.
In any event, that got me started on investigating the possibilities of using conventional high-quality dynamic drivers as dipoles. Quite frankly, I've always been very fond of certain characteristics that some electrostatic dipoles possess, yet I had never seriously pursued panel dipoles, in spite of their good qualities, because there was not enough dynamic low-frequency output for me. They also had a very tight listening spot, and were generally just too limited in what they could do.
Based on my initial experience with the PA columns, I set out to build a speaker that could perform similar to an electrostatic dipole but using conventional dynamic drivers in a manner that would avoid some of the limitations faced by large panel designs. It took several evolutionary iterations of the design to get a good understanding of which aspects of dipole operation are really important and which are not so critical, and why this is so.
During this development phase, I discovered how important the baffle shape and design were, as well as the frequency range over which dipole radiation was most beneficial. For instance, I found that dipole radiation of the treble region was not only unnecessary but, in fact, a disadvantage. Interestingly, you don't even see the rear-firing response from a dipole tweeter when you measure it on-axis, but when I listened to it in the room, I found that it caused some high-frequency "splatter" that didn't seem natural. I abandoned that approach and used a monopole dome tweeter from 2kHz on up and dipole radiation from the cone drivers down to 20Hz. I started out with a completely active system, with separate amps for all drivers, and used equalization as well. I equalized the speaker to be flat since a dipole on a small baffle has this natural 6dB/octave roll-off below a certain point. The first version was a pretty elaborate prototype.
Actually, at first I used a closed-box woofer because I didn't think I could get enough bass output from a compact dipole woofer system with a reasonable number of drivers. This is because of the acoustic short-circuit between front and back that a dipole represents. However, after sharing notes with Brian Elliott, a good friend and acoustician who had developed a splendid dipole bass system using six 12" drivers per channel, I began looking at the possibilities for dipole bass more closely. His system simply produced the most astoundingly natural low-frequency reproduction I had ever heard. However, I still stayed with the closed-box woofer concept a while longer. Then Don Barringer, another good friend and the recording engineer for the US Marine Band, reported great results in a normal-sized listening room using a speaker based on my design, but extending the dipole operation into the bass with two 12" drivers per channel equalized flat.
So, while I was refining the dipole midrange/monopole tweeter parameters of my speakers, I was actually a late-comer in taking the dipole concept all the way. As it turns out, and somewhat surprisingly to me, two big dipole woofers per channel does a very respectable job. Not quite like Brian's system with its total of twelve 12" drivers, but actually very good. On the other hand, we've also learned that there is very little difference between four and six 12" woofers per side, and with some creative mounting techniques, we've been able to mount four large drivers in a surprisingly compact dipole enclosure for use in larger rooms, and as a standard feature in our new top-of-the-line speaker system, the Beethoven.
Anyway, the production Dvorak is a five-piece, actively bi-amplified system comprising two main panels, two separate subwoofer cabinets, and an active crossover/equalizer. It covers the full audible range and beyond, from 20Hz to 25kHz. If a person has a small room or doesn't require deep bass below 40Hz, they can just use the main panel with a single amplifier, but the active crossover/equalizer is still required for the EQ of the two mid/bass drivers. We have also recently released the Vivaldi speaker, which has the same driver complement as the full Dvorak system but all mounted in one tall speaker. The system uses a passive design for both the crossover and equalization so it has a reasonably flat response to about 40Hz, below which the dipole's natural roll-off occurs. This system is ideal for those in small to moderate sized rooms who don't need the full 20Hz extension of the complete Dvorak system. It also requires just a single stereo amplifier.
Dickson: I suppose the directivity of the Dvoraks, particularly with the separate dipole subwoofers, presents some special considerations when these speakers are measured.
Linkwitz: One of the real problems was: How do you measure a dynamic dipole accurately? For instance, you cannot measure close to the cone because then you'll only see what the cone does essentially. You will not get the effect of the cancellation when the negative polarity rear wave combines with the forward positive wave so you have to measure some distance away. That really forces you into either an outdoor or anechoic-type measurement. It's very important to do it this way to get meaningful results. If the cabinet is large, or if the drivers are spaced very far apart, you have to have at least the same distance to your measurement microphone in order to capture the integrated sound coming from the rear, or off the edges of the cabinet, etc. The separate subwoofers are quite tricky to measure as well.
The bottom line on the Dvorak is that everything is based on a flat response under anechoic conditions with a moderately directional radiation throughout its full range. When I say that the dipole aspects of the speaker are directional, it's not in a very strong sense. For instance, the response is 1dB down at 30 degrees off-axis, 3dB down 45 degrees off-axis, 6dB down at 60 degrees, and has a null around 90 degrees. The monopole tweeter also maintains a similar directional characteristic because of the baffle design and the wavelengths of its frequency band. Of course, it differs from the dipole drivers in that it fires predominantly forward. The important thing is that the shape of the off-axis room response is very consistent with that produced on-axis, resulting in the open-sounding soundstage and the speaker's even tonal characteristics. Also the dipole "figure-eight" cosine directionality goes all the way from 2kHz down through the woofer's range to 20Hz. This directional deep bass is really pretty amazing! If you have another person to help you, play some low-frequency tones, then have your friend rotate the woofer cabinet and you can clearly hear the output null at 90 degrees off-axis!