Siegfried Linkwitz Page 6

Dickson: Yes, I've noticed I can hear increased focus in the low bass when I toe-in the woofers. This is one speaker where "stereo" bass may have some real meaning. Two of the Dvorak's sonic characteristics I find most striking are its dramatic reduction of room-induced colorations from the low bass through the midrange, and its ability to convey image height in correct proportion to the width and depth dimension of the soundfield. What factors do you think contribute most to these effects?

Linkwitz: These are primarily due to the dipole characteristics and the even room response. Since the speaker is moderately directional at all frequencies, more of the sound is directed toward the listener and less to the walls and ceiling. Therefore, less comes back from the room in the form of resonances or reverberation which will blend with and color the direct sound from the speaker...The active equalization is merely there to correct for the dipole cancellation that the raw drivers would have if you didn't compensate for the inherent 6dB/octave roll-off.

Now the image height is an interesting thing. I have to state that I don't fully understand all the psychoacoustics involved here, but I have found that it is important that the center of the radiating elements be at about ear height, and that the speakers have some vertical extension as well. I have built many small two-way minimonitors; while these systems can have very nice horizontal dispersion and excellent imaging, I've always felt that I was listening through a horizontal window, one that was very wide but with a height not much greater than that of the speaker itself. It's like listening through a horizontal sliver. Now vertically spreading out the driver's arrangement expands the vertical dimension of the soundstage and adds much more realism for me.

With respect to the reduction in overall room colorations that the Dvoraks provide, that has a lot to do with how the dipole characteristics are implemented. This comes back to the fact that the off-axis response is very well-behaved in this system. In other words, the design concentrates just as much on the off-axis performance as the on-axis. While I don't have any definitive proof, I strongly suspect that the erratic off-axis behavior of most panel speakers is what makes their room placement so critical, forcing a person to locate the panels in a place that minimizes reflections and changes how the off-axis sound couples with the room, in order to get a balanced output. On the other hand, quite frankly, I have not found the performance of either the Dvorak or the Vivaldi to be critically dependent on room placement compared to other speakers. There is still definitely an optimal placement in any given room, but you can get very satisfactory performance in a wide variety of locations, so this experience lends further credence to the value of a well-behaved off-axis response.

Dickson: While the Dvorak and Vivaldi represent a somewhat fresh approach to speaker design, they are mature designs. I'm very curious to hear about what projects you have planned for the near and more distant future.

Linkwitz: Recently, I've been doing extensive investigations into numerous drivers using some of the newly developed measurement techniques I alluded to earlier—especially the tests for nonlinear distortion artifacts, and those that help locate and define energy storage effects in drivers themselves. All this in a search for components that have even more clarity and transparency. What I had in mind was to see how much further this Dvorak concept could be refined.

We unveiled our new flagship, the Beethoven, at the recently completed 1996 WCES in Las Vegas. In addition to an all-new balanced electronic crossover, each main panel has a new silk-dome tweeter, two new 8" drivers and a pair of 10" dipole drivers—all low-distortion, high-excursion models. Both of the woofer cabinets for the new system contain four 12" dipole drivers, so obviously this system is designed for high output, very low distortion sound and will be considerably more expensive than the standard Dvorak. I must say we have been extremely gratified with the performance of the new system. So that's one project we are putting the finishing touches on now, and we are also thinking about a smaller, lower-cost version of a dipole speaker in the future.

A little further down the road, possibly over the next few years, I would also like to settle in my mind the importance of what I would describe as "linear phase." This refers to obtaining a result that is a more accurate replica of the time-domain wavefront. Some people seem to think that this is very important for reproducing clicks and transient-type sounds, and that may well be. From a common-sense point of view, it seems logical that you would want to have a true replication of the wavefront.

However, I'm not totally convinced because I have done a lot of experiments with phase-distorted signals. Basically, I've shifted the phase between different spectral components by running various signals through an "all-pass" filter, where the amplitude is unaffected when I change the phase response with frequency. When you look at these signals on an oscilloscope and change the phase, they look grossly different, so you'd think "surely this must sound different." But when you listen, you can't hear the difference, even though the time-domain waveform staring you in the eye looks so totally altered!

I did quite an investigation into this when I initially developed the Linkwitz-Riley crossover because it is not a linear-phase system, nor are the Butterworth crossovers, for that matter, except for the first-order slope. The experiments I've done so far have not convinced me that phase distortion in small amounts is audible. Now if the phase distortion is gross, you can definitely hear it, but the typical crossover is far from producing that much phase distortion. However, some people whom I respect seem to think this is something that could have audible consequences, so I'm keeping an open mind about it and want to determine once and for all its value, if I can. I must say that I have not heard an example of a speaker design that conclusively demonstrates the benefit of a linear-phase system.

Dickson: I imagine when you look at the total performance of a speaker with the various tradeoffs required to achieve a certain goal, you have to weigh their relative merits.

Linkwitz: This is true, you could question, for example, whether the extra stress on drivers and resulting distortions produced by a first-order system are not more audibly significant than the subtle improvements potentially created by its linear phase effects? However, it is possible, using digital techniques, to correct for the phase response as well, and my friend, Malcolm Omar Hawksford [of England's Essex University], who has done quite a bit of work in this area, has kindly offered to perform a phase correction for the first 10ms time record of the Dvorak's impulse response with his digital processor. It certainly would require a bit of horsepower to implement digital phase correction in the active crossover, but it could be done.

Again, it's not yet completely clear whether a digital crossover will buy you anything. It may buy something and that's the part I'm interested in. For instance, with this scenario we could combine the excellent on- and off-axis amplitude response of the existing Linkwitz-Riley crossover in the Dvorak with an after-the-fact digital correction of the time-domain, to achieve a linear-phase system.

You see, the digital time-domain correction would not affect the existing passive or active crossover response at all, it just would correct overall phase. As a matter of fact, at a recent AES convention, both Malcolm Hawksford and I attended a discussion about the use of very steep crossover filters with digital phase correction. Convincing arguments were presented showing that these extremely steep filters produce sonic anomalies, and consequently, are not desirable. Malcolm also stated that something like the Linkwitz-Riley fourth-order crossover was about optimum, even digitally implemented, when phase correction is applied. Anyway, I'm very interested to see how this research turns out.

Dickson: With the continual improvement in driver technology and refinements in other areas of audio design, it may be that these more subtle issues, like linear phase, will become more important in the future.

Linkwitz: I think that's a good way to look at it. You could say that you need to have a certain number of other things done correctly first before those effects come into play. I should also point out that the digital phase compensation I'm speaking of is very different from the digital room correction systems you may have read about. In any event, these are a few of the areas that we at Audio Artistry look forward to investigating and developing in the near future.

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