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Into the mind of Albert Von Schweikert: Part I of III

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During the course of reviewing the Von Schweikert VR-4 SR MkII in February and March of 2007, reviewer Douglas Schroeder spoke with Albert Von Schweikert extensively. The following article comprises the bulk of their enlightening conversations. DAGOGO is grateful to Albert for spending the extra time to delineate his philosophy of speaker design, share some of his inspiring experiences, and comment on Von Schweikert Audio.

DS: What were your primary considerations and goals in designing the VR4 series of speakers utilizing independent bass and M/T modules? What obstacles were you trying to overcome and what benefits were added compared to a traditional one-cabinet design?

AVS: During my experiments at Cal Tech, conducted between 1976 and 1979, we had two ongoing projects in our lab work:

a. The design of a small “reference” speaker to be used for psychoacoustic experiments;

b. A subwoofer to be used with our Quad electrostatic speakers to extend the bandwidth and enable higher power handling.

We were using a borrowed pair of Quads as a reference for the small 6” two-way speaker, but heard distortion when we drove the Quads with program material containing bass information. We found that limiting the electrostatic speakers to 100Hz and above would result in much cleaner sound, so the development of a “high-speed” subwoofer naturally led to the two-piece stacking cone module I now call the “VR-4” platform (although it was originally called the Vortex Screen, excellently reviewed by Robert Harley for Stereophile in their July 1989 issue).

In the course of these two student engineering projects, which we worked on concurrently, I discovered that the physical separation of the woofer cabinet from the midrange/tweeter cabinet resulted in cleaner sound. The “new-found” clarity turned out to be a reduction of modulation distortion of the midrange and treble caused by the powerful physical vibration of the woofer, so physical isolation helped immensely – not only was timbral quality upgraded, but more importantly, image focus was greatly improved.

In addition, two small cabinets have inherently less cabinet resonances (and hence coloration) than a single larger one, all things being equal, due to smaller and more rigid walls. Although two cabinets cost more to build than a single larger cabinet, the sound quality is well worth the additional expense and manufacturing difficulty.

DS: Why was the midrange/tweeter unit angled upwards instead of using the recessed method of Time Alignment? Isn’t there a slight loss of detail caused by listening to the drivers off their direct axis?

AVS: As with many engineering decisions, trade-offs occur between these two different mechanical methods of phase-aligning the sound waves from the transducers. Our older models used the “stepped” type of mechanical alignment, using felt blocks to absorb the reflections caused by the setback of the tweeter from the midrange. Note that the reflections from the step, if not absorbed, leads to a loss of image focus and there’s no net gain from using Time Alignment!

When we switched over to the wood cabinet style of the present VR-4 from the cloth wrapped original version, we decided that the felt blocks were not cosmetically appealing. In addition, many listeners who are sensitive to high frequencies being directly beamed at their ears report that listening to tweeters off-axis improves their smoothness.

(Note that listening to any speaker system without direct aiming of the tweeters at the ear is going to result in exactly the same effect: a perceived high frequency roll-off. However, we recommend that the listener aims the VR-4 directly at their ears to achieve flat frequency response and correct image focus).

Even though the VR-4 speakers may appear to have more detail when you stand up (which enables the listen to hear the tweeters on their direct axis), the response is actually tilted upwards towards the treble range, since we designed the frequency response to be flat for off-axis listening created by the tilt of the midrange/tweeter module. If you stand up to hear the “extra” treble boost, listening fatigue will quickly set in with listeners sensitive to high frequencies – especially if their systems have some grain, grit, or upper midrange hardness somewhere in the signal path ahead of the VR-4 speakers.

Another point: if your room has the average ratio of reflection to absorption, the super-high frequencies will be heard very clearly due to the bounce of these frequencies from the hard ceiling and walls; since you will be sitting in the reverberant far field, you will hear the same type of frequency balance as when sitting in a typical concert hall. Concentrated doses of high frequency detail may sound great for the first hour or two, but you will be quickly tired of this “in your face” type of treble response! Although this is known in some circles as “accuracy versus musicality,” I believe that people want to listen to music, not just treble detail.

DS: Please detail the driver selection for the VR-4 SR MkII. What brands are used and what led you to the determination to use each driver? What properties of each driver convinced you that they were the appropriate ones?

AVS: Over a 30-year period, I have held jobs at several different companies that made their own drivers (notably ESS Laboratory, home of the Heil Air Motion Transformer, or AMT). Later, I designed parts for KSC Industries, one of the largest driver factories in the world, and thus had access to every type of imaginable cone and/or diaphragm material and motor (drive system). Over a period of many years, I conducted empirical experiments to determine which types of transducers worked best at each of the three frequency ranges (bass, midrange, and treble). Although it would seem intuitive that a single diaphragm would work best in the interest of coherency, it is not physically possible for a small diaphragm to simultaneously “push” deep bass notes and also keep the treble notes clean.

Although small cones and planar diaphragms operating at very low volume levels do indeed sound good, they distort massively when the bass volume level approaches the SPL level of a live jazz group (not to mention an orchestra or rock band).

For the above reason, I split the frequency band in three parts and use transducers most suited to the frequencies being fed to them. I realize that many self-taught engineers and audiophiles believe that a simple speaker using one driver and no crossover MUST sound better than a very complex system like the VR-4 SR Mk2, but that is an extreme generalization NOT born out by empirical experimentation or direct A/B/X comparisons against a live musical source! Note that we use the simplest, classic, and “musically honest” speaker (at least in the midrange) in the world as our comparison reference: the Quad electrostatic. The VR-4 SR Mk2 is just as coherent, just as “fast” and just as “musical,” but has far greater clarity, dynamic range, and frequency extension than the Quad. To satisfy our doubts and to double check our biases, we have conducted several A/B/X comparisons with very keen listeners from the local audiophile clubs as our referees and judges – so we talk from experience, not opinion. Simpler is NOT always better! I wish this wasn’t true, because it would make my engineering job simpler as well. As always, we invite any interested Dagogo readers to visit our factory sound room in the San Diego area to verify our claims.

(The following is Mr. Von Schweikert’s elaboration on bass reproduction)

Many types of bass transducers (woofers) can achieve deep bass, but most of the designs on the market have undue coloration, especially ported bass reflex systems using floppy woofers.

“Boom” and resonant coloration on male voices is not only due to poor crossover points and/or resonant woofer loading, but is also caused by cone flex and high distortion motors. The most important factors in achieving deep and accurate bass are simple, really.

a. The designer must utilize cone materials which will not flex nor distort during heavy bass and long excursions. I have found aluminum or magnesium as the ideal woofer diaphragm, as both are thin and non-resonant metals. Besides being rigid and light in weight, the metal cone will transmit heat from the voice coil to the air, which reduces damage to the voice coil and SPL compression due to heating effects in the voice coil when over-driven. Paper, soft plastic, and Kevlar woofers have “soft” cones that will add up to 34% distortion caused by cone flexing, which generates “warped” sound waves. It is easy to measure the Young’s Modulus, which is the ratio of rigidity to weight, and thin metals measure (and sound) best. Note that midrange and tweeters DO NOT SOUND GOOD when made of metal, so this is not a “blanket” recommendation.

b. The ideal woofer has to have transient response equal to the midrange unit in order to enable perfect coherence at the crossover point. This level of “speed” is achieved by using low moving mass and a very strong motor which will start and stop the diaphragm as quickly as the music dictates. Our woofer uses the patented “Low Distortion Motor” system with a specially designed voice coil, internal pole piece, and matching top plate. Note that all transducers will generate distortion when driven past their limits, so the reduction of distortion in the woofer is paramount if “live” volume levels are to be achieved.

c. Of course, a carefully-engineered cabinet is necessary to permit the bass drivers to generate deep bass without undue coloration caused by resonant loading from the “one note” bass reflex-style tuning. An example of this is the simple, hollow box with a vent or port; usually the walls may be lined with fiberglass or foam, but the internal volume is empty. The hollow resonance does generate more bass volume, but is very boomy and colored as a result of the resonance of the hollow cavity.

The only cabinet tuning that enables fast transient response is the “transmission line” design, which we use in hybrid form (without the complex labyrinth found in less developed designs). Sealed boxes lose 50% of their efficiency, while our quasi-transmission line design is more efficient while improving the bandwidth (i.e., our tuning results in deeper bass while maintaining fast transients).

d. Most designs using multiple drivers force the woofers to reproduce the critical lower- midrange area, smearing the low-end of the male voice (and piano, etc) when the woofer cone goes into long excursions. Note that in 1978, the VR-4 design pioneered the use of extremely low crossover points (between 100 and 200Hz), eliminating midrange coloration generated by feeding voices and other lower midrange frequencies to the woofers. Other brands of conventional speakers run the woofers up as high as 500Hz, which greatly colors the human voice and the piano, unless a two-way design is used that has a decent bass/mid response, usually limited to a 6.5” woofer. Then the trade-off is limited bass, limited volume level, and some modulation of the midrange by the bass notes. Clearly, a three-way design is superior for midrange sound quality.

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