German Physiks Unlimited II Loudspeaker

King of the Bending Wave

Equipment report
German Physiks Unlimited MK II
German Physiks Unlimited II Loudspeaker

Lincoln Walsh isn’t exactly a household name in audiophile circles, but he literally turned driver design on its head in the early 1960s with his revolutionary bending-wave transducer. The Walsh driver looked like a giant inverted ice cream cone, terminated at the wide end and driven by a voice coil at the narrow one so that that sound was only radiated from its convex side. The cone was made thin enough so that its mode of operation transitioned from being piston-like in the bass range to bending wave in the lower midrange. Imagine vibrational waves rippling down the cone at a speed exceeding that of the speed of sound in air. By judicious choice of materials and variation of the cone’s stiffness profile over its length Walsh was able to achieve a coherent propagated wavefront perpendicular to the cone axis. Sonically the cone behaved much like a pulsating cylinder with all frequencies in phase with the input signal. Two other important benefits ensued from this design: wide-range operation from a single driver and an omnidirectional radiation pattern approximating that formed by an ideal pulsating sphere. The conceptual beauty of a bending-wave transducer resides in the fact that it exploits the non-rigidity of the diaphragm material, working with it rather than fighting it. Walsh experimented with several cone materials and obtained excellent results using a 0.002-inch thick aluminum sheet formed to give a 6-inch diameter at the large end. Sadly, Walsh passed on in 1971 before seeing his design released by Ohm Acoustics.

In the late 1970s Peter Dicks, an inventive German engineer, took up where Walsh had left off with the express goal of improving bending-wave driver performance. Many of his experimental cones used thin titanium sheets and after several years he managed to develop impressive-sounding prototypes. Initially industry interest, however, was nonexistent, and Dicks had to wait for nearly a decade before a loudspeaker company took up his design. Holger Mueller was the right man to commercialize the Dicks Dipole Driver (DDD). After all, he was not only an established conventional speaker manufacturer but also an owner of a pair of vintage Ohm Walsh Model F loudspeakers. He clearly understood that Dicks’ design surpassed the performance of the original Ohm Walsh driver. Mueller proceeded to license Dicks’ driver and worked for two years to refine the industrial design of a new loudspeaker while Dicks continued to perfect the driver. Finally, in 1993 German Physiks was formed to exclusively manufacture DDD-based designs.

Over the years the DDD design has continued to evolve and now offers increased excursion capability, power handling, and enhanced bandwidth to beyond 24kHz. A potential problem with the Walsh driver is reflected energy from the non-driven end of the cone. If the time required for the ripples to die down is substantial their impact can result in transient ringing. Ideally, vibrational waves should only traverse the cone once. The DDD closely approaches that objective by first minimizing the amount of energy that reaches the termination and by internally damping the cone and its termination. The use of a powerful ferrite magnet designed in-house together with an edge-wound voice coil ensures a respectable sensitivity spec. The linearity of the magnetic circuit is excellent, partly due to the use of an under-hung voice coil. According to Mueller, the current DDD design can be reliably used as low as 150Hz, but is typically crossed over at 170Hz. Of course such a low crossover frequency makes it a snap to integrate the DDD with a conventional woofer. One important point to remember about this driver is that it is optimized in dimensions and cone profile, and therefore cannot be effectively scaled up in size to increase efficiency. Several drivers need to be used in parallel in order to increase overall sensitivity. The Unlimited Mk II (U2) uses a single carbon-fiber DDD, which first became available in 2006 alongside the classic titanium type. Although both cone types are about equal in weight, they differ dramatically in thickness. The titanium cone is thinner by a factor of six than its carbon-fiber counterpart, and is tissue thin at only 0.025mm but is dampened by an equally thick synthetic coating. These materials are fragile in their sheet form and require great care in handling. As you can imagine, driver assembly is a slow and exacting process that is performed by skilled technicians. I have not auditioned the titanium DDD, but I am told that the carbon-fiber version is tonally a touch darker sounding and slightly less high in resolution. But on the plus side, carbon fiber is said to be more robust (prying fingers can damage the titanium cone) and is capable of being driven harder. An 8" woofer takes over below about 200Hz. It is floor-loaded in a sealed compact tower enclosure.

The Unlimited II is currently German Physiks’ entry-level loudspeaker. It started life in May 2011 as the Limited II—a limited production run of 100 pairs designed to test market interest. It solved the problem of how to rein in the cost of the previous “entry level,” the HRS-120, which was priced circa $25k in the U.S., without compromising sound quality. The DDD was a given as it cannot be dumbed down without sacrificing sound quality. In fact, the U2 deploys the same bending-wave driver found in all the other German Physiks models, including the top-of-the-line Gaudi. Since there was no significant cost saving possible with either the woofer or crossover, the cabinet remained the only cost-saving option. The switch from an octagonal to a square cabinet and from a wood veneer to a vinyl finish yielded considerable savings. The vinyl finish is actually a flooring vinyl that damps the cabinet, covered by a tough aesthetic veneer. As I see it, these were wise choices. In the bass range, where the wavelength is considerably larger than the effective dimension of the cabinet, an octagonal foot print has a small advantage in lower levels of cabinet resonance (owing to the smaller and thus stiffer panels), while standing waves in the lower midrange can easily be attenuated acoustically. As the Limited II sold out fairly quickly, distributors asked to have the design made a permanent part of the range. And so the design was re-introduced and dubbed in a humorous vein as the Unlimited Mk II. The only added touch afforded by the U2 is that it is available in four colors: black, white, grey and brown.

The nominal impedance is correctly stated to be 4 ohms, and the impedance magnitude does not dip below that value. The sealed box tuning is at about 40Hz. Impedance measurements indicate a significant peak around 9kHz, which I am told is due to an EQ network in the crossover designed to flatten the frequency response in this range. I’ll have more to say about this later on.

I’ve been a fan of planar dipole speakers for over 30 years. One of their attributes is the ability to generate a more realistic soundfield in a listening room than conventional box speakers. That omnidirectional designs possess the same sort of magic was made crystal clear to me way back in 1987 when I reviewed the Ohm Walsh 5. I lived through the studio monitor and mini-monitor craze of the 1970s and 80s, and while I appreciated the imaging precision of such designs I didn’t feel that they brought me any closer to being there, to experiencing a live acoustic space. Specifically, studio monitors are designed for narrow dispersion in the midrange and treble in an attempt to minimize the room’s acoustic signature. That, by itself, may be useful for a mastering engineer interested in judging exactly what the source material is all about, but as a music lover I would like to be immersed in the musical experience. And that can happen most effectively by getting the room involved in the playback process. Two-channel audio does a poor job of treating reverberant information, since all sound, both direct and reverb, is generated in a plane suspended between the two speakers, which grossly undermines the illusion of a natural acoustic. In a well-designed concert hall, the listener is enveloped in lateral reflections which convey the reverberant signature of the hall. Room reflections can immensely improve the illusion of a live performance, with the caveat that the reflected energy be delayed about 10 millisecond relative to the direct sound. As long as the reflections are outside of this critical time window, they will be perceived by the auditory system as spaciousness. Be prepared to give the U2 at least five feet of breathing space to the rear and sides in order to achieve the specified time delay, since each foot of travel yields about a one millisecond delay. You may get by with less than five feet if diffusers are applied to the adjacent wall surfaces. Furthermore, any speaker with a fairly uniform power response will most likely perform poorly in a dead end/live end listening environment. Sound dispersive wall treatments should be fine but highly absorptive wall treatments should be avoided for best results. Most domestic listening environments should prove to be perfectly suitable.

It only took me a few seconds to realize that this is a superbly coherent loudspeaker whose wavelaunch more closely mimics live music than the disjointed presentation of a typical multi-way speaker. While I don’t intend to condemn all multi-way designs, the truth is that the great majority simply fail to coalesce into a coherent whole. That’s a direct consequence of slicing and dicing the music spectrum into pieces for consumption by tweeters, mids, and woofers arrayed on a large baffle. Such speakers may integrate reasonably well at a few points in space but move the measuring mike or your head a few inches and the balance shifts significantly.