Museatex Hybrid 6 loudspeaker

I find quite appealing the image invoked by Museatex to describe its Real Time Ripple Effect (RTRE) loudspeaker line: a stone rippling the surface of a "still pond on a warm summer afternoon." Replace the stone with a voice-coil attached to the center of a stretched Mylar diaphragm and you begin to get a glimpse of the RTRE technology's conceptual beauty and promise. The idea of cohesively covering at least the midrange and treble with a single driver, without crossover filters, quickens my audio pulse.

It has been less than three years since Paul Burton and Highwood Audio of Canada (footnote 1) were awarded US Patent No.4,924,504 for a full-range speaker which achieves the propagation of a peaked wavefront from a large diaphragm—rather than a planar wavefront, as is commonly the case when the diaphragm is driven as a piston. A planar speaker, by virtue of being driven uniformly over its entire area, is constrained by the laws of physics to launch a plane wavefront, at least at those wavelengths that are small in comparison with the average dimension of the diaphragm. Although a plane wave bestows certain advantages upon the sonic reproduction, such as a less intense rolloff with distance (compared with that of a point source) and the re-creation of realistic image size, there are practical problems that require resolution. The most annoying of these is the tendency of a planar driver to beam the sound with increasing frequency. Because the radiated sound is increasingly concentrated on-axis, the treble is not only focused into a narrow sweet spot but also rises in intensity. Unless resolved (eg, via a curved diaphragm), the result is a speaker that is unnaturally bright on-axis and requires a head clamp for proper alignment of the listener with the speakers. By contrast, the RTRE approach is intended to elicit point-source behavior from a planar diaphragm.

A low-mass voice-coil is glued to the center of a stretched Mylar diaphragm. The pole piece and the rest of the magnetic circuit are anchored to a slotted frame behind the diaphragm. As it is applied to the center of the diaphragm, the drive signal generates a transverse ripple in the Mylar. As the diaphragm is mechanically displaced forward and backward by the passing ripple, sound is radiated into the surrounding air (see diagram).

The crucial thing to realize is that the ripple generates a time-delayed, lower-intensity image of the central source as it spreads out across the Mylar. The analogy of a pebble rippling the surface of a still pond is indeed physically appropriate, and helps to visualize the mechanical energy propagating across the Mylar. The radiated sound gets a head start at the center and lags increasingly with distance across the diaphragm. The result is a spreading hemispherical wavefront similar to that produced by a point source. Thus a large diaphragm is made to behave as a small virtual sound source; as a consequence, it should not suffer from treble beaming.

Actually, the basic idea of coaxing point-source acoustic behavior from a large diaphragm predates the RTRE loudspeaker. Credit for this development must go to Peter Walker, whose Quad ESL-63 electrostatic loudspeaker also simulates a virtual point source located behind the plane of the panel. This is accomplished by dividing the signal-bearing stators on either side of the diaphragm into annular rings. Each ring is fed via a delay line so that only the central section is fed a real-time signal. The outer rings are increasingly delayed with distance from the center. The resultant wavefront propagated from the diaphragm is hemispherical—as though a point source of sound were at work behind the panel. The main differences between the two approaches are that the Quad is driven over its entire area and the delay is achieved by electrical means, while the RTRE is driven only at the center and the delay is derived mechanically.

But as Mother Nature giveth, so she taketh away. There are two flies that must be plucked from the RTRE ointment: high-Q diaphragm resonances and the formation of standing waves due to edge reflections. Addressing these problems is a tough engineering challenge requiring considerable damping. A slotted MDF panel is mounted behind the diaphragm, which supports a reticulated foam layer. (With frontal lighting you can see through the grillecloth and the Mylar diaphragm to the damping material behind.) The air layer, together with the foam, damps or "brakes" the diaphragm motion. The slot pattern is such that more damping is present around the outside edge of the diaphragm as opposed to the central region of the transducer. Foam tape is also added to the periphery of the diaphragm to mass-load and damp upper-frequency transverse waves that make it to the edges. The nonuniform damping has the effect of reducing the total radiating area as frequency increases, which contributes to the treble radiating primarily from an annulus around the coil. This behavior helps the RTRE maintain an excellent polar response into the treble.

Ed Meitner of Museatex tells me that more than $1,000,000 has been spent refining RTRE technology. Yet, even at this point in its development, satisfactory full-range operation has not been achieved. Below a critical frequency the diaphragm behaves like a drumhead, which is, after all, what it wants to be. At present, low-Q operation of the diaphragm can only be maintained down to about 200Hz. This means that, at least for now, speakers using an RTRE drive-unit will need to use a conventional woofer to fill in the bottom octaves.

The hybrid 6
Visually, the Hybrid 6 strikes a stunningly elegant pose. Right out of the box, it reminded me of a baby Dahlquist DQ-30. Measuring 44.5" tall by 14.5" wide, it is neither intimidating nor awkward. The gray grillecloth, the beautiful oak veneer, and its slim and well-proportioned looks give the Hybrid 6 a high factor of acceptance by wives.

The Hybrid 6 combines a small (16" by 9") RTRE panel driver with a 5.25" graphite-loaded polypropylene woofer. The crossover frequency is nominally 450Hz, with first-order (6dB/octave) slopes used for both the high- and low-pass networks. The woofer is mounted in its own vented enclosure below the RTRE driver. The bass-reflex vent is located on the back above a single pair of binding posts (there is no provision for bi-wiring).

I used both the Fourier Components Sans Pareil and VAC 90C power amps during the evaluation. Speaker cable was TARA Labs' RSC, while interconnect cable was primarily the Expressive Technologies IC-1.

No guidance is offered by Museatex concerning a specific break-in period. The Hybrid 6 was, however, broken-in for several days before I commenced serious listening.

Set up in my reference room, the Hybrid 6es proved non-fussy in terms of placement. Getting them to reproduce convincing soundstage width was a piece of cake. Positioned with a moderate toe-in in the classic stereo configuration (with the speakers and listening seat at the corners of an isosceles triangle), the soundstage materialized as an organic whole. With some speakers, the left and right sonic fields appear to hover in the vicinity of the speakers, as if connected to them via an umbilical cord. This was definitely not the case with the Hybrid 6es. The soundstage broke free of the physical location of the transducers to float behind the plane of the speakers with a panoramic vista of the original space.

Sonic impressions
The realization came almost instantly that the Hybrid 6 just wasn't doing it for me. It was as if the Stagemeister had drawn the curtains shut and gone on vacation. The fire and passion of the musical message were lost. A fog permeated the soundstage to the point that I couldn't see far—certainly not to the back of the hall. Image outlines were fuzzy. The crisp focus that I've become accustomed to wasn't there, even compared to the much less expensive Signet SL260—reviewed in January 1993, Vol.16 No.1, p.192—which had just recently vacated the premises.

One of the glories of Belafonte at Carnegie Hall (BMG 6006-2-R) is its magical portrayal of the acoustic of old Carnegie Hall. Normally, it's child's play for me to step into the illusion of the hall. Now I felt on the outside, disoriented, working hard to transport myself to the original space. Scottie! Where are you? Please, beam me up! Alas, Scottie couldn't hear me. The audience in Carnegie started applauding, and I realized that something was really not right about the speaker's midrange and treble. When familiar sounds like applause sound wrong, something is seriously out of order.

Listening more critically to the midrange revealed just how messy it was. For one thing, it was too laid-back. Jennifer Warnes's rendition of "Ballad of the Runaway Horse" (on Rob Wasserman's Duets, MCA MCAD-42131) normally rivets me to my seat for its full 6:45 playing time. Heard through the Hybrid 6, the tonality of Warnes's voice was off. The upper mids sounded wooden, lacking verve. Tragically, much of the life of the performance and the sumptuous, reverberant halo surrounding Jennifer were bleached out.

Footnote 1: For a while, Highwood Audio made panel speakers sold under the Sumo brandname.
Museatex Audio Inc.
Calgary, Alberta T2B 1N5