|
Tip #24: (January 2001)
The BassZilla has finally arrived, after a gestation period of nearly two years. As the proud father of precocious twins, I'd like to share my new found sonic joy with you.
Two incarnations or flavors of the BassZilla are described in the kit plans. Each design preserves the coherency of the full-range driver by operating it full range without any high-pass electrical filters. However, a passive EQ network is used to deal with the inevitable break-up modes of full-range paper drivers. The "plain vanilla" version uses the Fostex FE208 Sigma, and exclusive of cabinetry, its estimated parts cost is in the range of US$1,000 US$1,100 per pair depending on the quality of the crossover network parts. The cost-no-object version is based on the Lowther DX4, and exclusive of cabinetry, its estimated parts cost is in the range of US$2,700 US$2,800 per pair depending on the quality of the crossover network parts. Commercial sources for drivers and parts are given at the end of the kit plans. These are easy to build kits, suitable for even the Do-It-Yourselfer with minimal woodworking skills. The BassZilla design concept breaks new ground in the deployment of full-range drivers. The inspiration for the BassZilla came about because of past personal frustration and dissatisfaction with traditional approaches such as rear horns. Take, for example, the Lowther paper twin-cone driver that was originally patented by Paul Voigt in England in the 1930s. Ironically, what was considered state-of-the-art some 60 years ago, has once again become topical as a superb musical transducer. These drivers are a fascinating blend of old and new technology that has garnered many loyal devotees worldwide. The myth persists that the Lowther-Voight driver is the perfect transducer. This may be close to the truth, but it isn't the whole truth. Every technology has its pros and cons, and the venerable Lowther is no different. Right out of the box, there are at least a couple of practical problems that need to be solved. The most obvious one is what to do about bass reproduction? The most popular solution for matching the midrange sensitivity of the Lowther in the bass range has been to use a rear horn. Not only are all of these rear horns folded to save space, but they are also of constant width. That is, the width dimension remains constant along the length of the horn - the flare taking shape only along the height dimension. Thus, well-known rear horns such as the Acousta 115 and the Medallion-II bear little physical relationship to straight circular cross-section horns that they attempt to emulate, and the entire design process becomes an art form. Jack Dinsdale, in his classic series of horn articles for Wireless World (1974) puts it as follows: "As soon as one departs from the straight horn of circular cross-section, the scientific design principles described cease to be relevant and become more of an approximate value " This follows on the heels of Wilson's admonition (1972) about the performance of folded horns: "It cannot legitimately be assumed that a horn incorporated in a cabinet has the precise characteristics of any particular type of straight horn, whether exponential, hyperbolic, catenary or tractrix, even though their dimensions have been used as guides in its construction. The multiple changes of direction, coupled with reflections and absorptions and internal resonances, are always such as to destroy any legitimate comparison. Every internal (horn) enclosure construction must be judged on its own merits as revealed by measurement and by listening tests." No wonder then that so many competing rear horn designs have surfaced over the years, with each designer claiming to have finally seen the Promised Land. As a practical size compromise, none of the folded horns being copied today are sufficiently long and of adequate mouth size to generate a bass cutoff frequency much below 100 Hz. I'm sure that you've heard it said that Lowther speakers don't have any bass. Well, at least in the context of the typical rear horn, that's absolutely true. There's not much extension below 100 Hz in an Acousta 115, based on both measurements and listening impressions. And that is NOT a satisfactory state of affairs for musical enjoyment. Bass response in the octave from 40 Hz to 80 Hz is essential for the realistic reproduction of many musical instruments. Another problem with using a Lowther in any type of horn, either rear or front design, is that of forcing a thin paper cone to operate in a high-pressure environment that exists in the compression chamber and throat of a horn. The pressure loading here is of sufficient magnitude to buckle and deform the cone, thus generating significant distortion products. That is why many pro-sound compression tweeters feature a light but very stiff titanium or beryllium cone. Finally, resonances and reflections from within the compression chamber can result in serious colorations. My own experiments with the Medallion-II rear horn and a Lowther PM2A driver clearly showed loss of clarity and image focus due to internal reflections within the small compression chamber. The addition of absorptive damping behind the driver noticeably improved the resolution of spatial outlines. The BassZilla solution is to cut through the Gordian Knot by eliminating the box at least in the midrange. We start by recognizing that full-range drivers are inherently fantastic midrange drivers with decent treble extension and resolution. But it's unrealistic to expect a 7 or 8-inch driver, barely out of puberty, to do a man's job. If you want definitive bass punch as well as high-efficiency, then a suitable 15-inch woofer should be enlisted. Once we recognize that the full-range should only cover the range above about 150 200 Hz, there is no reason to imprison it inside a box. Just as it would be heretical to enclose the QUAD-57 electrostatic speaker within a box, it would be sinful to compromise the midrange of an excellent full-range driver by subjecting it to standing waves and intense early reflections that characterize the interior of any box. The reason that the QUAD, or for that matter any other good planar speaker, sounds so non-boxy, is (duh!) that there is no box to obstruct midrange detail. In fact, the BassZilla deploys the full-range driver on an open baffle of sufficient dimensions to prevent bass cancellation down to a desired upper bass frequency, below which the Audax PR380M2 15-inch woofer takes over. But there is more to it than that. The reflected soundfield in a domestic listening environment is typically deficient in midrange energy. That is because most speakers begin to beam in the upper midrange, so that less power is radiated into the room above about 2 kHz. Since the soundfield at the listening seat is a composite of direct and reflected sound, any coloration in the reflected soundfield will affect the perceived balance. Note that at least 50% of the energy at the listening seat in most rooms is due to reflected sound. Using a full-range driver as a dipole radiator on an open baffle allows the backside of the driver to add 3 dB of additional midrange energy to the power response and gives a better midrange balance at the listening seat. As with other dipoles, the soundstage is also better fleshed out with a wonderful depth perspective providing that the speaker is positioned at least 5 feet form the rear wall - to generate late reflections (relative to the direct sound) outside of the critical initial 10 mS time window. Because of these design considerations, the BassZilla loudspeaker is capable of painting a palpable soundstage more spaciously and with far better focus than the same full-range in a rear horn. The second major issue that needs addressing in all paper twin-cone drivers is break-up resonances. For example, the Lowther cone is extremely thin to reduce moving mass and increase the acceleration factor. The flip side of all this is reduced stiffness and damping. The g forces generated due to cone acceleration in the upper midrange cause the Lowther cone to exhibit its first breakup resonance at 2.2 kHz. The cone literally decouples from the voice coil and vibrates in sections, rather than uniformly like a piston. This provides some high-frequency extension, which is further augmented by output from the whizzer cone. The first break-up resonance coincides with what has been called the singer's formant, a strong resonance in a soprano's vocal tract that allows her to project over an orchestra. Not surprisingly, the Lowther normally excels with female voice. The truth is thatstock full-range paper drivers are unlistenable due to high-Q break-up resonances. The kit plans show you how to properly tame these resonances and obtain the smoothest sound possible from the Lowther and Fostex drivers. I'll also show you how to unleash these drivers' full soundstaging potential by eliminating boxy effects in the midrange. Gain many of the benefits of planar transducers by operating your full-range as a dipole radiator. The 24-page Kit plans explain the design philosophy, and give detailed schematics and guidance for the construction of the baffles and crossover networks. Frequency response and impedance plots are included for both versions of the BassZilla. Technical support is available from Dick Olsher to the original purchasers of the BassZilla plans. The plans are available for sale ($US20) at the HiFi Authority.com Store. |
