Single Driver Ecstasy
By Dick Olsher
H. A. Hartley - a contemporary of Voigt and Lowther - devoted much of his life to perfecting the single-driver, full-range, loudspeaker. Since 1925 and through the 50s his professional passion was high-quality sound reproduction. He claims to have coined the term "high fidelity" in 1927 to denote sound quality becoming a music lover. Writing in the 50s he made a clear distinction between an audiophile and a music lover. In his book, a man with no interest in the mechanics of audio reproduction and concern only for the most faithful reproduction of the musical program material is considered a music lover. On the other hand, a fellow who is more interested in stunts or the overpowering reproduction of drums, triangles, or the components of "concrete music" qualifies as an audiophile.
There is a simple multiple-choice test any of us can take which is guaranteed to disclose one's true colors.
Question 1: Is your record collection smaller than 200 albums?
Question 2 : Are you more interested in bass extension or treble detail than midrange clarity?
Question 3: Do you count the number of individual singers in a chorus or routinely try to resolve the number of instruments in an ensemble?
If you answered "yes" to at least two of the above, let's face it, you're a confirmed audiophile, whose major interest is in the means and not the end.
Things haven't changed much in the last 40 years. And I believe that the dichotomy between audiophile and music lover priorities has been responsible for the modern obsession with multi-way speakers. Symptomatic of this has been the proliferation of exotic tweeters and woofers to the exclusion of midranges. Sadly, many audiophiles and audio engineers seem convinced that the road to hi-fi heaven lies at the frequency extremes.
The notion of attacking the loudspeaker problem with a horde of drivers would have seemed fantastic in the 30s. Back then the single-driver loudspeaker reigned supreme. The typical frequency response was in the range of 100 Hz to 8,000 Hz. While such a limited bandwidth may appear crude and primitive by contemporary standards, keep in mind what program material was like at that time. Wide-range radio reception was impossible because of adjacent channel interference. FM hadn't been invented yet, so you were stuck with AM hash. Phonograph records were no better and probably even worse. So it was wise not to open the window too wide. It was better to aim lower at an enjoyable level of performance, rather than to assail the listener with a constant barrage of rumble and hash.
As the quality of program material improved steadily into the 50s, the quest was on for "full-range" drivers. Eventually, speaker art in the 50s settled comfortably on multi-driver designs as an industry standard. However, as it turns out, our Mr. Hartley had already been there and done that in 1931! His may have been the first two-way system in production. Visitors to the 1931 National Radio Exhibition in London could hear this system consisting of an 18-inch woofer mated to a small cone tweeter. Public response was apparently quite favorable, but Hartley withdrew the design after a year because he thought it sounded horrible. He disliked an air-column resonance within the large cone and also the lack of coherence from two separate and distinct sound sources. It was back to full-range driver designs for Hartley. Sure, it was obvious to him that the widest frequency response was obtainable from a multi-driver design, and that if mere width of frequency response was the ultimate goal then such a system would be ideal. "Audiophiles want it," he said, "but I have no real evidence that music lovers want it." His original bandwidth desideratum for music lovers, laid down in the 30s, was perfect reproduction between the limits of 32 and 9,000 Hz. With 30 years worth of hindsight, he expanded the limits from 20 to 12,000 Hz. Only an audiophile, he argues seems to want to reproduce the sound of a triangle more triangular than the real thing.
It's not an easy trick to squeeze more treble out of a typical 8-inch cone. Speaker God has molded the laws of physics so that a driver is constrained to beam sound like a flashlight when its diameter approximates the wavelength it tries to radiate. For our benchmark 8-inch cone, the moment of truth arrives at a frequency of about 2 kHz. Another problem is that a large cone isn't stiff enough to withstand the extreme g-forces it is subjected to in the treble. Rather than maintain true pistonic action, it flexes and "breaks up" into multiple sections radially and tangentially, each acting on its own. Operating a driver into the breakup mode extends its frequency response all right, but since the cone sections do not radiate in phase, the frequency response becomes ragged. Simple-minded attempts to stiffen the cone lead to increased moving mass and a precipitous reduction in efficiency. I addition, inductance rolls off treble, and ultimately a woofer's HF response goes south rapidly because of its considerable voice coil inductance. It should be obvious by now that there's no free lunch when it comes to treble extension, and that considerable ingenuity is required to make a single driver sing sweetly to beyond 10 kHz.
A popular approach over the years has been the duo-cone design, where the cone is broken up by design into two concentric sections or rings, which are joined with enough compliance that the cone moves as a whole at low frequencies. In the treble, the smaller inner cone decouples from the outer section, resulting in increased treble output and dispersion because of the increase in stiffness and reduced size. Let's take a look at another design whose roots go back to the 30s. The Lowther-Voigt driver, as embodied in the Lowther PM6 and PM7 designs, uses Voigt's "twin-cone" design. Two paper cones are driven in concert by a clever dual-wound voice coil. The smaller, concentrically mounted paper cone, with the help of a phase plug improves treble extension - though it does not solve the problem of treble beaming. This is not a "whizzer" cone as some have surmised after a cursory exam of this unusual driver. Technically, a whizzer is not driven directly by the voice coil - rather it picks up and re-radiates treble energy from the apex of the main cone. The PM6A and PM7A use huge alnico magnets to generate 100 dB plus efficiencies in the midband while launching transients with startling electrostatic-like clarity.
Voice-coil activated full-range driver technology lay dormant until the 70s when the genius of Lincoln Walsh literally turned the art upside down. His driver, which looked much like an upside-down ice cream cone, represented a prime conceptual breakthrough. He realized that by driving the apex of an adequately stiff and properly shaped inverted cone it was possible to create a coherent source of radiation. The sound wave travels down the cone's surface faster than it can travel in air, so that the sound radiating from the surface of the cone remains vertically aligned with the sound radiating into the air. The really cool thing about this is that the sound source - the bending wave which flexes the cone as it travels from the apex - would normally be considered a source of distortion in a conventional cone; something to be damped out. The Ohm Walsh speaker line from Ohm Acoustics illustrates the benefits of the Walsh driver, even though a super tweeter is used to complement the output above 10 kHz.
The Walsh type driver reached its zenith of technical sophistication in the Dicks Dipole Driver (DDD) offered by German Physiks, and distributed in the US. The design process originated with a solution of the wave equation for phase velocity in a membrane and progressed through an experimental stage to optimize the membrane's response.
From Germany With Love
The Manger Sound Transducer offers similar benefits but uses a small disc diaphragm. I've actually not heard this driver for myself, but I've seen its praise sung on the Internet and certainly on paper its technical superiority is clear. This sonic marvel is capable of spanning a frequency range from 80 Hz to 33 kHz with a single diaphragm. So it's essentially full range, and a conventional woofer is recommended to round out the bass range. Its rise time is an astounding 13 microseconds (for the neodymium magnet version). Manger's bending-wave diaphragm is built up of three layers in the form of a thin disc, yet is flexible enough to bend under the force of a voice coil. As the disc is driven by the voice coil, its radiating area decreases with increasing frequency. The result is excellent dispersion and phase coherence so that transient response is said to be at the cutting edge. Transient perfection is such that the acoustic output of the transducer closely mimics the waveform of the input signal. Published impulse response data shows that the Manger is only equaled in transient perfection by the classic Quad ESL. Unfortunately the Manger is quite expensive at around $1,000 a piece. If you're still interested you may contact Manger for further information at: Industriestrase 17, D-97638 Mellrichstadt, Germany; Fax: 49-9776-7185.
Full-Range Drivers on a Budget
I've taken a close look at the full-range driver loudspeaker genre over the past couple of years, partly motivated by nostalgia and partly out of sheer curiosity. Interestingly, duo-cone designs are still being produced today by mainline driver manufacturers, targeted specifically at the PA and automotive markets. When tweeters are so dirt cheap, why would anybody bother using single drivers for hi-fidelity home loudspeakers? The rationale in a nutshell is coherence. I find it incredible that so many audiophiles swallow the multi- way speaker paradigm, hook line and sinker, without even a microsecond of critical thinking. With most multi-way speakers, I'm tempted to push to eject button on my listening seat. Think of what is involved here: chopping up the music into two or more ranges via a crossover network, feeding each portion to its respective driver, with the fond hope that the sound radiation from several points in space will somehow integrate into a musical whole. Often it just doesn't happen. On the other hand, single driver systems for all of their imperfections at least convey the music with a single voice.
A dramatic demonstration of the musical power of the single driver took place at the recent Milan Top Audio Hi-Fi Show in Italy (October 1996). Mozart Editrice, the publisher of [Costruire HiFi], an Italian gem of a DIY magazine (the Italian Sound Practices Magazine), demonstrated several of its kit projects - including an upcoming speaker kit, which was apologetically referred to as the "Joke." I was ushered into a small room, just a little bigger than a closet, where the "Joke" was stand-mounted against the wall. What the "Joke" turned out to be was an 8-inch Audax industrial-grade coated paper driver, actually a modified version of a duo-cone design (HT210A2), installed in a smallish vented box. Driven by a 2.5-watt amp kit, the "Joke" was one of the major highlights for me at the Show. It had plenty of rough edges, but there was also plenty of magic and boogie factor through the middle octaves - and that's no joke! I hope to obtain samples of the kit after the Winter CES, so look for a more detailed report in the near future.
Join the Club: The Lowther Acousta 115 Loudspeaker
There are many aficionados of single-ended triode amplification for whom the search for sonic happiness has ended with the Lowther driver. Its ranks of devotees have swelled over the years ... and justifiably so. My first exposure many years ago to a Lowther PM6C was quite memorable, and I'd periodically get the urge to taste the forbidden fruit. Finally, in 1995 I decided to take the plunge and ordered a pair of PM7A drivers from Tony Glynn, Lowther Club of America (http://www.lowther-america.com/ or via snail mail: P.O. Box 4758, Salem, Oregon 97302; Tel.: 503-370- 9115). Additionally, I bought a pair of finished cabinets through the Club. These enclosures are essentially copies of Lowther's rear horn-loaded Acousta 115 cabinet with only slight modifications (all for the better).
This was no ordinary transaction. Purchasing a Lowther can be a frustrating experience. It took some six months for the drivers to arrive, and it was only through Tony's personal intervention that the drivers arrived as early as they did. Also, instead of the 16-ohm version I asked for, 8-ohm drivers were delivered. I mention my personal experience to emphasize what my source in the UK reinforces, that Lowther is presently a marginal company with sporadic production and limited resources.
My time with the Lowther has been fairly limited so far. These are speakers that take a notoriously long time to break in. So with that caveat in mind, let me offer a synopsis of my first sonic impressions. Certainly, in the context of the Acousta cabinet, there is no deep bass and only limited mid bass. Treble extension is limited to about 12 kHz. The treble is pretty directional; you have to sit in a tight sweet spot to enjoy what there is. The midrange is, however, where the Lowther shines. It shares with the old QUAD ESL the same sort of magically cohesive presentation. The tonal balance is, however, considerably different. Whereas the QUAD is laid back, the Lowther is most definitely not. Part of its charm comes form a moderate peak in the upper mid/presence region that projects female voice into your lap. This is similar in tonal balance to the behavior of the Altec 755A driver, which I experimented with a couple of years ago. The Altec is no doubt a great PA speaker: it projects voice so vividly that more accurate transducers seem tame in comparison. No wonder it has such a devout cult following. Overall, the Lowther is a much better driver, with transient speed and control that runs circles around the Altec. Driven by a pair of Cary 805s, spatial outlines took on flesh and blood palpability. Transparency is quite remarkable, as is detail resolution. This is a high-resolution transducer that shines a bright light onto the soundstage. Dynamic nuances are voiced with a conviction matched only by a handful of other speakers on this planet. The Lowther sure pushed my emotional buttons. Together with the classic QUAD ESL, these loudspeakers underscore just how little progress music lovers have had to contend with in the past 50 years.