Pawel/Ensemble PA-1 & Reference loudspeakers
Quiet, please, Mr. Olsen is about to describe the disadvantages of multipledrive-unit systems. "The radiating surfaces must be separated by a finite distance, with the result that this system will exhibit peculiar directional characteristics in the overlap region where the sound radiation issues from both sources. To reduce this effect to a minimum, the overlap region must be confined to a very small range which requires an elaborate electric filter system for allocating the frequency bands of the units. The greater space required for the two loudspeakers is another important factor. The cost of two separate field structures and vibrating systems will be considerably greater than that of a single unit."
You see, the average radio receiver in those days was saddled with a single cone driver covering a frequency range of, say, 100Hz5kHz. Harry was offering the multitudes an inexpensive alternative: a 60Hz10kHz bandwidth driver in the form of a single corrugated cone. How did it sound? Pretty good, apparently. Harry Olsen, one of the great audio minds of all time, concluded with: "Listening tests are the ultimate criteria of the performance of any sound reproducing system. Comparison listening tests of this loudspeaker with a master reference system capable of reproducing the range from 35 to 16,000 cycles indicates that the reproduction of speech and music employing this loudspeaker compares favorably for all practical purposes with the master system." Amen, Harry! Many of us, no doubt dazzled by the latest acoustical measurement wonders, have forgotten this basic truth. When it comes to the evaluation of sound quality, the ear must reign supreme.
Despite Harry's reservations concerning two-way systems, the proliferation of inexpensive woofers and tweeters brought about an explosion of such systems. How many 8"-woofer/1"-tweeter speakers have come and gone? I'm sure that all of us at some point in time have owned or own such a system. The trend in the US has been toward largish cabinets of about 1ft3 volume with minimalist crossover networksgenerally first-order filters.
Just because I've used and favor fourth-order filters, should not be construed to mean that I'm biased against shallow-sloped networks. The reality of most off-the-shelf drivers dictates more frequency-selective types of filters. A first-order network produces significant overlap between the woofer and tweeter and pushes most drivers at least an octave outside of their comfort zone. The woofer is forced to contribute energy through the upper mids and lower treble while operating in its breakup mode, while the tweeter is forced to operate too close to its low-frequency resonance, with a resultant increase in harmonic distortion even at moderate volume levels. Typically, what you end up with is lots of grain, harshness, and soundstage confusiona veritable sonic headache.
The British finally did get it right. I refer, of course, to the BBC spec LS3/5a minimonitors. This was a seminal product that showed the way. The fact that it is still in production (JA owns a pair) attests not only to its commercial success but also to its status as a classic. The basic strategy was to forget about the deep bass, use a small woofer that was more comfortable handling upper-mid information, and, just as important, facilitate the use of a small enclosure. Of course, a small rigid box is much easier to build than a large rigid box. But even if the large box were just as rigid and just as well damped as the minimonitor, a small box has the inherent advantage of considerably reduced panel surface area. Thus, it radiates less of its panel resonances.
By physically staying small in the bass octaves, a little box actually offers more by producing less. When it comes to communicating low-frequency vibrations (including panel resonances) to air, big means more efficient. All other things being equal, the bass quality of a small box exceeds that of the big box. Here, I'm not talking about bass extension or punch, but bass detail and pitch definition. Midbass and upper-bass information can be greatly relieved of what have been charitably called "boxy colorations."
Another often-overlooked advantage of a small box is that it more closely approximates a point source of sound. Every enclosure has its own unique diffraction pattern that depends on its shape and size. Spherical or egg-shaped enclosures possess the smoothest diffraction patterns. A box, even if its edges are filed down, still generates quite a few frequency-response glitches. The point is that, for the small box, cabinet reflections are better integrated with the primary sound radiation. Front-baffle reflections are typically delayed less than about 0.5mswhich is important for imaging precision. A big box generates later-arriving reflections, which disperse image focus. No wonder that, as a family, minimonitors have earned high marks for their ability to delineate a soundstage and resolve spatial nuances.
So far I've extolled the generic virtues of minimonitors. Are there any other disadvantages besides the lack of deep bass? Unfortunately, for the average minimonitor, the answer is a big fat yes because of non-linear distortions. Paul Klipsch lists minimal distortion as one of the eight cardinal points of a good loudspeaker. Following his line of reasoning, it's easy to see why he logically ends up with a horn-loaded design. A horn greatly reduces diaphragm excursion, which is the key player in the distortion game. A small direct-radiator woofer works much harder than a big woofer. Life is already tough enough, even for a 12" woofer. Excursion demands quadruple with every halving of the frequency. On an acoustic watt output basis, a 5" woofer has to work much harder than its 12" counterpart. It pumps and pumps and then runs into its excursion limit. Small woofers are definitely excursion-limited, which is a kind way of saying that the suspension and voice-coil are driven into a non-linear region of operation. This is an "Alice in Wonderland" region where the output is not proportional to the input signal. The end result is the generation of distortion products: the appearance in the output of frequencies not originally present in the input. There's a whole zoo of them: even- and odd-order harmonics of original frequencies, and sum and difference products of two input frequenciesthe so-called intermodulation products.
There's another distortion animal, a bit more exotic and a favorite of Klipsch; namely, frequency modulation (FM). Beers and Belar introduced FM to the world in a 1943 paper. The basic idea here is that a woofer undergoing a large bass excursion is Doppler-shifting, or causing flutter in, the high frequencies that it is trying to reproduce at the same time. The interesting thing about FM is that it increases with the modulated frequency.
For example, a woofer undergoing an excursion of 0.125" produces FM sideband distortion of 1.2% at 300Hz. At 3kHz, the FM distortion climbs an order of magnitude to 12%. The audible result of FM, as with most forms of distortion, is a rough and grainy sound quality riding along with the music program. If you can see the cone pumping chronically, you're likely to be in trouble because of FMespecially with a woofer-midrange running fairly wide open.
On the basis of the above, you might conclude that the average minimonitor will lose its cool when driven fairly hard with wide-range orchestral music. You'd be right. Certainly, the LS3/5a has to be treated with kid gloves, or you'll be paying for a new woofer in a hurry. The Celestion SL600 is much better in this regardit can thermally sink quite a bit of power without fear of a Chernobyl-like meltdown. However, when the music really gets going, the SL600 does not. It congests and generally sounds strained as the music signal pushes it to the wall. The Ensembles do much much better in the area of dynamics, but I see that I'm letting the cat out of the bag.
Ensemble and Pawel Acoustics
The manufacturer of the PA-1 and Reference, Pawel Acoustics in Switzerland, does not get involved in the marketing of the line. That's handled by Ensemble, Inc. (also a Swiss Company). The PA-1 was distributed some years ago in the US by Goldmund as the Goldmund Prologue. The speaker has been improved since then and is now available here through the auspices of Graham Engineering.
When I ran into Bob Graham at the 1990 Winter CES, I not only got the latest scoop on his tonearm, but he could hardly contain himself about this wonderful little Swiss speaker he's importing. He confessed to initially being dragged into the whole affair "kicking and screaming." But once he heard the Ensemble PA-1, he became a believer. He kept comparing it to his MartinLogan CLSes in a light that made it clear that he actually preferred the PA-1.