BBC LS3/5a loudspeaker
Despite what must be a rather large amount of built-in bass boost (to compensate for the small size of the woofer), they are fairly efficient: We would estimate around 1¾%, which is comparable to an average acoustic-suspension system. Maximum safe output level is around 95dB SPL (sound pressure level) at a listening distance of up to 15', which is about as loud as a symphonic crescendo from 10' behind the conductor. This is with full-range program material; the limiting factor on power input is the "woofer" (because of its bass boost), so when the speakers are used with a subwoofer (crossing at 60 to 80Hz), they are capable of a clean 100 to 105dB, which is enough to give any masochist a most gratifying case of permanent ear damage.
Judging by their size, one's first thought is likely to be that these will work just dandy up on the wall, right below the ceiling and toward the room corners, where standing-wave resonances in the room will help augment the speakers' thin bass. But their size is very deceptive. These arenot thin-sounding. In fact, they produce an overall balance similar to that of a pair of large systems when they (the Rogers) are located on 30"-high stands, right out near the center of the floor. (This bass-balance design is consistent with the BBC's recent research findings which showed that the smoothest bass response is obtained when a speaker is as far as possible from room boundaries.)
It is because these speakers are so well-balanced when they are out in the room that they may well produce too much bass when placed against a wall, particularly when located near the junction between three room surfaces. In corners, they are (in most rooms) intolerably boomy because they are designed for out-of-corner placement, and because that location excites the maximum number and amplitude of standing-wave resonances (footnote 2) in the room.
The close proximity of room surfaces (or, worse yet, of a box or shelf under the speakers) also causes diffraction interference—the chopping of deep holes in the frequency response due to selective cancellation of certain frequencies. The smaller the speaker enclosure, the less audible are these diffraction effects and the smoother the system sounds. But nearby corners and surfaces can spoil the advantage of the small enclosure.
Another advantage of a small sound source is that it tends to radiate sound waves as expanding spheres rather than as a planar wave (as from large screens). Human ears react in a seemingly paradoxical manner to a spherical sound field: The reproduced sound seems, much bigger than its source, yet the angular localization of sounds across the "stage" between the speakers (ie, the imaging) is dramatically improved. In fact, the apparent audible size of these tiny speakers is almost laughable; we had the feeling that it just could not possibly be.
Adding to the illusion of a large speaker system, is the remarkable low-end performance, which is not really all that deep (subjectively flat to a bit below 57Hz in our rooms) but sounds deeper than it is because the response is actually pretty flat down to there (rather than drooping), and the bass detail is astonishing from 5" woofers. The speakers gave such a startling account of themselves at the low end that we were not acutely aware of the lack of deep bottom until deeper notes (as from bass drum or the bottom range of the string bass) that we knew were on the recording failed to come through.
High-end performance is quite remark able. The speakers have a very slightly rising response above about 5kHz (fig.1), but because there is no audible peak at the top, the rise does not cause any sizzling or spitting, but tends rather to exaggerate slightly the extreme high-end energy in the program, adding a bit more sibilance to voices, a bit more shimmer to cymbals, and a bit more overall airiness to the sound than is actually in the program material (footnote 3).
Footnote 1: American visitors to England consistently report that the BBC transmits superb-quality sound and that unlike the US (where practically everything aired is canned), Britons are privileged to hear frequent live broadcasts of orchestra concerts. It is thus reasonable to assume that the BBC engineers know good sound when they hear it.
Footnote 2: A standing wave occurs when a sound wave travels to one room surface and back again in exact synchronism with the reproduced frequency, causing an energy buildup (a response peak) between the surfaces.
We were not able to determine how far out the high end goes, because our oscillator's upper limit is 20kHz, but the smoothness and the ability to reproduce hard transient information (triangles, castanets, etc.) suggested that 27kHz might not be a bad guess for the upper rolloff point. Overall, the monitors have an extraordinary feeling of depth and breadth to their sound. Inner details were excellently rendered, and there were no audible vowel-like colorations that we could detect. The only thing we could the Monitors on (other than the low-end range) was a subtle tendency to lighten or "stretch upwards" the vowel structure of some instrumental sounds. The effect could be likened to the difference between the lower range of an oboe and the upper range of an English horn, although the extent of the difference was less.
Qur comments about the sound of these speakers have thus far been based on its performance with what we feel (as of this writing) to be the best available power amplifier: the Audio Research D-150. (The new Audio Research equipment has just been announced, but will not have been auditioned by the time we return our Monitors.) We freely acknowledge the idiocy of driving a $430 pair of speakers with a $2700 power amplifier, but we can report that these Rogers/BBC speakers did just as well (if not slightly better in one one respect: a less "hot" high end) when driven by a Dual 76A. It is both a tribute to, and a liability of, these speakers that the lesser their driving equipment, the worse they sound. They reproduce the high-end roughness of solid-state amplifiers mercilessly, and most such amplifiers tend also to dry up their bass end a bit so that they lose some of their nice bottom fatness. (Placing them nearer to room boundaries then helps to compensate).
As for room placement: Except for the possible detrimental effects of standing waves, these are refreshingly uncritical of room placement. They are, as we said, best used on stands, and any carpenter can make up a pair. The speakers should be about 6' apart and 10-12' from the listening area, and each should be toed inwards, by about 5 degrees.
Optimally placed, and driven by suitably high-quality electronics and signal sources, these are a perfect way of getting sound that is comparable to that from Quad electrostatics, at far lower cost and with added bonuses of slightly smoother high end, better stereo imaging, a broader listening area, and considerably greater apparent (that is, audible) size. Their low-end output is, in most rooms, deeper and fuller than that of the Quads, but like the Quads, their major weaknesses are limited sound output and lack of extreme bottom. Above 60Hz, the Rogers BBC minimonitors outperform the vast majority of systems costing upwards of $500 per channel. And with the addition of subwoofering, via M&Ks or Janises, these could provide perfoorance comparable to that of some of the best systems com mercially available, regardless of cost.
The only real handicap that we can see to owning a pair of these is that your tineared friends won't be as impressed as they would be if your speakers took up the whole East Wall of your living room.
Footnote 3: Stereophile's subjective frequency response curves show how the transducer under test sounded to us, rather than how it measured. The vertical scale on each curve is the same, and is scaled so that a barely perceptible audible deviation from flat frequency response is reflected in a barely perceptible visual deviation of the response curve.
Fig.1 Rogers LS3/5a, "subjective" frequency response.
A Followup appeared in December 1977 (Vol.4 No.1):
Subsequent experience with these remarkable little speakers has strengthened our feeling that that they should not be used with solid-state power amplifiers. The speakers need a slight high-end softening, and are more likely to overload (the woofers bottom, making an alarmingly loudbang) from the average solid-state amp. An Audio Research Dual 51A is ideal (with a solid-state amplifier for the woofers if you choose to bi-amp), while an upgraded Dynaco Stereo 70 does very well. The Audio Research D76A is fine but has more power (and a higher price) than is necessary.
Output level is limited to around 85dB with a high-powered, wideband solidstate amp, 95dB with tubed electronics and no subwoofer. With a subwoofer and the Rogers rolled off below 70 or 100Hz, listening levels of over 100dB can be obtained without stress.
If you missed our full report on these in the last issue, a summation: Superbly balanced sound overall; very subtly nasal in some rooms; slightly rising (above 5kHz) but very smooth and extended high end; no deep bass but deficiency not noticeable on most program material; very good detail; extremely large apparent soundsource; very good stereo imaging; limited output level.—J. Gordon Holt
 | ||
No comments:
Post a Comment