PMC IB-1S loudspeaker Page 2

I found the IB-1S's bass response puzzling, and not what I expect from a transmission line. The bass in general was certainly extended, quite full, and balanced with the midrange. On "Hillbillies from Outer Space" there was a satisfying kick to the percussion. Taking on Dead Can Dance's Into the Labyrinth (Virgin 45383-2) for system fuel, the low, droning bass of "Yulunga" was wonderfully resonant and superbly defined, the sense of airy space exhilarating. However, the deeper, more percussive lows failed to make adequately emphatic punctuation.

Similarly with the beefy Previn/LSO recording of Rachmaninoff's Symphonic Dances (EMI CDM 7 69025 2): the cellos and bass fiddles, even the timpani, sounded clean and impressive, but the bass-drum whacks seemed wimpy. Not that the bottom-feeders weren't reproduced; they were. It was just that they did not balance adequately with the rest of the bass. Considering the bass performance and the previously noted box vibrations, it's not surprising that the IB-1S could add some weight to male voices. Still, this addition was very slight and often attractive, and orchestral balances were not significantly colored.

A little testing
To get a better appreciation of what was happening, I a little informal testing with the Sound Check 2 Audio Test and Demonstration CD (footnote 2). The double-jewel-box set contains a CD and a built-in microphone/LED sound pressure level meter. Although the LED meter has only 3dB resolution (some interpolation is possible) over a +12/–15dB range, it's fairly consistent and very convenient for quick testing. The CD has various interesting demo tracks and a selection of test signals, the most useful of which are the low-frequency warble tones (footnote 3). One major advantage of warble tones over sinewaves is that they permit you to assess LF response with reduced interaction from standing waves. This makes mike placement for informal measurements much less critical for informal measurements of LF response under non-anechoic conditions.

With the IB-1S, 1/3-octave warble tones centered at 30Hz were at the 1kHz reference level and could shake the room. Warble tones at 25Hz were audible, but at 20Hz were inaudible unless the gain was greatly increased. Unfortunately, at those levels the woofer was hitting its stops and complaining loudly, something it never did with music. Strangely, I found the output at 40Hz to be 3dB lower than at 30Hz, with full recovery to reference levels by 60Hz. My measurements may not be duplicated in your listening room (or correlate with JA's measurements) since the circa-30Hz response is probably specific to my listening room. However, these results indicate that there is no problem with the IB-1S's bass extension.

I can, therefore, only speculate as to why the IB-1S lacks visceral impact at the bottom end, the very area where a TL should excel. One possible explanation is that interaction with the output of the midrange driver blunts the leading edges of bass transients giving the impression of "slow" bass at the very bottom. ("Fast bass" is an oxymoron; what makes a woofer sound "fast" is the accurate reproduction of the rising edge of a complex musical tone by the higher-frequency drivers or by a wider pass-band for the woofer.)

Another possibility relates to the enclosure size. While certainly not a minimonitor, the IB-1S box is relatively small for a TL with a 10" driver, which limits the critical volume directly behind the woofer and the equally critical TL tunnel area. (My old TLs, with a similar-sized woofer, had about three times the internal volume.) Unless there is a sufficient volume behind the woofer and the beginning of the line is also suitably large, the speaker can perform as if it were loaded by that small volume, with a resultant fairly high resonant frequency.

Quickie comparison
Rereading my notes, I fear that I've been less favorable to the PMC IB-1S than it may deserve. I may have dwelled too much on their foibles because of their general excellence. The IB-1S's strengths are its superb harmonic integration across the frequency spectrum and its almost complete lack of grain and blurring. In addition, its rendering of fine dynamic modulations is as good as any. For comparison, consider that the PMC is priced the same as the Ruark Equinox that I reviewed in February 1997 (Vol.20 No.2), but is considerably larger and more robust.

In the final movement of Sallinen's Symphony 6 (Kamu/Malmo SO, BIS CD-511), the composer makes extensive use of a potent battery of percussion to comment on and underline the orchestral drama. Both the Ruarks and the PMCs created a rich, detailed soundstage when playing this recording. While both speakers were quite satisfying, the Equinoxes dissolved into the acoustic space but were not capable of generating the sheer mass of a large symphony orchestra at realistic levels. Moreover, their reproduction of the deep bass was a valid suggestion rather than a clear statement. I found I could turn up the gain on the PMCs and revel in the weight, color, and precise placement of all the instruments, including the timpani and lower strings. I would have preferred better definition in those lowest tones and a bit more image width, but, overall, the IB-1Ses were simply capable of a more realistic re-creation of a full orchestra.

Conclusions
Even though I've been quite critical of it, I found the IB-1S to be excellent. Under appropriate conditions, it provided a remarkably lucid and enjoyable view of the recorded event. Never did it obscure detail or flinch from reproducing lifelike, and sometimes fearsome, sound levels. Its heritage and application as a studio monitor are fully justified, and can be appreciated in the listening room. If you think that a truly pure midrange is the exclusive province of minimonitors and electrostatics, you must hear the IB-1S.

Nonetheless, I respected this speaker more than I loved it. Perhaps the powered version's active crossover provides some effective compensation, but these superb components simply deserve to be better housed. I'd like to see them in a still bigger enclosure that permits the woofer to breathe through a more generous line, places the midrange at the listener's ear level, and presents a narrower frontal area for better HF dispersion. The IB-1S is pretty damn good, but I think there's a truly great speaker inside, just waiting to get out.



Footnote 2: I obtained the complete kit for $l76 from InterStudio Limited at www.interstudio.co.uk/isl/. The US distributor is Ultra Systems, 1 Walters Lane, Box 570, Point Pleasant, PA 18950. Tel: (215) 297-0227. Web: www.surfnetwork.com/ultrasystems.—Kalman Rubinson

Footnote 3: Stereophile's Editor's Choice CD also has low-frequency 1/3-octave warble tones and other tracks to help set up speakers in rooms.—John Atkinson

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