PMC IB1 S Loudspeaker

      Date posted: October 24, 1997


Sugg. Retail: $6000 (CAN) ($4500 U.S.) pr
Size: 29″H x 13″W x 18 1/3″D
Distributor: Bryston Ltd.,
P.O. Box 2170, 677 Neal Drive,
Peterborough, Ont. K9J 7Y4
(705) 742-5325 FAX 742-0882

(Reprinted from the Fall 97 Audio Ideas Guide)

      PMC is that most unusual of speaker makers that sells its pro monitors to consumers altered only in exterior finish. Other pro companies like JBL usually have entirely separate lines of speakers for each market, while consumer speakers, such as certain models from Paradigm or Totem occasionally get sold into the pro market as nearfield monitors. I’ve been personally very pleased at the success of PMC (We used the small LB1 as location monitor on our organ CD, The Sadler Selection, and it surprised everyone present with its ability to reproduce 32-Hz pedal notes in the nearfield) just because what they are doing is improving the state of monitoring by making speakers that are both audiophile-quality accurate reproducers as well as fulfilling the studio requirements of high SPLs and very clear articulation.

      Sometimes in the pro market both are achieved at the expense of frequency response accuracy, but that is not the case here. But more on performance below. First, a description of this quite large speaker.

      The transmission line IB1S is the first PMC model to incorporate a new flat Carbon Fibre/Nomex sandwich woofer diaphragm (being flat, it is not a cone) driven by “an edge wound 3″ Kapton voice coil system and an immense magnet.” The driver’s moving section is described as “incredibly light and immensely rigid” on the product sheet. This fact, and the transmission line bass, which unlike a port pressurizes the air and speeds the bass, make for a system that should have much faster bass than any reflex system.

      The tweeter and midrange are both fabric domes, the latter 3″ in diameter. Both are ferrofluid damped for high power handling, the tweeter’s diaphragm made of silk by Vifa. Each is recessed in a vestigial horn to reduce diffraction across the baffle. Crossover frequencies are 380 Hz and 3.8 kHz, maintaining the phase integrity of the critical midrange decade. Impedance is a nominal 4 ohms, while sensitivity is rated at 89 dB, quite low for a studio monitor. That seems to be the only compromise made to ensure audiophile performance.

      This system is rated to achieve 116 dB at 1 metre, in the process able to handle 500 watts of amplifier power. Tri-wired terminals also allow tri-amping, these excellent quality gold-plated posts recessed on the rear panel.

      As can be seen, the painted black finish is pretty industrial looking; not everyone will appreciate the PMC logo embossed (even branded) on both sides of each speaker. This will not be seen on the rosewood finished consumer model, which will be otherwise identical to what we have here. That may not reduce its visible bulk, which beside our tower-style Veritas speakers seemed rather matronly. PMC Frequency Response

      Looking at the measurements we see a very linear Pink Noise Sweep (PNS), showing a little less midrange and treble energy than the Summed Axial Response (SAR) that overlays it. This is because the IB1S (I’m starting to call this speaker the Ibis, but a flower name won’t make it look any more delicate) has controlled dispersion evident in the axial curves below. Thus it loads the room a little less at higher frequencies as shown in the PNS; for a monitor this is a good thing, and I think, for a hi-fi speaker also, because it is more of a point source, and will image more accurately.

      On axis, in fact over a 30o radius, the IB1S is a little bright at the top, but very linear in the midrange, +/-2 dB or better over the range of the midrange dome; with bi-amping the treble can be easily smoothed, up 3 to 4 dB at 10 kHz. I noticed in listening preceding measurement a slight mid-bass dryness, and this does indeed show up in the curves at 200 Hz. Bass is extended, but not quite as much as I expected, down 2 dB at 40 Hz, and 6 dB at 30; careful room placement should bring these up a few dB, and the prodigious output capability of this piston bass driver coupled to the transmission line should allow mighty pedal power to well over 100 dB. Sometimes measurements can be a little deceptive, and while it might take a subwoofer to bring us below-20-Hz bass, the IB1S is only 10 dB down at 20, and should be able to play very loud at this frequency with a little low end EQ. As with the Waveform Mach 17, a trio of Bryston 3B STs and a 10B crossover could make these speakers a state-of-the-art dynamic system, though costly at close to an additional $6000 for these electronics. PMC Impedence

      Even with the use of its own 4th order crossover the IB1S maintains excellent impedance smoothness, not quite reaching the rated 4 ohms, 4 1/2 at 100 Hz, and between 8 and 9 ohms over much of the range above, with bass peaks at about 13 and 16 ohms. This benign impedance, coupled with a very accurate electrical phase curve (seen below the impedance), means that the speaker is an extremely easy load to drive for virtually any amplifier, though only a solid state one with lots of current will exploit its immense dynamic range potential.

      I listened to them with a single 3B ST, and was very impressed. They don’t sound as bright as the measurements suggest when faced straight ahead, and the overall tonal character was quite similar to that of the Energy Veritas v1.8 except for that slightly leaner mid-bass. The IB1S’s are both neutral and revealing, quick and dynamic, especially in the lower octaves, and image with an accuracy and depth that is uncanny. A bipolar may be more spacious in that outside-the-speaker fashion, and a dipole might exaggerate the out-of-phase information, but a true monitor like this reveals all that is on the recording with an even-handed balance. This is my preference in reproduction of music, as longtime readers know.

      On female voice it was very natural as were male and choral voices. Acoustic guitar was very quick and clean with just a bit of extra zing; however, strings sounded rounded and right, without an extra sheen, and sibilance was well controlled. It appears that only close-miked instruments with higher-frequency overtones will show up the slight treble rise of the IB1S.

      The best way to set them up seems to be quite wide apart so that the listener is on an axis of between 20 and 30o. They’ll throw a big image, with lots of depth, and very precise lateral palcement of voices or instruments. I preferred to listen with grilles on, these contoured on their inside edges to control diffraction, the cloth a very light weave that has no effect on high frequencies.

      The IB1S will realistically portray the full dynamics of an orchestra or a rock concert without the compression that almost all consumer speakers exhibit above 100 dB, and is therefore suited to even the biggest of listening rooms. Its character is only a little on the analytical, perhaps to a greater degree than the Waveform, but then, the Mach 17’s electronic crossover can be used to modify that character somewhat (but not too much) by adjusting bass and midrange settings.

      Because of the very linear midrange between 300 and 5000 Hz, this speaker has a remarkable ability to get voices and individual musical instruments right, and doesn’t wilt under tenor power or piano fortissimo at high sound levels. I’ve long maintained that a dome midrange has the advantage of acting as a piston, driven at its periphery as it is, without the inevitable breakup modes at high levels of a cone driven from a centre voice coil.

      The PMC IB1S is a speaker that does so many things well that it tends to acoustically disappear if you close your eyes, leaving just the music, bathed in a palpable soundstage that extends beyond and behind the speakers, not drawing attention to itself, but drawing the listener into the music. That’s why it’s a very appreciable value for its price, a real pro in the very best sense.

Andrew Marshall

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