Monitor Audio Platinum PL200 loudspeaker Measurements

Sidebar 3: Measurements

I performed the quasi-anechoic measurements of the Monitor Audio Platinum PL200 with DRA Labs' MLSSA system and a calibrated DPA 4006 microphone. The grille was left off for the measurements, as that was how Bob Deutsch auditioned the speakers. The Platinum PL200's sensitivity is specified as a usefully high 90dB/2.83V/m; my estimate was actually a little higher, at 90.4dB(B)/2.83V/m, though this is within experimental error of the specification. This speaker will play loud when driven by amplifiers of low or moderate power. However, its electrical impedance (fig.1) does drop below 6 ohms in the lower midrange and mid-treble, with a minimum value of 3.6 ohms at 150Hz. The amplifier used should be 4 ohm-rated.

Fig.1 Monitor Audio Platinum PL200, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)

The traces in fig.1 have a sharp discontinuity at 280Hz, suggesting a cabinet resonance of some kind at that frequency. However, when I investigated the cabinet's vibrational behavior with a piezoelectric-film accelerometer, instead of a panel resonance at 280Hz, I found a small mode at 191Hz on all surfaces, as well as resonances, also low in level, at 410Hz (fig.2), 480Hz, and 510Hz. The first mode is low enough in level, and the last three high enough in frequency, that they should add no coloration.

Fig.2 Monitor Audio Platinum PL200, cumulative spectral-decay plot calculated from output of accelerometer fastened to center of side panel level with upper woofer (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz).

The saddle between the two low-frequency impedance peaks in fig.1 suggests that the rear-facing port, with its bullet-shaped central insert, is tuned to 43Hz, which is a little higher than I was expecting for such a large, heavy speaker. The red trace in fig.3 shows the output of the port, measured in the nearfield; it does indeed peak between 30 and 60Hz, though a sharp drop-off above 150Hz suggests that something is going on in that region. There is a slight peak at 280Hz, but this is well down in level, as are a couple of higher-frequency modes. The two woofers behave identically, and their summed nearfield output (fig.3, blue trace) features the expected minimum-motion notch at the port tuning frequency. (This is where the pressure on the back of the cone from the port resonance holds the cone stationary.) Higher in frequency, a discontinuity is apparent just below 300Hz, which is both a sign of some kind of acoustic resonance and the same frequency as the glitch in the impedance traces.

Fig.3 Monitor Audio Platinum PL200, acoustic crossover on tweeter axis at 50", corrected for microphone response, with nearfield responses of woofer (blue) and port (red) plotted below 300Hz and 700Hz, respectively.

Above 350Hz, the traces in fig.3 were taken in the farfield on the tweeter axis. The woofers cross over to the midrange at 710Hz with asymmetrical acoustic slopes, the woofers rolling out a little faster than the midrange driver. The green trace shows the response of the midrange-and-tweeter section; it is basically flat, though the ribbon tweeter looks as if it is a couple of dB too high in level, and its output extends beyond the 30kHz cutoff of this graph. Fig.4 shows the overall response of the Platinum PL200, averaged across a 30° horizontal window centered on an axis level with the center of the tweeter. It is both smooth and flat, though again, the region covered by the ribbon tweeter is slightly too high in level. As Bob wrote, "To the extent that the PL200 had a distinctive sound, it was in the direction of a slight brightness." In the bass, about half of the response bump in the upper and midbass is an artifact of the nearfield measurement technique. It does look as if the Monitor's bass alignment is a little on the rich side—I note that Bob was impressed by the PL200's low frequencies, though he also correctly noted that it rolled out above the low-bass region.

Fig.4 Monitor Audio Platinum PL200, anechoic response on tweeter axis at 50", averaged across 30° horizontal window and corrected for microphone response, with complex sum of woofer and port nearfield responses plotted below 300Hz.

The Platinum PL200 offers a wide, even dispersion in the horizontal plane (fig.5), though with a slight drop-off at the top of the midrange unit's passband. The ribbon tweeter's radiation pattern is a little narrower than I was expecting between 7 and 18kHz, which in small or lively rooms will work against the tweeter's being a little hot. However, it offers very wide dispersion above 20kHz, which might bother dogs and small children. In the vertical plane (fig.6), a sharp suckout develops at the upper crossover frequency of 3285Hz for listening axes much above the top of the cabinet. The response doesn't change significantly until more than 10° below that axis, however, which is perfect for a speaker of this height. (Without the integral plinth's spiked feet, the center of the tweeter is 36.5" from the floor.)

Fig.5 Monitor Audio Platinum PL200, lateral response family at 50", normalized to response on tweeter axis, from back to front: differences in response 90–5° off axis, reference response, differences in response 5–90° off axis.

Fig.6 Monitor Audio Platinum PL200, vertical response family at 50", normalized to response on tweeter axis, from back to front: differences in response 15–5° above axis, reference response, differences in response 5–15° below axis.

Turning to the time domain, the PL200's step response (fig.7) indicates that the tweeter and midrange drive-units are connected in inverted polarity, the woofers in positive polarity. Each unit's step smoothly blends into that of the next lower in frequency, which correlates with the good frequency-domain integration of their outputs seen in fig.4. The cumulative spectral-decay plot on the tweeter axis (fig.8) is superbly clean, especially in the tweeter's passband. It's no surprise that RD found that, "when it came to transparency, the PL200 . . . performed like a champ." This an impressively well-engineered, beautiful-looking loudspeaker.—John Atkinson

Fig.7 Monitor Audio Platinum PL200, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).

Fig.8 Monitor Audio Platinum PL200, cumulative spectral-decay plot on tweeter axis at 50" (0.15ms risetime).

COMPANY INFO
Monitor Audio Ltd.
US distributor: Kevro International, Inc.
902 McKay Rd., Suite 4
Pickering, Ontario L1W 3X8, Canada
(905) 428-2800
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