Sidebar 3: Measurements
I used DRA Labs' MLSSA system and a calibrated DPA 4006 microphone with an Earthworks microphone preamplifier to measure the Monitor Audio Platinum 300 3G's farfield frequency behavior and dispersion. For the nearfield responses, I used an Earthworks QTC-40 mike, which has a ¼" capsule and thus doesn't present a significant obstacle to the sound. I also used this microphone for the in-room behavior. The impedance magnitude and electrical phase angle were measured with Dayton Audio's DATS V2 system.
Footnote 1: With both ports blocked with the foam bungs, the impedance magnitude was typical of a sealed-box alignment, with a single low-frequency peak at 49Hz. Footnote 2: See EPDR is the resistive load that gives rise to the same peak dissipation in an amplifier's output devices as the loudspeaker. See "Audio Power Amplifiers for Loudspeaker Loads," JAES, Vol.42 No.9, September 1994, and stereophile.com/reference/707heavy/index.html. Footnote 3: Using the FuzzMeasure 3.0 program, a Metric Halo MIO2882 FireWire-connected audio interface, and a 96kHz sample rate, I average 20 1/6-octave–smoothed spectra, individually taken for the left and right speakers, in a rectangular grid 36" wide by 18" high and centered on the positions of my ears.
Footnote 4: See the blue trace in fig.7 here.
Fig.1 Monitor Audio Platinum 300 3G, ports open, electrical impedance (solid) and phase (dashed) (2 ohms/vertical div.).
Monitor Audio specifies the Platinum 300 3G's free-field sensitivity as 88dB/2.83V/m, which was confirmed by my measurement. The Platinum 300 3G's impedance is specified as 4 ohms, with a minimum magnitude of 4.0 ohms at 3.75kHz. With the ports open (footnote 1), the impedance remains between 4 ohms and 7 ohms for most of the audioband (fig.1, solid trace), with minimum values of 4.2 ohms at 124Hz and 3.75kHz. The electrical phase angle (dashed trace) is occasionally high; the effective resistance, or EPDR (footnote 2), therefore drops below 4 ohms between 27Hz and 330Hz and below 3 ohms between 66Hz and 128Hz and between 2.4kHz and 5.9kHz. The minimum EPDR values are 2 ohms at 85Hz and 1.85 ohms at 4.2kHz. The Platinum 300 3G is a relatively demanding load for amplifiers; tubed designs will best be used from their 4 ohm output taps.
Fig.2 Monitor Audio Platinum 300 3G, cumulative spectral-decay plot calculated from output of accelerometer fastened to center of sidewall level with lower woofer (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz).
The traces in fig.1 are free from the small discontinuities in the midrange that would imply the existence of cabinet resonances. When I investigated the enclosure's vibrational behavior with a plastic-tape accelerometer, I found some resonant modes on the sidewalls level with the woofers (fig.2). As well as being low in level, these modes have a high Q (Quality Factor), which will work against this behavior having audible consequences.
Fig.3 Monitor Audio Platinum 300 3G, acoustic crossover on tweeter axis at 50", corrected for microphone response, with the nearfield responses of the midrange unit (green), woofers (blue), and ports (red), respectively plotted below 400Hz, 300Hz, and 600Hz.
The saddle centered on 33Hz in the magnitude trace in fig.1 indicates that this is the tuning frequency of the two ports on the Monitor Audio's rear panel. The ports behaved identically, and the red trace in fig.3 shows their response measured in the nearfield. The output reaches its maximum just below the tuning frequency, and while some low-level peaks are present between 250Hz and 600Hz, the upper-frequency rollout is otherwise clean. The two woofers also behaved identically, and their nearfield output (blue trace) has the expected minimum-motion notch at the tuning frequency of the ports. The woofers cross over to the midrange unit (green trace) close to the specified 650Hz, with a clean upper-frequency rollout. The farfield response of the midrange unit and tweeter is flat in the low- and mid-treble regions but rises in the top octave, with a 6dB peak at 16kHz. I repeated this measurement with the Earthworks microphone and MLSSA set to its highest, 40kHz bandwidth and found another narrow peak an octave higher at 32kHz.
Fig.4 Monitor Audio Platinum 300 3G, anechoic response on tweeter axis at 50", averaged across 30° horizontal window and corrected for microphone response, with the complex sum of the nearfield responses plotted below 300Hz.
The complex sum of the woofer and port responses in fig.4 (black trace below 300Hz) has a 6dB rise in the midbass output. This is due to the nearfield measurement technique, which assumes that the drive units are mounted in a true infinite baffle. The Platinum 300 3G's low-frequency alignment is close to maximally flat; the output is down by 6dB at the specified free-field extension of 25Hz. The black trace above 300Hz in fig.4 shows the Platinum 300 3G's quasi-anechoic farfield response averaged across a 30° horizontal window centered on the tweeter axis. The response is even overall, with a very slight lack of energy between 1.5kHz and 2.5kHz and that rising trend above 7kHz. The pair matching between the two samples was excellent, any difference between them lying well within ±0.5dB.
Fig.5 Monitor Audio Platinum 300 3G, lateral response family at 50", normalized to response on tweeter axis, from back to front: differences in response 60–5° off axis, reference response, differences in response 5–60° off axis.
Fig.6 Monitor Audio Platinum 300 3G, 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.
Fig.5 shows the Platinum 300 3G's horizontal dispersion, normalized to the response on the tweeter axis, which thus appears as a straight line. The radiation pattern is even and doesn't start to narrow until 10kHz. The slight lack of low-treble energy in the on-axis response fills in to the speaker's sides. Fig.6 shows the speaker's radiation pattern in the vertical plane, again normalized to the tweeter-axis response, which is 40" from the floor. My ears are 4" lower than that, but the Monitor Audio's response 5° below the tweeter axis is identical.
Fig.7 Monitor Audio Platinum 300 3G, spatially averaged, 1/6-octave response in JA's listening room with the ports open (red) and with the ports blocked (blue).
The red trace in fig.7 shows the Monitor Audio Platinum 300 3Gs' spatially averaged response in my listening room with their ports open (footnote 3). The blue trace in fig.7 shows the spatially averaged response with all the ports blocked with the supplied foam plugs. With the ports open, the Platinum 300 3Gs maximally excite the lowest-frequency mode in my room, the frequency of which is close to the port-tuning frequency. With the ports closed, the in-room response slopes gently down below 100Hz. The peaks and dips in the lower midrange will be due to interference from reflections of the woofers' output from the floor; above that region, there is a reduction in the speakers' output in the treble, due to the increased absorption of the room's furnishings as the frequency increases, though this is not to as great a degree as with my KEF LS50s (footnote 4).
Fig.8 Monitor Audio Platinum 300 3G, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).
Fig.9 Monitor Audio Platinum 300 3G, cumulative spectral-decay plot on tweeter axis at 50" (0.15ms risetime).
In the time domain, the Platinum 300 3G's step response (fig.8) indicates that the tweeter and woofers are all connected in positive acoustic polarity, the midrange unit in inverted polarity. The tweeter's output arrives first at the microphone, and the decay of the tweeter's step blends with the start of the midrange unit's step, which implies an optimal crossover implementation. The Platinum 300 3G's cumulative spectral-decay plot (fig.9) is very clean overall. (Ignore the apparent low-level ridge of delayed energy just below 16kHz, which is due to interference from the MLSSA host PC's video circuitry.)
The Monitor Audio Platinum 300 3G's measured performance is similar to that of the earlier Platinum PL300 II, which Robert Deutsch reviewed in November 2016. However, the new speaker's tweeter is connected in positive acoustic polarity, which eliminates a suckout in the upper crossover region above the tweeter axis. The horizontal dispersion is better controlled with the new speaker, and, as mentioned in Monitor Audio's promotional material, the on-axis output has a touch more energy in the top two audio octaves.—John Atkinson
Footnote 1: With both ports blocked with the foam bungs, the impedance magnitude was typical of a sealed-box alignment, with a single low-frequency peak at 49Hz. Footnote 2: See EPDR is the resistive load that gives rise to the same peak dissipation in an amplifier's output devices as the loudspeaker. See "Audio Power Amplifiers for Loudspeaker Loads," JAES, Vol.42 No.9, September 1994, and stereophile.com/reference/707heavy/index.html. Footnote 3: Using the FuzzMeasure 3.0 program, a Metric Halo MIO2882 FireWire-connected audio interface, and a 96kHz sample rate, I average 20 1/6-octave–smoothed spectra, individually taken for the left and right speakers, in a rectangular grid 36" wide by 18" high and centered on the positions of my ears.
