Sidebar 4: Measurements
I used DRA Labs' MLSSA system and a calibrated DPA 4006 microphone to measure the Monitor Audio Gold 300's frequency response in the farfield and an Earthworks QTC-40 mike for the nearfield responses. I left off the loudspeaker's grille for the measurements.
Although Monitor Audio specifies the Gold 300's sensitivity as 90dB/W/m, my estimate was almost 3dB lower, at 87.3dB(B)/2.83V/m, perhaps due to the fact that the Gold 300's impedance is specified as 4 ohms. (An input of 2.83V is equivalent to 2W into 4 ohms.) The impedance magnitude (fig.1, solid trace) does stay close to 4 ohms from the upper bass through to the mid-treble, with minimum values of 3.7 ohms at 117Hz and 3.57 ohms at 1kHz. There is a current-hungry combination of 5 ohms and –39° electrical phase angle (dotted trace) at 77Hz, and the phase angle exceeds +40° above 10kHz, presumably due to the inductance of the MPD tweeter's drive system. The Monitor Audio needs to be partnered with an amplifier that is comfortable driving low impedances.
The Monitor Audio Gold 300's horizontal dispersion, with each trace normalized to the tweeter-axis response, is shown in fig.6. The contour lines in this graph are evenly spaced in the midrange and low treble, which implies stable stereo imaging, and a slight excess to the sides balances the lack of presence-region energy in the on-axis response. The speaker becomes increasingly directional in the top two octaves, which might make the highs sound a little too mellow in large rooms. In the vertical plane (fig.7), the Monitor Audio's on-axis balance is maintained over a relatively wide listening window, but 15° above the tweeter axis a suckout starts to develop in the lower crossover region.
Turning to the time domain, the Gold 300's step response on the tweeter axis (fig.8) indicates that the tweeter and midrange unit are connected in negative acoustic polarity, the woofers in positive polarity. (I confirmed this by looking at the individual nearfield outputs.) More important, the decay of the tweeter's step blends smoothly with the start of the midrange unit's step and the decay of that driver's step blends smoothly with the start of the woofers' step. The outputs of the Gold 300's drive-units may not be coincident in time, but they are time-coherent, which suggests optimal crossover implementation. The Monitor Audio's cumulative spectral-decay plot (fig.9) is clean overall.
Fig.1 Monitor Audio Gold 300, electrical impedance (solid) and phase (dashed) (2 ohms/vertical div.).
The impedance traces are free from small discontinuities that would imply the presence of panel resonances, and the enclosure did seem inert to the knuckle-rap test. When I investigated the cabinet's vibrational behavior with a plastic-tape accelerometer, only the areas of the sidewalls level with the upper woofer had resonant modes in the midrange (fig.2). On the back panel, a strong mode was present just below 800Hz (fig.3), but the area affected, between the top of the cabinet and upper port, was small.
Fig.2 Monitor Audio Gold 300, cumulative spectral-decay plot calculated from output of accelerometer fastened to center of back panel (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz).
Fig.3 Monitor Audio Gold 300, cumulative spectral-decay plot calculated from output of accelerometer fastened to center of sidewall level with upper woofer (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz).
There is a saddle in the impedance magnitude trace between 30Hz and 40Hz, which suggests that the tuning frequency of the two ports lies in this region. (Both ports behave identically, as do the two woofers.) The woofers' nearfield response (fig.4, blue trace) has its minimum-motion notch at 37Hz; this is the frequency at which the back pressure from the port resonance holds the cones stationary. The output of the ports (red trace) peaks broadly between 25Hz and 65Hz, implying excellent low-frequency extension, and while some midrange spuriae are present, they are low in level.
Fig.4 Monitor Audio Gold 300, acoustic crossover on tweeter axis at 50", corrected for microphone response, with nearfield woofer (blue) and port (red) responses respectively plotted below 355Hz and 650Hz.
The woofers cross over to the small-diameter midrange unit (fig.4, green trace) close to the specified 650Hz, with what appears to be a third-order filter slope, and their upper-frequency rolloff is free from peaks. The output of the midrange unit and tweeter on the tweeter axis is impressively flat, though there is a slight depression in the presence region and a second-order rolloff above 18kHz. Fig.5 shows the Monitor Audio's farfield response, averaged across a 30° horizontal window centered on the tweeter axis. The trace below 300Hz shows the sum of the nearfield woofer and port outputs, taking into account acoustic phase and the different distance of each radiator from a nominal farfield microphone position. The rise in response in the upper bass is due almost entirely to the nearfield measurement technique, the Gold 300's reflex tuning being maximally flat. The speaker might sound bass heavy in small rooms.
Fig.5 Monitor Audio Gold 300, anechoic response on tweeter axis at 50", averaged across 30° horizontal window and corrected for microphone response, with the complex sum of the nearfield woofer and port responses plotted below 300Hz.
Fig.6 Monitor Audio Gold 300, 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.7 Monitor Audio Gold 300, 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.8 Monitor Audio Gold 300, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).
Fig.9 Monitor Audio Gold 300, cumulative spectral-decay plot on tweeter axis at 50" (0.15ms risetime).
Monitor Audio's Gold 300 offers excellent measured performance.—John Atkinson
