Sidebar 5: JA's Measurements
I measured a Monitor Audio Gold 300 6G loudspeaker with the serial number 900005. I used DRA Labs' MLSSA system, a calibrated DPA 4006 microphone, and an Earthworks microphone preamplifier to measure the speaker's quasi-anechoic frequency- and time-domain behavior in the farfield (footnote 1). I used an Earthworks QTC-40 microphone, which has a small ¼" diameter capsule, for the nearfield responses.
Footnote 1: I measured the Gold 300 6G with the tensioning bolts fully tightened. Footnote 2: 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: A nearfield measurement assumes that the baffle extends to infinity in both horizontal and vertical planes, which means that the loudspeaker is firing into hemispherical space rather than a full sphere. See this discussion.
Fig.1 Monitor Audio Gold 300 6G, ports open, electrical impedance (solid) and phase (dashed) (2 ohms/vertical div.).
The Monitor Audio loudspeaker's voltage sensitivity is specified as 87dB/2.83V/m; my B-weighted estimate was 1dB higher. Monitor Audio specifies the Gold 300 6G's nominal impedance as 4 ohms, with a minimum value of 3.8 ohms at 365Hz. Measured with Dayton Audio's DATS V2 system, the impedance magnitude (fig.1, solid trace) was higher than 4 ohms throughout the audioband, with a minimum value of 4.1 ohms between 157Hz and 175Hz. The electrical phase angle (fig.1, dotted trace) is high in the midrange and bass frequencies, which means that the effective resistance, or EPDR (footnote 2), drops below 4 ohms from 75Hz to 440Hz, between 860Hz and 1.1kHz, and from 2.9kHz to 5.6kHz. The minimum EPDR values are 2.13 ohms at 106Hz, 2.9 ohms from 279Hz to 315Hz, and 3 ohms at 4kHz. Consequently, the Gold 300 6G needs to be partnered with an amplifier that doesn't have a problem delivering current into low impedances.
Fig.2 Monitor Audio Gold 300 6G, ports closed, electrical impedance (solid) and phase (dashed) (2 ohms/vertical div.).
Fig.1 was taken with the loudspeaker's twin ports on the rear panel open. Repeating the impedance measurement with the ports blocked, there was now a single magnitude peak in the bass centered on 59Hz (fig.2). This is the sealed-box tuning frequency of the woofers.
Fig.3 Monitor Audio Gold 300 6G, cumulative spectral-decay plot calculated from output of accelerometer fastened to the center of a sidewall level with the upper port (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz).
When I rapped the Monitor Audio enclosure's sidewalls with my knuckles, I heard a quiet "plunk." Using a plastic-tape accelerometer, I found a resonant mode at 438Hz on the sidewalls (fig.2), but this is relatively low in level and has a high Q (Quality Factor); both characteristics will reduce the possibility of this having audible consequences.
Fig.4 Monitor Audio Gold 300 6G, acoustic crossover on tweeter axis at 50", corrected for microphone response, with the nearfield responses of the midrange unit (blue), woofers (green), and ports (red), respectively plotted below 500Hz, 350Hz, and 450Hz.
The saddle centered on 36Hz in the Gold 300 6G's impedance magnitude trace suggests that this is the ports' tuning frequency, which is not very different from the specified 35Hz. The sum of the ports' nearfield responses (fig.3, red trace) peaks just below the tuning frequency and rolls off cleanly at lower and higher frequencies. The two woofers behaved identically; their summed nearfield response (green trace below 350Hz in fig.3) had the expected reflex tuning notch at 36Hz. In the farfield, the woofers cross over to the midrange unit (fig.3, blue trace) at the specified 800Hz and roll off smoothly above that frequency. The tweeter's output is even in the low treble but gently rises in the top two audio octaves.
Fig.5 Monitor Audio Gold 300 6G, 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 (black) and of the woofers with the ports blocked (red) plotted below 300Hz.
This can also be seen in the Gold 300 6G's farfield response, averaged across a 30° horizontal window centered on the tweeter axis and taken without the grilles (fig.4). (The response on the midrange axis was identical to that on the tweeter axis.) The complex sum of the woofers' and ports' nearfield responses is shown as the black trace below 300Hz in this graph, while the response of the woofers with the ports blocked is shown as the red trace. The boosted upper bass in both traces is due to the nearfield measurement technique (footnote 3). As expected, the sealed-box output rolls off earlier at low frequencies than the reflex output.
Fig.6 Monitor Audio Gold 300 6G, 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 6G, 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.
The speaker's horizontal radiation pattern, normalized to the response on the tweeter axis, which therefore appears as a straight line, is shown in fig.5. The dispersion is generally well-controlled through the midrange and presence region, though the radiation pattern narrows in the mid-treble and above. This will tend to compensate for the gentle top-octave rise in response seen in figs.3 and 4. Not toeing in the speakers all the way to the listening position should optimize the Monitor Audio's treble balance. Fig.6 shows the Gold 300 6G's dispersion in the vertical plane, again normalized to the response on the tweeter axis. The speaker's output in the audioband doesn't change significantly above and below that axis, which is useful considering that at 40" from the floor with the speaker sitting on its outrigger stands, the tweeter is a few inches higher than the ear height of a listener in a typical chair.
Fig.8 Monitor Audio Gold 300 6G, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).
Fig.9 Monitor Audio Gold 300 6G, cumulative spectral-decay plot on tweeter axis at 50" (0.15ms risetime).
In the time domain, the Monitor Audio's step response on the tweeter axis (fig.7) indicates that the tweeter and woofers are connected in positive acoustic polarity, the midrange unit in negative polarity. The tweeter's output arrives first at the microphone followed by that of the midrange unit, with the decay and overshoot of its step smoothly blending with the start of the midrange step. In turn, the decay of that step then smoothly blends with the start of the woofers' step. This all implies an optimal crossover topology. Other than some low-level delayed energy at the top of the midrange unit's passband, the Gold 300 6G's cumulative spectral-decay, or waterfall, plot (fig.8) is very clean. (As always with these plots, ignore the ridge of delayed energy close to 16kHz, which is due to interference from the measurement computer's video circuitry.)
In many respects, the Gold 300 6G's measured performance is similar to that of Monitor Audio's earlier Silver 300, which Kalman Rubinson reviewed in March 2018. But the new speaker's enclosure is significantly better-damped, and while the earlier speaker's high-frequency response was flatter, the resonant frequency of the Gold 300 tweeter's diaphragm lies above 30kHz.—John Atkinson
Footnote 1: I measured the Gold 300 6G with the tensioning bolts fully tightened. Footnote 2: 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: A nearfield measurement assumes that the baffle extends to infinity in both horizontal and vertical planes, which means that the loudspeaker is firing into hemispherical space rather than a full sphere. See this discussion.
