Sidebar 3: Measurements
I used DRA Labs' MLSSA system and a calibrated DPA 4006 microphone with an Earthworks microphone preamplifier to measure the MoFi SourcePoint 10's farfield frequency behavior and dispersion. I used an Earthworks QTC-40 mike for the nearfield and in-room responses and Dayton Audio's DATS V2 system to measure the impedance magnitude and electrical phase angle.
MoFi specifies the SourcePoint 10's anechoic sensitivity as a high 91dB/2.83V/m. My estimate was within experimental error of that figure, at 90.3dB(B)/2.83V/m. The SourcePoint 10's impedance, specified as 8 ohms with a minimum magnitude of 6.2 ohms, is higher than 8 ohms for most of the audioband (fig.1, solid trace), dropping to 6.3 ohms at 143Hz. Though the electrical phase angle (dashed trace) is occasionally high, the effective resistance, or EPDR (footnote 1), is generally benign. It does drop below 4 ohms between 83Hz and 130Hz and between 234Hz and 608Hz, with a minimum value of 3 ohms at 99Hz and 406Hz. The SourcePoint 10 won't be a difficult load for amplifiers, though tubed designs will probably be best used from their 4 ohm taps.
The black trace above 300Hz in fig.3 shows the SourcePoint 10's quasi-anechoic farfield response, averaged across a 30° horizontal window centered on the tweeter axis. The response is respectably even, though there is a gentle rising trend above 3.5kHz. The pair matching between the two samples was excellent, meeting ±0.5dB limits between 400Hz and 20kHz. The trace in fig.3 was taken without the skeletal grille. Repeating the response measurement with the grille increased slightly the amplitudes of the small peaks and dips in the response above 3kHz but didn't change the overall balance in the treble.
Fig.4 shows the SourcePoint 10's horizontal dispersion, normalized to the response on the tweeter axis, which thus appears as a straight line. The radiation pattern doesn't start to narrow until 10kHz, and the on-axis suckout centered on 3.2kHz tends to fill in to the speaker's sides. Commendably, in light of the comments I made at the beginning of this review about the problems with "10" two-ways," there is no discontinuity between the radiation pattern at the top of the woofer's passband and that of the tweeter at the bottom of its passband. Fig.5 shows the speaker's radiation pattern in the vertical plane, again normalized to the tweeter-axis response. With its symmetrical drive-unit configuration, the MoFi speaker's dispersion is very similar to that in the horizontal plane. And again, its output doesn't start to become directional until the top octave.
In the time domain, the SourcePoint 10's step response (fig.7) indicates that the tweeter and woofer are both connected in positive acoustic polarity, with the tweeter's output arriving first at the microphone. The decay of the tweeter's step smoothly blends with the start of the woofer's step, which implies an optimal arrangement of the drive units' physical placement, coupled with the phase behavior of the crossover filters. The SourcePoint 10's cumulative spectral-decay plot (fig.8) is very clean overall, though some delayed energy is present at the bottom of the tweeter's passband.
Footnote 1: 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 2: 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.
Fig.1 MoFi SourcePoint 10, electrical impedance (solid) and phase (dashed) (2 ohms/vertical div.).
The traces in fig.1 are free from the small discontinuities in the midrange that would imply the existence of cabinet resonances. Nevertheless, when I investigated the enclosure's vibrational behavior with a plastic-tape accelerometer, I found a resonant mode at 387Hz on the sidewall (fig.2). This mode has a high Q (Quality Factor), which will work against audibility. (A resonance needs to be stimulated with a signal at the same frequency for the same number of cycles as the Q to be fully excited.) On the other hand, at 387Hz this resonance is close to the frequency of the note G above Middle C (390Hz), which will increase the possibility for the resonance having audible consequences, unless the radiating area is small.
Fig.2 MoFi SourcePoint 10, cumulative spectral-decay plot calculated from output of accelerometer fastened to center of sidewall (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz).
The saddle between 40Hz and 50Hz in the magnitude trace in fig.1 indicates that the tuning frequency of the two ports on the rear panel lies in this region. The red trace in fig.3 shows the response of the ports, measured in the nearfield. The output reaches its maximum at the tuning frequency of 41Hz, and the upper-frequency rollout is clean. The woofer's nearfield output (blue trace) has the expected minimum-motion notch at the port-tuning frequency, and while the complex sum of the woofer and port responses (black trace below 300Hz in fig.3) has a 5dB rise in the midbass response, this will be mostly due to the nearfield measurement technique, which assumes that the drive units are mounted in a true infinite baffle. The SourcePoint 10's low-frequency output is down by –6dB at the port-tuning frequency, very close to the specified extension of 42Hz.
Fig.3 MoFi SourcePoint 10, anechoic response on tweeter axis at 50", averaged across 30° horizontal window and corrected for microphone response, with the nearfield responses of the woofer (blue), and ports (red), and their complex sum (black), respectively plotted below 300Hz, 400Hz, and 300Hz.
Fig.4 MoFi SourcePoint 10, 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.5 MoFi SourcePoint 10, vertical response family at 50", normalized to response on tweeter axis, from back to front: differences in response 45–5° above axis, reference response, differences in response 5–45° below axis.
The red trace in fig.6 shows the MoFi SourcePoint 10s' spatially averaged response in my listening room (footnote 2). For reference, the blue trace in fig.6 shows the spatially averaged response of the Mission 770s ($5000/pair with stands), which I reviewed in November 2022. Both speakers' in-room responses are almost identical from 100Hz to 1kHz, though the MoFis excite the lowest-frequency room mode to a slightly great extent. While the Missions have the expected gentle slope down in response in the treble, which will be primarily due to the increased absorption of the room's furnishings as the frequency increases, the SourcePoint 10s have a flat in-room response up to the top audio octave. This will be due in part to the slightly rising trend in the on-axis response but also to the wide, even high-frequency dispersion in both the horizontal and vertical planes.
Fig.6 MoFi SourcePoint 10, spatially averaged, 1/6-octave response in JA's listening room (red) and of the Mission 770 (blue).
Fig.7 MoFi SourcePoint 10, step response on tweeter axis at 50" (5ms time window, 30kHz bandwidth).
Fig.8 MoFi SourcePoint 10, cumulative spectral-decay plot on tweeter axis at 50" (0.15ms risetime).
The combination of the MoFi SourcePoint 10's slightly rising on-axis response and its wide dispersion results in an in-room behavior that I would have expected to sound bright. Yet, its tonal balance wasn't so much bright as clean and extended in the top octaves.—John Atkinson
Footnote 1: 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.
