Sidebar 3: Measurements
I used DRA Labs' MLSSA system and a calibrated DPA 4006 microphone to measure the MBL 120's frequency response in the farfield, and an Earthworks QTC-40 mike to measure the nearfield responses. I used Dayton Audio's DATS V2 system to determine the MBL speaker's impedance.
Designer Jürgen Reis wrote in an email that the MBL 120's sensitivity on an axis level with the top of the tweeter magnet—the recommended listening axis—was 79dB/2.83V/m. Though my estimate was slightly higher than this, at 81dB(B)/2.83V/m, this is still significantly lower than average. However, as the MBL 120 is an omnidirectional loudspeaker, its in-room subjective sensitivity will be somewhat higher.
I had understood from Reis that the 120's impedance is 4 ohms. However, the solid trace in fig.1 indicates that the impedance magnitude remains between 5 and 10 ohms for almost the entire audioband. The minimum magnitude is 4 ohms between 66Hz and 74Hz, and the electrical phase angle (dashed trace) becomes increasingly capacitive below 80Hz, due to the presence of a second-order electrical high-pass filter. The "equivalent peak dissipation resistance" (EPDR, footnote 1) is an easy-to-drive 5 ohms or higher from the upper bass to the low treble but drops to 2 ohms between 39Hz and 79Hz, with a minimum value of 1.5 ohms at 58Hz. As is often the case, this loudspeaker will work best with amplifiers that are comfortable driving 4 ohms and below.
Fig.4 shows the MBL's horizontal radiation pattern, with the off-axis responses normalized to the response on the tweeter axis, which thus appears as a straight line in the center of the graph. Some narrow ridges and gullies are visible above 10kHz, but these are likely inconsequential. The off-axis suckout just below 1kHz will be due to the difference in arrival times of the two side-mounted woofers at extreme angles. Other than that, the MBL 120 maintains its output throughout the midrange and mid-treble up to 90° off-axis: This is a true omnidirectional loudspeaker. The MBL's vertical dispersion is shown in fig.5, with the off-axis responses shown up to 15° above and below the tweeter axis. Some off-axis peaks and dips are visible in the high treble, and a suckout develops in the upper crossover region 15° above the recommended axis. For optimal performance, do not listen to this loudspeaker while standing.
In the time domain, the MBL 120's step response on the tweeter axis (fig.6) indicates that all four drive-units are connected in positive acoustic polarity. The tweeter's step arrives first at the microphone; the decay of its step blends smoothly with the positive-going start of the midrange's step, which confirms the good blend of their outputs in the frequency domain. The MBL 120's cumulative spectral-decay plot (fig.7) indicates a clean decay in the region covered by the midrange unit, but the decay of the tweeter's output is disturbed by some low-level ridges of delayed energy.
Footnote 1: EPDR is the resistive load that gives rise to the same peak dissipation in an amplifier's output devices as the loudspeaker. See Eric Benjamin's "Audio Power Amplifiers for Loudspeaker Loads," JAES, Vol.42 No.9, September 1994, and stereophile.com/reference/707heavy/index.html.
Footnote 2: In this respect, the 120's top-octave response is very similar to that of the MBL 101E Mk.II that MF reviewed in April 2012; see fig.2 here.
Fig.1 MBL 120, electrical impedance (solid) and phase (dashed) (2 ohms/vertical div.).
The traces in fig.1 are free from discontinuities that would imply resonances of some kind. I investigated the enclosure's vibrational behavior with a plastic-tape accelerometer and found that all the surfaces were relatively inert. Fig.2, for example, is a cumulative spectral-decay plot calculated from the accelerometer's output when it was fastened to the center of the front baffle. While a couple of modes can be seen close to 1kHz, these are very low in level.
Fig.2 MBL 120, cumulative spectral-decay plot calculated from output of accelerometer fastened to center of front baffle (MLS driving voltage to speaker, 7.55V; measurement bandwidth, 2kHz).
The impedance magnitude trace of a reflex loudspeaker usually includes a "saddle" that indicates the tuning frequency of the port. This is obscured in fig.1 by the behavior of the high-pass filter. The blue trace in fig.3 shows the nearfield output of the woofers on the MBL 120's side panels. That trace has a minimum-motion notch, which is when the port resonance holds the cone stationary, centered at 37Hz. The port's output (red trace) peaks higher in frequency than I was expecting, between 50Hz and 100Hz, though the upper-frequency rolloff is very clean. The presence of a high-pass filter in the woofer circuit means that the ultimate rolloff of both the woofers and port is close to 24dB/octave rather than the usual 12dB/octave.
Fig.3 MBL 120, anechoic response on listening axis without grille at 50", averaged across 30° horizontal window and corrected for microphone response, with the nearfield responses of the woofers (blue) and port (red), respectively plotted below 300Hz and 550Hz.
The black trace below 300Hz in fig.3 shows the complex sum of the nearfield woofer and port responses (the latter taking into account the fact that the port is on the back panel). The boost in the response in the upper bass, as with the woofers' output (blue trace), will be due in part to the nearfield measurement technique, which assumes that the radiators are mounted in a baffle that extends to infinity in both vertical and horizontal planes. (For reasons of consistency over the three decades I have been measuring loudspeakers, I don't apply a correction based on the loudspeaker's geometry to my nearfield responses.) Even so, the MBL 120's low frequencies are somewhat exaggerated in level.
I measured the MBL 120's farfield behavior on the recommended axis, level with the top of the tweeter magnet. The response, averaged across a 30° horizontal window centered on that axis, is shown as the black trace above 300Hz in fig.3. Other than a small rise in the lower midrange, the response is even before rolling off above 14kHz. The output is down by 6dB at 15kHz, close to the 15.5kHz that Reis mentioned in his email (footnote 2).
Fig.4 MBL 120, lateral response family at 50", normalized to response on listening axis, from back to front: differences in response 90–5° off axis, reference response, differences in response 5–90° off axis.
Fig.5 MBL 120, vertical response family at 50", normalized to response on listening axis, from back to front: responses 15–5° above axis, reference response, responses 5–15° below axis.
Fig.6 MBL 120, step response on listening axis at 50" (5ms time window, 30kHz bandwidth).
Fig.7 MBL 120, cumulative spectral-decay plot on listening axis axis at 50" (0.15ms risetime).
The MBL 120's measured performance confirms that this is a true omnidirectional design. Its low-frequency behavior, where an overdamped reflex alignment is combined with an under-damped second-order high-pass filter with a corner frequency slightly lower than that of the port tuning, suggests that the MBLs will sound rich.—John Atkinson
Footnote 1: EPDR is the resistive load that gives rise to the same peak dissipation in an amplifier's output devices as the loudspeaker. See Eric Benjamin's "Audio Power Amplifiers for Loudspeaker Loads," JAES, Vol.42 No.9, September 1994, and stereophile.com/reference/707heavy/index.html.
