Magnepan Magneplanar MG3.6/R loudspeaker Measurements

Sidebar 3: Measurements

The Magnepan's estimated voltage sensitivity was on the low side, at 83.5dB(B)/2.83V/m. However, in a typical room the speaker's quasi-line-source vertical dispersion should make it sound a little louder than might otherwise be expected. (The in-room loudness of a true line source falls off in a linear manner with distance, rather than as the square of the distance, as is the case with a point source.) But it should be noted that BD did need a good beefy amplifier to drive the Maggies to useful levels.

The speaker's impedance (fig.1) approximates a resistive load of around 4 ohms over much of the audioband. However, there is a slight magnitude peak centered at 1.6kHz, due to the crossover between the ribbon and the midrange diaphragm. The minimum value is 3.3 ohms at 10kHz, which is not going to be problem for any good amplifier to drive, while the increasingly positive electrical phase angle at the top of the audioband is, I assume, due to the residual inductance of the ribbon driver. There is a small wrinkle in the trace between 50Hz and 60Hz, which is probably due to the tuning of the woofer diaphragm.

Fig.1 Magnepan MG3.6/R, electrical impedance (solid) and phase (dashed). (2 ohms/vertical div.)

This can be seen to the left of fig.2, as the big peak in the mid-bass. This is a nearfield measurement, which will exaggerate the behavior of the bass panel [see the letters at the end of this reprint—Ed.]. This does not necessarily mean the speaker will "boom" in an out-of-control manner—BD commented that the MG3.6/R's low frequencies sounded "powerful, clean, and articulate" down to about 35Hz in his room—but such measured bass behavior does appear to be characteristic of panel speakers. Yet the midrange diaphragm does not have a response peak apparent. It neatly covers the 200Hz to 1.2kHz region, with relatively steep rolloffs above and below that bandpass. From this graph, the ribbon tweeter seems both to be set a little low in level, and comes in rather high in frequency. I imagine that the narrow peaks and dips in its response, are due to local interference effects. They should therefore not have any subjective consequences.

Fig.2 Magnepan MG3.6/R, acoustic crossover on tweeter axis 36" from the floor at 50", corrected for microphone response, with the nearfield woofer and midrange responses each plotted below 300Hz.

The Magnepan's overall response, measured on the ribbon axis 36" from the floor (ie, halfway up the ribbon) and averaged across a 30 degrees lateral window is shown in fig.3. The microphone was at a 50" distance, which results in a significant proximity effect with such a physically large speaker. This accounts for much of the downward response tilt evidenced between 200Hz and 2kHz in this graph. The level mismatch between the midrange diaphragm and the ribbon tweeter is still evident, but I wonder how this will manifest itself at a normal listening distance. (Circumstances dictate that I use a 50" microphone distance for my acoustic measurements.) The logistics of the magazine's relocation to New York meant that I could not perform in-room measurements in BD's listening environment, but I suspect that the MG3.6/R's behavior will be better behaved in a room.

Fig.3 Magnepan MG3.6/R, anechoic response on-axis at 50", averaged across 30 degrees horizontal window and corrected for microphone response, with the complex sum of the nearfield woofer and midrange responses plotted below 300Hz.

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