Magnepan Magneplanar MG3.6/R loudspeaker Measurements part 2
Fig.4 Magnepan MG3.6/R, lateral response family at 50", from back to front: response 90 degrees-5 degrees off-axis on woofer side, reference response, response 5 degrees-90 degrees off-axis on tweeter side.
Fig.5 Magnepan MG3.6/R, vertical response family at 50", normalized to response on middle of tweeter axis, from back to front: differences in response 10 degrees-5 degrees above axis, reference response, differences in response 5 degrees-15 degrees below axis.
In the time domain, the Magnepans' step response (fig.6) indicates that the ribbon tweeter and woofer diaphragm are connected in positive acoustic polarity, the midrange diaphragm in negative polarity. Because the drive-units are mounted side-by-side, this will swing the main lobes to the woofer side of the panel, hence Magnepan's placement instruction. The cumulative spectral-decay plot (fig.7) shows an initially clean decay in the treble, but then some hashy behavior. This, I believe, is not due to the presence of resonances but to early reflections from the physically large radiating areas.
Fig.6 Magnepan MG3.6/R, on-axis step response at 50" (5ms time window, 30kHz bandwidth).
Fig.7 Magnepan MG3.6/R, cumulative spectral-decay plot at 50" (0.15ms risetime).
As I have written before in these pages, measuring physically large speakers with in-room quasi-anechoic techniques is in some ways a fruitless task. The usual assumption, that the measuring microphone is very much farther away than the largest dimension of the speaker being measured, is clearly wrong. Yet without access to a large anechoic chamber costing many hundreds of thousands of dollars, in-room measurement techniques are all we have to rely on.—John Atkinson