Magnepan Magneplanar MG3.6/R loudspeaker More Comments
Siegfied Linkwitz comments
Editor: The review of the Magneplanar MG3.6/R in the August Stereophile caught my attention. I am a proponent of open-baffle speakers because of their room acoustic advantages and the absence of sound coloring boxes. So I looked with great interest at figs.2 & 3 on page 89 showing individual driver frequency responses and their summation.
The nearfield measurements of woofer and midrange in fig.2, presumably taken only an inch or so from the driver surface, are a valid set of data. You also could have measured the tweeter at such close range and obtained useful information. Where things fall apart is in fig.3 when you form the complex sum of nearfield measurements and the 50" tweeter "farfield" measurement. This curve does not represent the frequency response a listener might experience at any distance and is therefore extremely misleading.
The nearfield frequency response of an acoustic source is only proportional to its farfield response if the source is small, ie, omnidirectional, and if it is in free-space. Summing a driver diameter corrected woofer nearfield response to a farfield midrange response works for a small monitor on a stand, but already has errors when the speaker is larger and the woofer is close to the floor—when the conditions move away from free-space or anechoic.
The Magneplanar is clearly not a point source and, being open-baffle, it has an acoustic short circuit between front and back. This causes a 6 dB/octave low-frequency roll-off in the farfield response. So from all open baffle nearfield measurements you have to subtract first a 6dB/octave (= 20dB/decade) slope before you can sum the data with other farfield measurements. When you apply this correction to the MG3.6 woofer response you see that it flattens from 400Hz to 60Hz and shows a peak at 47Hz. Similarly the midrange has to be corrected before you can use it for the composite response. The actual room response is still different from this composite, though, primarily due to the effect of the floor on woofer radiation.
You might consider to add a measurement taken with a 50ms time window at your listening position, spatially averaged and half-octave smoothed to include the room. I think as a measurement that allows true comparison between speakers, this would be more useful than the composite data that are correct only in a few special cases.
I hope this letter helps your readers to understand the difficulties in describing a loudspeaker by measurements.—Siegfried Linkwitz, Corte Madera, CA, www.linkwitzlab.com.
Mike Gough comments
Editor: I read with interest the measurement section of the Magnepan Magneplanar MG3.6/R review in the August 2000 Stereophile, specifically John Atkinson's comments on the midbass peak of the nearfield response. The following may be of interest.
Some years ago, while designing the SCM8 dipole surround speaker (the triangular one) for B&W's original THX Home Theatre System, I was discussing with Quad's Peter Walker the problems of coping with the bass roll-off imposed by front-to-back cancellation of dipole designs. I was having a problem meeting the (then) THX bass extension with such a small enclosure, but did not want to revert to monopole in the bass (as so many do).
Peter told me of a technique he used on the Quad electrostatics, which I was ashamed I hadn't also thought of, which was to engineer an underdamped bass alignment. That gave a basically rising response with decreasing frequency down to the nominal cut-off frequency, which compensates the roll-off due to dipole cancellation. This underdamped characteristic, of course, shows up in a nearfield measurement, but not in the far field. It is not apparent in the midrange panel because it is not needed. The dipole cancellation starts at a frequency defined by the smallest dimension of the panel and this is the same for all sections in a common panel size. The midrange panel operates above this frequency.
So such a nearfield peak is often a deliberate part of the design of dipoles (of which panel speakers are an example). Mind you, both Peter and I went for much more modest peaks. The dipole imposes an extra roll-off rate of 6dB/octave. You can add a second-order Q=1 to a first-order at the same frequency to get close to a third-order Butterworth or, for a more extended "flat" response; a second-order Q=2 added to a first-order at twice the frequency gives something akin to a Tshebychev with a 1dB ripple. The Magnepan peak does seem a little excessive, but it all depends how it interacts with the modes of the listening room.
This technique does open the debate as to what the ear actually hears. A Q of 2 has a pretty abysmal transient response and the question is whether the dipole "equalisation" ameliorates that effect in the total response. As both mechanisms are minimum-phase, I suspect and believe that that indeed happens. As it is ultimately third-order, though, the response will have an inferior low-level transient behaviour to a well-adjusted second-order. It should have some similarity to the series C (capacitor) closed-box alignments we used while I was at KEF with Laurie Fincham. There the -3dB point was lowered by putting a capacitor in series with an acoustic alignment with Q of 1. In those days we wanted to protect speakers from turntable rumble.—Mike Gough, Senior Product Manager, B&W Loudspeakers Ltd.