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• rev. 359 - V

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.

I'd like to get away from 'alignments'(whatever that means?) and have a simple system (equation) like the sealed box equation.

How about a 'Pi Alighned Open Baffle'

Equalization from electronics will do the same as a speaker system with specific mechanical Q as will a Helmholtz resonator or other acoustic device that provides the same curve. These things are all just ways of describing conditions, and that's really all that is meant by the word "alignment" as used in loudspeaker configurations. It's basically just a label that is used to describe a particular set of circumstances.

So no matter what you do, the system will have a particular set of characteristics, and those can be said to be it's "alignment." It is neither good or bad, just a way to describe the system.