Compensating the Diplole Phase- Cancellation

An ESL panel radiates sound waves from both the front and the back, which are exactly opposite in phase. And whenever opposite-phase waves meet, they cancel each other.  Thus; wavelengths long enough to reach around the panel meet at the panel edges, and the portions that overlap beyond the panel, cancel out. This effect gives dipole speakers a characteristic figure-8 radiation pattern.       

The 3db down point [frequency] occurs where the wavelength is about four times the minimum dimension of the speaker (width of panel + frame), and it increases progressively for lower frequencies.  The flat panel also beams the shorter wavelength frequencies, making those sound louder, and the combined effect is a 6db/octave roll off  in the far field, for frequencies below the cancellation threshold.  

Fortunately, these effects can be compensated using either a 6db/octave shelving circuit to pull down the frequencies above the threshold, or by boosting the frequencies below the threshold.  I use a Behringer DEQ2496 EQ to compensate the roll off and beaming effects, but any decent-quality equalizer would suffice.  If using a DSP crossover with parametric EQ, its EQ could be used in lieu of a separate equalizer.  In any case, equalization will be required to fill in the mid-bass region-- from where the panel begins rolling off to where the woofer takes over.    

With equalization, the off-axis cancellation still occurs, of course, but the now-boosted lower frequencies are projected forward enough, within the on-axis lobe, to reach and envelop the listener with balanced sound.