The Crossover and Diaphragm Resonance

Just like a drum head, a taught ESL diaphragm has a loud natural resonance, which typically falls between 50-110Hz, depending on the tension and span between supports. If you tap the panel with your finger, the sound you hear is the diaphragm vibrating at its resonance frequency.

We want to avoid playing the panel at or near the resonance frequency, and one big advantage of a hybrid design is that we don't have to.

My panels' resonance is around 80Hz, and in order to avoid exciting the resonance, the rule-of-thumb is to set the crossover frequency at least two octaves above the resonance if using a 24 db filter slope, or at least one octave above the resonance if using a 48db filter slope.

I currently use a 200Hz/48db crossover. The steep filter slope allows setting the crossover low enough to keep the woofer's energy out of the midrange (where a woofer performs poorly but an ESL excels), and still high enough to avoid playing the panel near its resonance.