Slam what Am

Slam is an enjoyable trick that ported speakers do better than sealed (acoustic suspension) speakers. Not everyone cares about slam — you don’t really want it for acoustic music like classical, jazz, or folk — but for pop music, it’s part of the fun. Tuned ports deepen the bass, but cause it to be blurred and unnatural, among other problems, problems that are bad for the sound of non-pop music, but they do promote slam. Once I finally noticed the correlation, I had an idea of what was creating it. 
Ports don’t put a stiff air spring behind the woofer to control it below the woofer’s resonance. When the woofer gets an impulse in a ported speaker, it has nothing to stop it from hitting its mechanical limits. This makes a blast you can feel when a kick drum is kicked, for instance, even at less than ear splitting levels.  
You can simulate this by putting a slightly resonant bump into the frequency response. It has to go in a bit higher in the bass than you’d expect, doesn’t have to be huge, and doesn’t cause an obvious problem with boominess. I messed around with EQ to do this until I was happy with the results, and it’s now one of the optional profiles we can program into the active speakers. We shipped one pair so far with it recently. 
What’s all this arcing stuff, anyhow?
Arcing is thought to be a general risk for an electrostatic, and it is for some, even under what you might think are ordinary conditions. Those speakers need electronic limiting, which is an improvement over arcing, but has some drawbacks: transients are clipped off or compressed when playing at high volume, which sounds “pinched” and might cause an audible crunch as the protection circuit kicks in, and some protection circuits can overload and damage an amplifier. 
Some electrostatics, like ours, use insulated electrodes. Depending on the type and thickness of the insulation, this will prevent arcing to various degrees. A JansZen uses insulated electrodes that prevent arcing at a minimum of twice the voltage that causes ionization (four times the amplifier power).
One key to making this work well is to do it without reducing the appearance of field voltage in the air gap under normal conditions, which would otherwise reduce sensitivity. With the amount of physical protection that we use, ionization of the air can occur safely, and merely makes the electrodes glow blue/lavender. This looks cool, but: you can’t see it through the grill; it’s not good to do it for long because the transducers get hot enough to melt; and the ozone it generates is toxic, so please don’t try it at home.
Anyway, ionization makes the electrostatic get quieter, which throws the tonal balance off kilter and lets the owner know to turn it down. It’s nice to have built in notification like this, because electrostatic speakers don’t sound loud when they get loud, and that’s because they don’t produce the tell-tale distortion that other speakers do when they get loud.

FWIW, the reason they get quieter when overloaded is because ionized air is electrically conductive and shorts out the bias charge on the membrane, and also reduces the impedance of the transducer. After turning down the volume, the sound will immediately go back to normal.

Here’s the beauty part: when an overvoltage condition is brief enough, there isn’t time for the air to ionize. Because of that, with transients from hammer strikes, string plucks, etc., you get the full sound of that transient, even though the speaker is theoretically being vastly overpowered at those instants. As a result, we recommend amplifier power that is well above what’s needed to produce the maximum sustained sound level; this gives you the distortion-free headroom needed for reproducing transients during already loud passages, i.e., for maximum realism and overall fidelity. 
As ever, thanks for your attention. 
David A. Janszen, President | JansZen Loudspeaker Co., Ltd.