ESL vs subwoofer

These are just some random points that came to my mind:

The driver in a subwoofer doesn't need to be speedy. Typically there's a low pass filter with a corner frequency of about 80 Hz.

The assumption in the beginning is another problem ("SPL is directly and only related to volume of air displaced"). Especially if we talk about ported subwoofers or passive radiators.

The driver in a subwoofer just needs to play low frequencies, so the full excursion can be used for those low frequencies. So a direct comparison of area * excursion doesn't work.

You can get high (~100) dB SPL from even a 10" sub.

Electrostatics are almost always dipole so the amount of air moved is only 1/2 the issue. Since the front and back are out of phase, you get cancellation of the long waves so in practice, electrostatics need much more excursion in the bass than your hypothesis to sound linear and extremely low bass is almost impossible without either an extremely large panel or baffle. Proximity to a rear boundary only exasperates the issue by causing more and earlier cancellation.

Xnor is correct about the speed of bass notes. Fast enough will do and boxing the rear wave or tuning it in a dynamic doesn't have the same issues. Playing all frequencies does not limit the bass capability of a panel in any way. Higher frequencies would be the equivalent of ripples on a long wave with the total energy or excursion not fundamentally different.

An electrostatic will show lower distortion to it's low frequency limit (depending on coupling Xformer etc.) but has a very difficult time making low bass. A Quad 2805 is pretty linear to about 36 hz and rolls at 18/oct below due to cancellation. The significant increase in size of a 2905 (4 bass panels instead of 2 though all 6 make bass) only gets a few more hz in extension.

There have been huge panels that do go low but you end up with a wall of speaker in front of you. I guess someone could make an electrostatic subwoofer in a box but how big and how complex when we already have something for that. I used to hate subwoofers 30 years back but there's been some good ones made since then.

The problem with a single driver playing all frequencies (full range) is high intermodulation distortion.

Also, besides those ESLs using tapped delay lines (like the ESL-63) they behave as a line source instead of a point source like cone speakers.

Here's what Linkwitz writes about the ESL-63:

Nowadays we have ESLs with curved panels and other techniques to improves maximum bass levels and extension. So no, big numbers (like area * excursion) don't automatically give you big bass.

Yes on it being volume limited. Dynamic range is a poor way to describe that because if they play loud enough for you (for most), they have fab dynamics. I'm speaking newer versions here. They are topologically the same but everything has been improved, especially the transformers and protection circuits. They are effectively like a point source emanating from a foot behind the speaker and nothing like a line source. It's concentric rings and nothing in line. The 6" disc represents the propagation from that point source traveling thorough that point in time and space. It also means it has uniform dispersion relative to plain as opposed to the very different dispersion that line sources and to a lesser extent even dynamics have vertically vs horizontally. The polar response of dynamics goes to pot vertically so sitting is also required for optimum character if they got the driver timing correct to begin with.

Best character from the ~6" full range center disc is limited to sitting between them for best response ( 3 folks) but who doesn't when listening seriously. The way panel speakers roll highs more slowly over distance mitigates the high frequency off axis roll for casual listening but nothing is perfect and I don't understand how that fits the discussion of bass capability.

If you look at frequency response graphs of the quad, you'll see high frequency roll. It built into the speaker with a cap and resistor because it wouldn't sound correct with more at the listening position. There is also similar down tailoring to make the bass linear but that's for a different reason.

 I think you'll find that panel speakers(especially segmented ones) should be measured at 2 to 3 meters distance instead of the common 1m to get the best picture. 1m is used to help with room reflections when measuring dynamics but isn't a normal listening distance. In fact, it also totally screwed up a stereophile measurement of a Dunlavy that uses reinforcement of it's  5' spread woofers for low bass.

You are correct that dynamics are preferred for bass reproduction. For other ranges, it's up to the consumer. I don't like most panels for a variety of reasons but think the Quad is quite good but like all speakers, not perfect. That isn't what this discussion was about. I just used the Quad as an example since it''s the best known and most acclaimed example of modern electrostatics.

You can't curve a diaphragm and give it a large surface area. Phisically impossible. It needs to be segmented not to bow too much (it still does). Those modern electrostats that you speak of with bass, have none unless you add a large flat woofer panel as I described or add a dynamic. Much poorer bass response than a quad with the same panel area. They also run 1/2 the voltages, necessary larger gaps and much thicker diaphragms to make up for their design. Less control. They behave like line sources with better dispersion but their high frequency character is quite odd due to how large the radiating area is relative to bandwidth. Odd sounding to say the least. Soundlab does a better job with their segmented flat array. Let me put it this way, The qued is called the 63 because that is when Peter Walker came up with the design. It was made for 20 years because the technology wasn't there. The tech required for MLs or SounLabs was available before that.They just happened not to be. What you prefer is up to you but the quad is the most advanced tech panel out there.

If bass starts to compress at higher volumes the dynamic range is lowered. I'm not talking about how 'dynamic' it sounds if have the volume lowered, but the ratio between loudest undistorted output and noise floor.

On line vs. point sources:

You're right, this is kinda off-topic. But I thought it was worth mentioning because the initial post sounds like ESLs are the holy grail.

To me dynamic contrast is more important than dynamic range when the device plays loud enough for your needs. It's how well one can discern something quiet while playing something loud. Electrostats in general, do this well. Obviously, if it just doesn't play loud enough for you, it's absolutely dynamically limited for your needs and the qaud will be so for some folks but they are a minority. Definitely something to consider when looking at a product.