What is it exactly that is under so much pressure?

[Pleyel]

I don't think a Phantom would explode if you open it, as I don't expect any static pressure difference vs outside air.

I have no idea about how the 1.2 ton force to seal the box is meaningful for us, beyond being a parameter of  the manufacturing process.

Regarding the internal pressure, I did the math earlier when this figure was released, and could confirm:
Nominal volume is 3 liter (for each woofer). Excursion is +/-12 mm, as read on the web.
Volume change, each way, is 1.2 cm * pi * (10 cm)^2 = 377 cm3 = 0.377 liter peak
Relative volume change is 0.377 / 3 = 12.6% peak (+/-12.6%)
PV = nRT => assuming temperature is constant, relative pressure change = relative volume change = +/-12.6%
Rounding atmospheric pressure to 10^5 Pa, amplitude of pressure = 12.6 kPa pk
RMS pressure = peak pressure / sqrt(2) = 8.9 kPa RMS
https://en.wikipedia.org/wiki/Sound_pres...sure_level = 20 * log10 (pRMS / 20 µPa) = 173 dB
So the 174 dB in Devialet's white paper looks correct, and pretty unusual in less loud and/or compact speakers.
Hope it helps.

deleting my previous answer as it appears that I was wrong and Pleyel was right. My bad not to have verified before replying.

Jean-Marie

No pb, Jean-Marie.
Btw, measuring change in distance between left and right woofers (2x +/-12 or 13 mm = 26 mm) and summing their volume (2x3 l = 6 l) give the same relative pressure change, so the same sound level.
Don't hesitate to correct my math, as I'm not an "Expert"...
I discovered a rough estimate of sound levels, expressed in relative pressure change in %, which I find easier to visualize than dB SPL. Usual sound levels correspond to a very small relative change (0.01% pk is already a loud 111 dB SPL).
I also realized the absolute, theoretical maximum level, when pressure ranges from 0% to 200% of average pressure with a sine wave, but the same formula gives 20*log10 (1/1.414 * 1000 hPa / 20 µPa) = 191 dB SPL, that is 3 dB less than the 194 stated by the Wikipedia article, as if we shouldn't divide by sqrt(2) to convert (peak) amplitude to RMS: any clue, someone?

Anyway, thanks all for a great laugh about the pressure of different departments...