Several different concepts are being discussed with confusion between them. I'll try to clarify the issues. First, let's understand the terms and what we are discussing. This is somewhat simplified because I'm trying to hit the essence.
Sealed-Cabinet Types
Infinite baffle is a sealed box which does not use as a restoring force the air compressed by the woofer's backwards movement (backwave). The backwave, instead, is intentionally dissipated and lost inside the cabinet by expanding around baffles and into batting such that it cannot put pressure on the woofer. The suspension on the woofer, of the normal type, instead serves as the restoring spring. So while an infinite baffle cabinet is sealed (nearly but not completely sealed, though, as we'll later see), is is not acoustic suspension. The infinite baffle cabinet is normally significantly larger than an acoustic suspension or ported (bass reflex, transmission line, aperiodic vent or ARU, etc.) cabinet. Bozaks are the most familiar infinite baffle cabinet, but Electro-Voice also made them. Think big box.
Acoustic suspension: is a sealed box which uses as a restoring force the air compressed by the woofer's backwave. The rearward wave compresses air inside cabinet and this forces the woofer to its resting position. The driver's surround must be very soft and floppy so that it does not resist this restoring pressure and moves faster than it would otherwise do. Acoustic stuffing does not dissipate the backwave; it, instead, serves as an acoustic brake to slow the propagation of the waveform, allowing greater excursion and slowing the frontward movement, but it does not reduce the interior cabinet pressure. So while an acoustic suspension cabinet is sealed exactly like an infinite baffle, it is not functioning in the same way because in acoustic suspension the backwave is required for proper functioning.
Perfectly Sealed Box
When any sealed box, be it infinite baffle or acoustic suspension, is perfectly sealed then the air pressure internal to the box differs from the air pressure external to the box. The interior cabinet air would be trapped at the time of manufacture, and its volume fixed. As a result, as the external pressure increased or decreased the cone would be moved in or out, changing its resting position.
Air Exchange
To avoid having the resting position vary with air pressure—air pressure varies with altitude and with storms, so it constantly changes, which is how a barometer works—every cabinet must therefore have a means to permit air exchange between the interior and exterior. The mechanism of exchange must be small to avoid having the backwave exit through that orifice and thus (a) (in infinite baffle and acoustic suspension) interfere with the front wave or (b) (in acoustic suspension) reduce the restoring force on the woofer by reducing the interior cabinet pressure.
At the same time, the orifice must be sized large enough such that air does not rapidly move through it or the orifice will squeak, like one of those rubber squeeze toys for dogs or children. A pinhole will squeak.
Dust Cap as Exchange Orifice
The dustcap is a nice way to exchange air from the interior of the cabinet to the exterior, and this also cools the voice coil preventing it from overheating and even burning out. It provides a nice surface area so the rate of air flow through any spot is small.
Cabinet Sealing
Despite the need for tiny amounts of air exchange, in both acoustic suspension and infinite baffle the cabinet best functions with a nearly perfect air seal. This is why a gooey caulking like Mortite (tm) or other filled butylene rubber was historically used around the drivers and the back. It is why the surround must be resealed after some decades, because the butylene rubber in the surround has oxidized, hardened, flaked off, or otherwise degraded. The drivers must be perfectly sealed to the baffle, the back must be perfectly sealed to the cabinet, and the surround must be perfectly sealed to prevent air from moving through it. The only air exchange is through an opening kept small and diffuse, and that is the dust cap.
As has been demonstrated, the dust cap must be a bit porous to allow some air flow to equalize the air pressure and to cool the woofer voice coil which can become quite hot.
Conclusion:
Never believe what you read on the internet without critically analyzing it. Nearly every bit of what is discussed about vintage speaker "improvements" using adhesives and caulks is pernicious and dangerous nonsense. Don't let people convince you that their cult is correct. Reserarch, verify, and think about every single proposed change. Replacing bits of a vintage Acoustic Research or other acoustic suspension speaker is expensive, time consuming, and leaves you with a different speaker than the one you had.
Until you understand—deeply and thoroughly understand, not casually but hundreds or thousands of hours of education and understanding—how a loudspeaker functions, don't modify it. IF YOU LACK THIS LEVEL OF UNDERSTANDING THEN DO NOT MODIFY IT.
Edit: fixed three typos and a missing comma that were pointed out to me. Thanks to those who found them. Hopefully that's it.
