Interesting subject, this damping of the energy from the rear of the diaphragm on a compression driver. Going back to the beginning, the Western Electric 555W used a wad of lamb's wool in the chamber behind the diaphragm, and vented to the atmosphere though a wire screen. Collectors call these early ones "mesh backs." Later 555s used a solid plate in lieu of the screen. Edward Wente's patent paper for the Fletcher Horn System compression driver, the original 4" diaphragm driver, illustrated a rear cover which incorporated screened vents. The production W.E. 594A used a sealed rear cover.

The earliest Lansing compression drivers, based on Wente's early work, provided a number of holes in the rear cover, usually covered with cloth. Later ones were sealed tight.

I once made on-axis measurements of several JBL 2440 drivers on a JBL 2350 radial horn and could measure no difference whatsoever whether the rear cover was on or off.

The original Lansing small format driver, the 801 of 1937, used a sealed rear cover with a felt pad glued to the inside of the rear cover. Later Lansing and Altec Lansing 802s followed suit.

It is interesting to note that the earliest thinking in regard to a compression driver was that the rear volume should vent to the atmosphere; certainly there was no intention of a tuned chamber as some have thought in recent years. I think our forefathers eventually arrived at the conclusion that sealing the rear volume made no measurable difference, and improved reliability by sealing out contaminants.