Felt in 802 rear cover

[mah]

Would 1/8 inch just on the back surface be enough?

Marshall

[Altec Best]

Yes should be as long as it is the dense felt.

[mah]

Probably medium-dense. Easy to add a bit more if needed

[GM]

Measure the distance from the back of the diaphragm to the cover, divide it into ~6780". This will be ~the average standing wave frequency, which should be way up the driver's response curve and since acoustic power falls at 1/f with increasing frequency, it will be next to nothing, so even thin hobby craft felt is sufficient to damp them.

GM

[Steve Schell]

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.

[bowtie427ss]

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.

Newton's 3rd law would suggest that a broader range of parameters needs to be measured, no?

Certainly rear chamber loading/tuning affects excursion of the moving assembly.

In my own experience i've heard damping/stuffing in the rear chamber influence the timbral or sonic character to a degree as well.

I'm certainly no expert here, nor am i challenging Mr. Schell as i have a great deal of respect for his knowledge, experience, and contributions. Just trying to stir some discussion on a somewhat cloudy topic.