902 rear covers = 802 rear covers?

[gearjammer]

Just my crazy idea, but I would not run without a rear cap on my hf.drivers.

[gearjammer]

Just my crazy idea, but I would not run without a rear cap on my hf.drivers.

[Steve Schell]

The small format Altec driver rear cover traces its origins back to the Lansing 801 field coil driver of 1937. These early ones were stamped from steel, and most have the felt pad on the inside. The die cast covers probably began with the Altec cover with the star pattern on back, circa 1944. All these covers, with their three attachment screws, are interchangeable as far as I know.

I have always preferred the sound of compression drivers with the back covers removed for hi fi purposes. There seems to be a bit of additional clarity and reduced confusion in the sound, although on axis response mesurements show zero difference with the cover on or off.

If a driver is used inside a low frequency cabinet the cover must be used to protect the diaphragm from the l.f. pressure changes.

Interestingly, early drawings of the Bell Labs and Western Electric compression drivers indicate rear covers with screened vents, so venting the back of the diaphragm refects the original thinking of the inventors. The first version of the W.E. 555W used a screen over the rear exit, behind a chamber stuffed with lamb's wool. Also, the patent paper for Wente's driver for the Fletcher System (later to become the W.E. 594A and JBL 375/2440) indicates a rear cover with large screened areas.

The only circumstance where the cover makes a big difference is when a large format compression driver is pressed into service for bass duty on a long, slow flare bass horn. This approach can yield the ultimate in clear, accurate, boom-free bass reproduction, but the diaphragm cannot be hampered by a tiny enclosed rear volume. I first learned this when trying an Emilar EC-600 driver (6" diaphragm) on a bass horn. There was little output below 100Hz. until I removed the rear cover, then the bass came on strong.