|Amperite RB - switch on rear|
In fact the knob is connected to a cloth-covered metal plate which slides up and down, blocking the rear of the mic. This is a mechanical method of altering both the pickup pattern and the frequency response.
|Inside the Amperite - sliding plate, blurry photo :-(|
When open, the mic will be a normal figure-8 pattern. Closing the vent will reduce pickup at the rear, and also make the mic sound brighter by reducing the proximity effect.
Well done Amperite!