Friday, May 12, 2006
The acoustic tube (I also refer to it as the redirect tube) is a vertical plated brass tube brazed into the pole pieces in the 77DX. The radius of the tube OD fits the inside radius of the pole pieces and confines a small area of air behind the ribbon, which is only about an inch long. The top of the tube is closed, and the bottom of the tube is positioned above the entrance hole in of the Acoustic Labyrinth.
The acoustic tube has two slots cut into it: One is at the entrance, just behind the ribbon, and the other rear slot is on the opposite side of the ribbon, toward the rear. The rear slot is milled to have a flat surface on which the shutter rests and rotates.
The shot above shows a modeled acoustic tube and ribbon motor assembly used to characterize the geometry and acoustic Z of the basic 77DX design. One apparent shortcoming of the 77DX is reflection from the back of the shutter loading the adjacent ribbon. Here we have placed the ribbon about 2X the distance from the back of the tube, added an acoustic matching network, and tuned the acoustic load until the response is fiarly flat looking at a distant broadband source.
Posted by Bob Crowley at 7:32 PM