It is well known that high efficiency loudspeakers are required for public address and other sound reinforcing applications. In many cases only the speech articulation range from 500 cycles upward need be projected and space limitations for the speaker may be severe. The -MIS- loudspeaker shown in Fig. 1 meets such requirements, being less than 6" in diameter and 4" deep. The construction may be seen in the cross section drawing. A small phenolic cone is drive at the rim by a dynamic driving coil, which is immersed in a strong magnetic field. The convex side of the cone is directed towards the throat of the horn. This throat is constituted by a labyrinth having three ducts, the area of the ducts at the end next to the diaphragm being a carefully selected fraction of the area of the diaphragm. This causes the horn to provide acoustic radiation resistance loading on the diaphragm. The horn area expands at a generally exponential rate and is folded to form three traverses. The ducts constitute the first traverse; the second duct is annular and is formed between the housing and a conical -reflector,- and the third duct is also annular and extends forward between the reflector and the outer shell.
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