580 THE BELL SYSTEM TECHNICAL JOURNAL, JULY 1951 



is coUimated into a fairly sharp beam. High frequencies emanating from this 

 horn would be projected through a rather small angular region and such a 

 horn by itself would not be a very desirable general purpose loud speaker. 

 Figure 22 shows the same horn with a slant plate divergent lens (described 

 in reference 2) placed in front of its aperture. The transformation of the fiat 

 phase Ironts into circular fronts and the wider angle of coverage can be 

 readily seen. A frequency of 9 KC was used for these tests. 



When a large cone type loud speaker is operated at the higher frequencies 

 it too becomes quite directive. Figure 23 is a radiation pattern of a 12-inch 



Fig. 23 — A large aperture loud speaker (12 inch) also becomes directive at high frequencies 

 as evidenced by the flat wave fronts in the beam. / = 8.5 KC. 



loud speaker at 8.5 KC. The minor lobe formation is noticeable, indicating 

 the presence of a central lobe surrounded by a region of low intensity. 



Microwaves and Sound 



The lenses employed in the preceding photographs were originally con- 

 ceived and constructed for use at the very short radio wavelengths known 

 as microwaves.^ The strip lens is, in fact, a small scale model of the type 

 used in the antenna systems of the New York-Chicago microwave relay 

 circuits of the Bell System for telephone and network television. A similar 

 type of dual-purpose delay lens using disks instead of strips is shown in the 



8 W. E. Kock, "MeUllic Delay Lenses," Bell Sys. Tech. Jour., 27, 58 (1948), 



