270 BELL SYSTEM TECHNICAL JOURNAL 



The horn, shown in the upper part of Fig. 5, comprises several 

 separate channels, each of which has substantially an exponential 

 taper. Toward the narrow ends these channels are brought together 

 with their axes parallel, and are terminated into a single tapered tube 

 which at its other end connects to the receiver unit. Sound from the 

 latter is transmitted along the single tube as a plane wave and is 

 divided equally among the several channels. If the channels have 

 the same taper, the speed of propagation of sound in them is the same. 

 The large ends are so proportioned and placed that the particle motion 

 of the air will be in phase and equal over the mouth of the horn. This 

 design gives a true spherical wave front at the mouth of the horn at 

 all frequencies for which the transverse dimensions of the mouth 

 opening are a large fraction of a wave-length. 



As the frequency is increased, the ratio of wave-length to transverse 

 width of the channels becomes less, and the sound will be confined 

 more and more to the immediate neighborhood of the axis of each 

 channel. The sound then will not be distributed uniformly over the 

 mouth opening of the horn, but each channel will act as an independent 

 horn. To have a true shperical wave front up to the highest fre- 

 quencies, the horn would have to be divided into a sufficient number 

 of channels to make the transverse dimension of each channel small 

 compared with the wave-length up to the highest frequencies. If it 

 is desired to transmit up to 15,000 c.p.s., it is not very practical to 

 subdivide the horn to that extent. Both the cost of construction and 

 the losses in the horn would be high if designed to transmit also 

 frequencies as low as 200 c.p.s., as is the case under consideration. 

 However, it is not important that at very high frequencies a spherical 

 wave front be established over the whole mouth of the horn. For 

 this frequency region it is perfectly satisfactory to have each channel 

 act as an independent horn, provided that the construction of the 

 horn is such that the direction of the sound waves coming from the 

 channels is normal to the spherical wave front. 



The angle through which sound is projected by this horn is about 

 60 degrees, both in the vertical and in the horizontal direction. For 

 reproducing the orchestra two of these horns, each with a receiving unit, 

 were used. They were arranged so that a horizontal angle of 120 degrees 

 and a vertical angle of 60 degrees were covered. These angular exten- 

 sions were sufficient to cover most of the seats in the hall with the loud 

 speaker on the stage. The vertical angle determines to a large extent 

 the ratio of the direct to the indirect sound transmitted to the audience. 

 The vertical angle of 60 degrees was chosen purely on the basis of judg- 

 ment as to what this ratio should be for the most pleasing results. 



