. Elements of acoustical engineering. Electro-acoustics; Sound. CURVED SURFACE SOURCE 35 blance to those of an arc of the same angular spread. For example, the angular spread of the horn of Fig. in the plane containing the line AA' and the axis is 87|°. This may be compared to the arc of Fig. In this case X '4, X 2, X, 2X, 4X and 8X will correspond to 145, 290, 580, 1160, 2320 and 4640 cycles. The angular spread in the plane containing the line BB' and the axis is 52|°. This may be compared to the 60° arc aso'v lOOO/v. Fig. Directional characteristics of the 15-cell cellular ho
. Elements of acoustical engineering. Electro-acoustics; Sound. CURVED SURFACE SOURCE 35 blance to those of an arc of the same angular spread. For example, the angular spread of the horn of Fig. in the plane containing the line AA' and the axis is 87|°. This may be compared to the arc of Fig. In this case X '4, X 2, X, 2X, 4X and 8X will correspond to 145, 290, 580, 1160, 2320 and 4640 cycles. The angular spread in the plane containing the line BB' and the axis is 52|°. This may be compared to the 60° arc aso'v lOOO/v. Fig. Directional characteristics of the 15-cell cellular horn shown in Fig. in a plane containing the line A-A' and the axis of the center horn. The polar graph depicts the pressure, at a fixed distance, as a function of the angle. The pressure for the angle 0° is arbitrarily chosen. of Fig. with the same relation between the wavelengths and frequencies as noted above. It will be seen that there is a marked resemblance be- tween corresponding frequencies. Of course, there is some variation due to the fact that the frequencies do not correspond exactly. Further, there is some difference in the angular spread. For most spherical sur- faces of this t)^e the directional characteristics in various planes corre- spond very closely to the directional characteristics of the corresponding arc. Directional systems are used for sound ranging both in air and in water. For the general subject of sound ranging and signaling, see Stewart and Lindsay, " Acoustics," D. Van Nostrand Company, New York City, and Olson and Massa, " Applied Acoustics," Blakiston Company, Philadelphia. Equations, and are applicable to plane and curved acoustic diffraction gratings. As in the case of optics the angle of the maxima shifts with the frequency. Gratings have been used in systems for the analysis of sound. The audio fre- quency is used to modulate a high frequency oscillator (50,000 cycles). The output of the oscillator d
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