28 



ACOUSTICAL RADIATING SYSTEMS 



several wavelengths in length in order to yield a " wedge-shaped " di- 

 rectional characteristic. 



RADIUS=A 



Fig. 2.6. Directional characteristics of a 120° arc as a function of the radius and the wave- 

 length. The polar graph depicts the pressure, at a large fixed distance, as a function of the 

 angle in the plane of the arc. The pressure for the angle 0° is arbitrarily chosen as unity. 



2.6. Circular Ring Source. — The directional characteristics ^'^^ of a 

 circular ring source of uniform intensity at all points on the ring is 



^a — /o 





2.27 



where Ra = ratio of the pressure for an angle a to the pressure for an 

 angle a — 0, 

 Jo = Bessel Function of zero order, 

 R = radius of the circle, in centimeters, and 



a = angle between the axis of the circle and the line joining the 

 point of observation and the center of the circle. 



The directional characteristics of a circular ring source as a function of 

 the diameter and the wavelength are shown in Fig. 2.7. The shapes are 

 quite similar to those of a straight line. The characteristic is somewhat 

 sharper than that of a uniform line of length equal to the diameter of the 

 circle, but has almost the same form. 



9 Stenzel, H., Elek. Nach. Tech., Vol. 4, No. 6, p. 1, 1927. 



'» Wolff, I. and Maker, L., Jour. Acous. Soc. Amer., Vol. 2, No. 2, p. 201, 1930. 



