We see that when the delay line of an electrostatic loudspeaker is configured so that it imitates an oscillating sphere, it is not the delay line but the discretization of the stator into rings of finite width that produces irregularities in the pressure response of Fig. 15.11. However, a very smooth response may be obtained using an analog delay line, as shown in Fig. 15.14. Although this does not produce a constant directivity pattern up to the very highest frequencies, broad directivity is maintained throughout the vital midrange and lower treble, as shown in Fig. 15.13. Eq. (15.19) is a useful formula for the voltage sensitivity when such a delay line is used, although Walker's voltage equation of Eq. (15.20) still applies for f   <   c/(πa).