INSTRUMENTS FOR THE NEW TELEPHONE SETS 349 



stand a second grid of insulating material is added. The holes of this 

 grid have a mass and resistance, Wo^o, which must be taken into account 

 in arriving at an overall picture of the factors affecting response. 

 These holes are coupled to the moisture-resistant membrane, m\r\, by 

 means of the stiffness 5o of the enclosed cavity. The cavity stiffness, 

 Si, couples the membrane to the diaphragm. 



There are two types of response frequency measurements in general 

 use; namely, pressure response measurements in which a constant 

 sound pressure is maintained at the face of the transmitter throughout 

 the frequency range covered by the test, and field response measure- 

 ments in which a sound field of constant intensity is established at each 

 frequency before inserting the test transmitter. Pressure response is 

 used principally for purposes of analysis whereas field response usually 

 affords a better measure of the performance of the transmitter under 

 the conditions of actual use. 



The pressure response of the new transmitter measured with a con- 

 stant sound pressure at the grid, and the response computed from the 

 equivalent circuit are shown on Fig. 6. The transmitter used in this 

 test was artificially aged by an amount equivalent to two years of 

 service in order to simulate more nearly plant conditions. While the 

 computed curve departs slightly from the measured curve at certain 

 frequencies, due to the inadequacy of some of the basic assumptions, 

 such as those which were made relative to the impedance of the 

 granular carbon, the agreement in general is so good as to provide a 

 powerful tool for predetermining the response characteristics of 

 transmitters under development and a useful method for evaluating 

 the reaction of one element of the transmitter on another. Reducing 

 the transmitter to an equivalent electrical circuit also has proved 

 invaluable in determining the causes of variations in transmitter 

 performance observed during manufacture. 



As previously mentioned, the response characteristics of a trans- 

 mitter under conditions of actual use may differ from those obtained 

 with a constant pressure at its face. In general this is due to the fact 

 that the diffraction effect of the transmitter as an obstacle in the sound 

 field has not been taken into account. While not as readily pre- 

 determined as the pressure response, the field response can be easily 

 measured by inserting the transmitter in a sound field of constant 

 intensity. An artificial voice ^ is used in this measurement as a sound 

 source and the electrical input is adjusted to maintain a constant 

 sound pressure at the guard ring at each frequency before inserting 

 the instrument under test. Response curves for the new and early 

 types of transmitter obtained in this way also are shown in Fig. 6. 



