252 THE BELL SYSTEM TECHNICAL JOURNAL, APRIL 1951 



to the transmitter terminals is obtained by means of contact springs sup- 

 ported in a separate plastic cup which serves also as a controlled acoustic 

 cavity for the transmitter and as an acoustic shield between the transmitter 

 and receiver. Such a shield is necessary, as otherwise the transmitter and 

 receiver would be directly coupled acoustically. 



Transmitter 



While the transmitter unit is similar in structural design in some ways to 

 the transmitter of the previous handset/ it differs in many important details. 

 The diaphragm of the new unit is rigidly clamped at its periphery, thus 

 increasing the output in the upper frequency range as compared to the 

 paper clamped diaphragm transmitter of previous design. This is essential, 

 to achieve a quality of transmission that approximates the orthotelephonic 

 objective. 



The simple conventional system, consisting of a clamped diaphragm, 

 back cavity and carbon chamber, has a response characterized by a single 

 sharp resonant peak; whereas it was desired to provide a gradual increase 

 in output with frequency with a broad maximum in the region of 3000 cps. 

 This might be accomplished by a sufficiently damped structure with its 

 resonance in the region of 3500 cps, but only at the expense of efficiency. In 

 the new transmitter the desired response is obtained with high efficiency by 

 coupling the diaphragm to a doubly resonant system composed of the cavity 

 within the unit behind the diaphragm and the chamber between the unit 

 and the plastic cup. These two cavities are connected by holes covered by 

 woven fabric having carefully controlled resistance to the flow of air. 



The equivalent circuit of such an acoustic system and its acoustic im- 

 pedance characteristic as a function of frequency for some limiting and 

 typical values of the component impedances are shown on Fig. 10, where 



53 is the stiffness of the chamber in the transmitter behind the dia- 



phragm, 



54 is the stiffness of the chamber formed by the plastic cup, 

 M4 is the mass of the air in the holes coupling the two cavities, 

 R4 is the acoustic resistance of the coupling holes. 



The stiffness impedance of the cavity S3 behind the transmitter dia- 

 phragm acting alone is shown by Curve 1. Curve 2 shows the impedance of 

 both cavities S3 and S4 combined, with zero leakage impedance between 

 them. Curve 3 shows the impedance of the acoustic system composed of both 

 cavities coupled together by an impedance having typical value of mass 

 but zero damping resistance, while Curve 4 shows the characteristic of the 

 same system with coupling impedance having zero mass and a typical 



