AN IMPROVED TELEPHONE SET 



253 



value of damping resistance. Finally, Curve 5 shows the acoustic impedance 

 of the two-cavity system in which values are assumed for both the mass 

 and resistance of the coupling impedance, such as would occur in the trans- 

 mitter. 



The acoustic design of such a system requires exact control of the indi- 

 v^idual elements to prevent large irregularities in the overall transmitter 

 response. The control of R4 is particularly critical as illustrated by Curve 3, 



200 



100 

 80 



60 



l40 



O 



2 30 



20 



30 40 



200 400 600 1000 2000 



FREQUENCY IN CYCLES PER SECOND 



Fig. 10 — Transmitter acoustic impedances. 



4000 6000 10,000 



which shows the large decrease in acoustic impedance at resonance with a 

 corresponding increase at anti-resonance of the system when the damping 

 resistance R4 becomes very small. This will cause a sharp peak and dip 

 respectively in the transmitter response at these frequencies. 



Correct balance of the acoustic impedances, as illustrated in Curve 5, 

 will result in an acoustic network having an impedance at low frequencies 

 approaching that of the combined cavities, Curve 2, with gradual trans- 

 formation as the frequency increases, reaching the impedance of the single 

 smaller cavity, Curve 1, at high frequencies. 



