752 BELL SYSTEM TECHNICAL JOURNAL 



which shows that the "normal" substitution voltage equals the open circuit 

 voltage for all types of acoustic measurements and for any value of electric 

 impedance loading the microphone. It is shown that the method recently- 

 proposed by some authors of removing the acoustic load from the micro- 

 phone when applying the substitution voltage results in a substitution volt- 

 age which does not equal the open circuit voltage. It is also shown that a 

 formula for the response of a transducer derived for a system in which the 

 microphones are open-circuited may be used when the microphones are 

 terminated by finite electrical impedances, by replacing the generated open 

 circuit voltages in the formula by the corresponding "normal" substitution 

 voltages. 



Consideration is given to the restriction in the delinition of the pressure 

 response of a transducer made necessary by the fact that the pressure on a 

 microphone diaphragm is a function of the electrical impedance terminating 

 the microphone. 



An experiment is described which involves a microphone coupled to a 

 chamber, the acoustical impedance of which is high relative to that of the 

 microphone. The results of this experiment agree with the conclusions of the 

 analysis. 



A Note on Filter-Type Traveling-Wave Amplifiers? J. R. Pierce* and 

 Nelson Wax. A small-signal analysis of systems in which an electron beam 

 interacts with a circuit composed of discrete filter elements is given here. 

 The effects of a line beam interacting with a series of gaps, which are capaci- 

 tive elements of a filter structure, are calculated, and it is shown that an 

 admittance can be introduced which arises from the presence of the elec- 

 trons. This admittance is in parallel with the gap capacitance, and thus 

 will alter the propagation factor of the filter circuit. It is shown that travel- 

 ing-wave solutions exist for the combination of electron beam and filter 

 circuit, and that there is a solution which has a positive real part, indicating 

 that gain will be exhibited. 



' Proc. LR.E., V. 37, pp. 622-625, June 1949. 

 * Of Bell Tel. Labs. 



