538 BELL SYSTEM TECHNICAL JOURNAL 



traiismiucr. I'mlouhtedK' lliis damping is nol only \arial)le for 

 different indi\iduals but depends upon the pressure and the nature 

 of the flesh and bone structure in the vicinity of the point of applica- 

 tion for any one individual. Thus the frequency characteristic of 

 the transmitter is somewhat dependent on the conditions of use. 

 The frequency of maximum response is slightly above 200 cycles, 

 and the nature of the response-frequency cur\e indicates that the 

 vibratory system is highly damped. A discussion of the overall 

 frequency characteristic of the stethophone, including the trans- 

 mitter, is given in a later section of the paper. 



It is obvious that variations in the pressure of application will 

 introduce disturbing noises in the audible frequency range. Suitable 

 means have, therefore, been provided to eliminate the communica- 

 tion to the vibratory system of hand tremors, slight movements of 

 the patient, and friction noises of the fingers on the case of the trans- 

 mitter. 



Another source of extraneous noise is the rubbing of the transmitter 

 cord on the clothing or on other surfaces. A stift cord is \cry ob- 

 jectionable from the standpoint of transmission of friction noises. 

 Insulation from these noises has been i)rovided b\- a very flexible 

 section of cord at the transmitter end. 



."). .AMri,iiii;i< 



The three-stage am|)litk'r I'liiploys one Wi-sicrn I'.lectric 102-D and 

 two Western Electric 101-1) \acuuni tubes. As shown in Fig. 4, the 

 input transformer T\ connects the transmitter to the grid of the first 

 tube which is coupled to the second tube through a resistance potentio- 

 meter. The second and third lubes are coupled through a trans- 

 former T2. The output circuit of the last lube may be connected 

 to the load directly from its plate circuit for high impedance loads, 

 or through an output transformer T'i for low impedance loads. The 

 plate circuit of the second tube is tuned by means of a condenser CI 

 in order to retain high amplification at the low end of the fre<iiiency 

 scale. 



A \ery flat characteristic is obtained over the range of interest, 

 the maximum \arialion being only about 3 TU (See Fig. 5). A 

 total gain of about 80 TU is pro\idcd, that is. a power amplifica- 

 tion of about one hundred million times. With .ui amplification of 

 ")() TU, about the same loudness is observed in a single receiser in 

 the output circuit of the stethophone as is heard by the direct use 

 of the open bell stethoscope. This lca\'es a rcser\e amjilification 



