540 



BELL SYSTEM ll-Xl I XICAL JOLRX.ll. 



The stethophone is equipped with live fillers whose cut-off fre- 

 quencies are based on careful anaK'ses of about 100 hospital cases 

 of heart murmurs, rales and breathing sounds. These analyses 

 showed that the sounds of pathological interest to the physician can 

 be grouped into fairly definite frequency regions. When sounds in 

 a particular range of frequencies are of immediate importance, they 

 may be emphasized by suppressing sounds outside of this band. 



The frequency characteristics of the filters are shown in Fig. G. 



FREQUENCY IN CYCLES PER SECOND 



Fig. 6. — Loss characteristics of the five fillers 



For conNenicnce, tiie rut-oft frecjuency has been defined as that 

 frequency at wliicli tlii' I'licrgy is reduced to aiiproximatcK' 1 10 

 of its original \,iluc. 



The low-pass liltir with a cut-off trec|uency of 180 cycles is ol 

 primary use for ri|)iii(hiring tlu' normal heart sounds and fetal heart 

 sounds in cases wiiere the rate alone is desired. Most of the energy 

 of these sounds is below 100 cycles. With this filter most of the 

 common inti'rfiTing noises, inrluding the sounds of llu' luiinaii \nici', 

 are excluded. 



The low-pass 400 c\cle filter is i)arlicularl\- useful for ob.serxing 

 presystolic and certain low-pitched systolic and diastolic murmurs. 



The low-pass ().")() ex cle filter has been found the most valuable 

 of all fise filters. With it, most high-pitched murmurs, low-pitched 



