686 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION I 



Z 



O 'o- 



I- 



< 70 



Z> 



UJ 80— 



h- 



3 456769100 2 3 4 5 6 7B9ipOO 



FREQUENCY - cycles per second 



Otdb 



X 40 SO 



Attenuation 



100 XO XO 4X1 SOD Vs 



curves of the six filter'^ 



FIG. 2. A schematic representation of the frequency response chsiracteristics of several phonocar- 

 diographic systems. A: Lcatham's high-, mid- and low -frequency systems. B: Mannheimer's mul- 

 tiple filter system. These various systems are needed because the wide intensity range of cardiovas- 

 cular sound, particularly the very intense low-frequency components and the relatively faint higher 

 frequency components cannot be displayed simultaneously in a single oscillogram, A second motiva- 

 tion is the desire to obtain approximate information on the frequency composition of heart sounds 

 and murmurs by the examination of oscillographic displays of various frequency bands. The system 

 of Maass and Weber is similar to that of Mannheimer. Luisada's selective phonocardiography uses 

 a filter which cuts off those components above as well as below a certain frequency band. [Courtesy 

 of Williams & Wilkins (12).] 



frequency response characteristics, precisely as one 

 would predict from spectral phonocardiography. A 

 single channel phonocardiograph can be used if 

 necessary, since the five separately filtered displays 

 can be made seriatim from magnetic tape or directly 

 from the patient. 



Two central questions regarding amplification and 

 filtration for phonocardiography are, /) at what 

 point should equalization (the pre-emphasis of 

 higher frequency components to compensate for the 

 12 db decrement with frequency) be performed, and 

 i?) should low-cut, high-pass filters, or band-pass'' 

 filters be used. 



Filter Systems in Spectral Phonocardtugraphv 



The filter system in the commercially available 

 sound spectrograph (Kay "Vibralizer"), which was 

 adapted for phonocardiograpliy and has been used 

 almost exclusively, is of the heterodyne type. The 

 sounds to be analyzed are essentially passed through 

 a filter mechanism, the tuning of which is changed 

 progressively in coordination with the write-out of 



' Luisada & Zalter (g) have used band-pass filters with a 

 decrement of 24 db per octave at each end (Krohn-Hite Corp., 

 Cambridge, Mass.). Satisfactory results should be achieved, 

 for example, with the following five octave bands: 30-60, 

 60-120, 120-240, 240-480, and 480-960. 



50 



100 200 400 



Frequency in c p s 



800 



FIG. 3. .\n idealized multifilter system for phonocardiog- 

 raphy. Line AB represents the natural intensity decrement of 

 cardio\ascular sound with frequency. Five high-pass, low-cut 

 litters with the shoulder at successively higher frequencies are 

 shown. Standardization of amplitude to a constant level is 

 represented by the vertical arrows. The length of each arrow 

 represents the amount of attenuation or amplification necessary 

 to bring all signals to the standard level. The interrupted line 

 represents, in the case of each filter, the pass characteristics of 

 the filter. 



