534 BELL SYSTEM TECHNICAL JOURNAL 



li\- Dr. \i. C". ("al)(>i/ it is to be expected that many of the moderately 

 high and high pitched murmurs can be heard more distinctly with 

 the Bowles than with the open bell stethoscope. On the other hand, 

 for many faint pathological sounds such as presystolic murmurs 

 wliirh are composed primarily of relatively low frequencies, the 

 oijen bell stethoscope is more satisfactory for observation. The 

 latter introduces less distortion so that with it all sounds are retained 

 more nearly at their original relative intensities. These remarks 

 are, of course, confined to the particular designs of stethoscopes shown 

 in Figs. 1 and 2. It should be noted that as the length of the rubber 

 tubing is increased, the fundamental peak of Fig. 3 moves down- 

 ward in frequency, and the transmission at higher frequencies becomes 

 poorer. In order to retain the very high pitched components of 

 certain heart and chest sounds, the use of long rubber tubing should, 

 tiuTcfiire, be a\c)i<k-d. 



The common stethoscope ser\es as a convenient means of obser\- 

 ing body sounds. If the available energy from a single chest piece 

 is subdix iiicd in order to supp]>- several indi\iduals, howe\"er, the 

 sounds (]bs(i\i(l by eacli are much fainter. In cases where the 

 sounds <if paili()liJi;ir,il interest are sufficiently near the threshold 

 of audibility llie ii>e nl a multiplicity of observing tubes renders 

 these .sounds inaudible. This is often the case. 



For teaching purposes or for consultation, it is extremeh' desirable 

 tf) ha\e multiple listening units. In the past, it has been necessary 

 to handle the students of large classes either singly or in small groups. 

 This method naturally limits the number of cases that can be demon- 

 strated and makes ii impossible to gi\e each student as much practice 

 as has been found necessary for iiiin to become familiar with the 

 more ob.scure sounds. Aside from iliese factors, it has not been 

 feasible for a large group to cibser\e simultaneously- with the in- 

 structor the i^eculiarities .uid cli.iniies in murnuu-s of a transient or 

 evanescent character. 



With the development of \acuum tube ani|)liliers, liie imssibiiilies 

 of lepriidui ing and magnifying body sounds electrically were con- 

 sidered. Ii ajjpeared that a device might be provitled which would 

 be useful not only in teaching but also in diagnosis, as an aid to 

 |)h>'sicians of subnormal hearing, in the reproduction of the \er\- 

 faint fetal heart sounds or e\en in fields be\'ond the scope ol the 

 ordinary stethoscope. 



' R. t". Cabot, " Pliysicial Diagnosis," Chap. \'l, V)li. 



