50 Applied Biophysics 



of hearing with frequency. A piezoelectric microphone may be 

 switched into circuit to facihtate speaking to a partially deaf 

 person undergoing test. They are valuable in the diagnosis 

 of deafness and the accurate prescription of hearing aids. Many 

 a physician who prides himself on his skill with his stethoscope 

 would be surprised at his audiogram if he were tested with an 

 audiometer. 



Hearing aids employing modern small piezoelectric micro- 

 phones, miniature valves and batteries, and compensating tone 

 circuits, can be of great value to the deaf. Tone-compensated 

 and automatic volume controls have increased the usefulness of 

 these instruments. Lately there has been a tendency for 

 wireless specialists to "fit" deaf persons with hearing aids ; this 

 is a dangerous practice. One must remember that many deaf 

 persons will not benefit at all by the use of these aids. 



There are many types of amplifying stethoscopes working on 

 the thermionic-valve amplifier principle. Instruments have been 

 introduced for the graphic registration of the heart sounds, which 

 incorporate such devices. Olson ^^ introduced a new acoustic 

 stethoscope which transmits all frequencies over the range 

 from 40-4,000 cycles per second without discrimination or ap- 

 preciable attentuation, whereas an ordinary stethoscope has an 

 effective range of only 200-1,500 cycles per second. There is a 

 marked falling off in the frequency response of an orthodox 

 acoustic stethoscope below 200 cycles per second. A filter 

 control is incorporated in the instrument described by Olson. 

 The arrangement used for comparing the response characteris- 

 tics of this stethoscope with others is as follows : Sound vibra- 

 tions were developed in the human body by means of a sub- 

 aqueous loudspeaker fed by an audioamplifier and audiosignal 

 generator. An artificial ear was first held directly against the 

 opposite side of the body to secure a reference characteristic, 

 and different stethoscopes in turn were then ihtroduced between 

 the artificial ear and the body. 



The recording and reproduction of sound is of interest to 

 the physician. Such records are of value foi; teaching and re- 

 search purposes, Henriques -^ described an apparatus for record- 



