16 Sound and the Ear /I : 3 



sound pressure level increases with the switch released and decreases 

 with the switch depressed. The entire setup then hunts for the thresh- 

 old, continuously crossing and recrossing it. A recording pen is 

 attached to the variable attenuator. The pen writes on a calibrated 

 chart, recording the instantaneous setting of the attenuator. Another 

 motor drives both the chart and the oscillator so that a record is obtained 

 of threshold level versus frequency. This level is recorded without any 

 effect of the examiner. 



The Bekesy audiometer is very successful in limiting the role of any 

 operator other than the subject. It also gives a continuous record of 

 threshold versus frequency instead of values only at discrete points. It 

 presents the threshold curve directly in a graphical form. However, it 

 has several disadvantages. It is slower than an audiometer operated 

 at discrete frequencies by an experienced technician. It is impossible 

 with the Bekesy audiometer to distinguish between losses in a certain 

 frequency range and apparent losses due to extraneous physiological 

 noises such as swallowing. Using the discrete frequency audiometer, 

 the operator crosses and recrosses the threshold, thereby eliminating the 

 effect of extraneous physiological noises. Finally, the Bekesy audio- 

 meter depends on the skill of the subject and his understanding of the 

 instructions. Both of these will vary from person to person, introducing 

 a nonhearing variable into the apparent threshold. 



An ideal compromise would be an instrument similar to the Bekesy 

 audiometer but operating only at discrete frequencies. If the instru- 

 ment could remain at one frequency until the threshold stays constant 

 for, say, 15 seconds and then shift automatically to the next frequency, 

 it would encompass most of the advantages of both the discrete frequency 

 and the Bekesy audiometer. Unfortunately, this becomes so complex 

 electronically that the author knows of only one audiometer of this type, 

 and it takes a skilled electronic engineer to keep it running. 



The information obtained from a speech audiometer is different from 

 that found by using a pure tone audiometer. In a speech audiometer 

 various test words are presented at a constant sound pressure level. 

 Some persons who have appreciable pure tone hearing losses at certain 

 frequencies do not show any hearing loss for speech. Conversely, other 

 people, with normal pure tone thresholds, have marked speech hearing 

 deficiencies. The problem of recognition of speech is much more com- 

 plex than hearing a pure tone. Understanding speech involves the 

 function of several parts of the brain. Actually, speech can still be 

 understood if any two continuous octaves of the audible spectrum are 

 presented and the rest of the energy filtered out. The quality of the 

 speech will be altered, but it is still understandable. (Even up to 50 

 per cent of every syllable or word can be removed. The remainder 



