I : 3/ Sound and the Ear 



15 



The pure tone hearing threshold tests described above depend on the 

 accuracy of the apparatus and the technique of the operator, as well as 

 on the hearing of the person being tested. By suitable calibrating 

 techniques, the equipment can be standardized so that the sound 

 pressure levels are accurately known to within 1 db (that is, about 

 ± 10 per cent in the actual sound pressure). It is difficult to improve 

 on this by more than a factor of 2. The effect of the operator is harder 

 to remove. He must present successively lower and then higher sound 

 pressure levels to the subject. If he starts far above the threshold, the 

 subject becomes familiar with the tone and will distinguish it at lower 

 sound pressure levels than if the operator started below the threshold. 

 The operator must cross and recross the threshold until, in his judgment, 

 he has found a stable value. 



One very ingenious attempt to remove the effect of the operator was 

 introduced by von Bekesy. His audiometer includes the person being 

 tested as part of a feedback loop in an automatic control device designed 

 to keep the sound pressure level at the ear close to the threshold. The 

 system is illustrated in block diagram form in Figure 6. The output 



Ear Phone 



Z 



Oscillator 

 z. 



Attenuator 



Motor Driving 



Chart and 

 Oscillator Dial 



Record 



When depressed, motor 

 increases attenuation. 



When re/eased, motor 

 decreases attenuation. 



- Pen: Moves up and 

 down indicating 

 attenuation. 



Chart: 

 Moves horizontally 

 indicating frequency. 



Subject: 



Depresses switch when 

 he hears sound. Releases 

 switch when he does not. 



Figure 6. Block diagram of the Bekesy audiometer which 

 records the threshold of hearing without influence of any 

 operator other than the subject. 



of an oscillator is fed through a variable attenuator to the earphones. 

 The subject is given a switch which he depresses when he hears the tone 

 and releases when he does not. The switch is connected to a reversible 

 motor which drives the variable attenuator in such a fashion that the 



