I : 3/ Sound and the Ear 17 



when compressed to eliminate the blank times is still understandable.) 



The speech threshold measures a person's ability to participate in a 

 conversation or listen to a lecture. It depends as much on the functional 

 condition of the brain as it does on the action of the ear. In contrast, 

 the pure tone threshold indicates to a greater extent the action of the 

 ear itself. 



As people grow older, the pure tone thresholds are raised, particularly 

 at higher frequencies. For people of all ages, these thresholds are raised 

 by exposure to loud noises. The latter effect is reversible if only occa- 

 sional exposures occur but is quite irreversible after years of continuous 

 exposure. It is not worth while here to go into the details of current 

 estimates on criteria for levels at which, say, 5 per cent of the persons 

 will be appreciably deafened after years of exposure. The currently 

 accepted levels are lower than those which exist in many factories today. 



The relationship of pleasure to audible frequency range is very 

 complicated. In these days of high fidelity, stereophonic sound, and 

 extended frequency ranges, one might guess that the greater the fre- 

 quency range, the more pleasing. This does not seem to be the case. 

 Older people find hearing aids which correct their high frequency losses 

 make music sound harsh and unpleasant but that flat response ampli- 

 fiers increase their satisfaction in listening to music. In other words, 

 what the listener is used to hearing is enjoyable. 



Other types of information can be gleaned from experiments similar 

 to those used to obtain the pure tone threshold curves. One test is to 

 ask the subject to match in loudness tones of different frequencies. On 

 the basis of these results, equal loudness curves can be drawn. They are 

 illustrated in Figure 7. The lowest is the threshold curve itself. As 

 the sound pressure level is raised, the equal loudness curves tend to 

 flatten out, approaching straight lines by the time the sound pressure 

 level at 1 kc has reached 100 db. 



Another test is to ask the subject to choose just noticeable differences 

 in loudness. A change of this nature is sometimes referred to as a 

 difference limen, abbreviated DL. When the sound pressure level is 

 60 db or more above the threshold of hearing the DL is of the order of 

 0.5 db 6 throughout most of the auditory range. At lower sound pressure 

 levels, the DL's are greater. At 30 db above threshold they are about 

 1 db; they are as large as 6 db near threshold. 



Similarly, difference limens, or just noticeable differences, exist as 

 the frequency is varied. At very low frequencies, a 0.5 cps change is 

 detectable. In the middle frequency range (around 1 kc), the normal 

 person can notice a 3 cps change. At the very high frequency end of 

 the audible range, changes greater than 25 cps are necessary before a 



6 That is to say, it is between 0.25 and 1.0 db. 



