554 BELL SYSTEM TECHNICAL JOURNAL 



TABLE 14 



Sound Level Meter Readings, with Flat Weighting, in db Above .0002 

 Dyne per Square Centimeter 



steady noise equal in magnitude to the maximum noise levels was 

 computed to be 11 db at 440 cycles, 3 db at 880 cycles, and zero at 

 higher frequencies.* The interpretation of these results is in some 

 doubt because the people in the booths tended to be more quiet when 

 the test level approached threshold, and also because the disturbing 

 effect of sounds of irregular character may not be properly indicated by 

 masking computations based on experiments with steady sounds. 

 Accordingly a more direct method of evaluating the disturbing effect 

 of noise was tried. 



Members of Bell Telephone Laboratories who had taken the test at 

 the Fair under routine conditions at various times during the season 

 were retested at the Laboratories after the Fair closed. The same 

 equipment and procedure were used, except that only one person was 

 tested at a time under conditions free from any disturbing noise except 

 that created by the observer himself. On the average, the tests indi- 

 cated more acute hearing during the retest at the Laboratories, par- 

 ticularly at the low frequencies. The average shift was 2.9 db at 440 

 cycles, 1.4 db at 880 cycles, 1.1 db at 1760 cycles, and negligible at 

 the higher frequencies. Since the test at the Laboratories was given 

 last in every case these shifts may have been partly due to improve- 

 ment with practice. However, they serve to set an upper limit to 

 the average disturbing effect of noise. This comparison is based on 

 tests of 150 ears of 106 people, whose average age was 39 and whose 

 average hearing acuity was somewhat better than an equivalent age 

 group at the Fair. 



The equipment for the tones hearing tests consisted of eight machines 

 at New York and two at San Francisco. These machines were main- 

 tained in an equipment room some distance from the test booths, one 

 machine being connected to each booth. Each machine consisted of 

 a phonograph reproducer, amplifier, attenuation network, and seven 



* These values of masking apply to an ideal observer having a threshold approxi- 

 mating the minimum audible pressure curve of Fig. 6. Since a great majority of 

 observers at the Fairs had higher thresholds, the masking would be correspondingly 

 less for them. 



