SCIENTIFIC RESEARCH APPLIED TO THE TELEPHONE 263 



maximum intensity of sound that the ear is capable of handUng. 

 This curve was determined by noting that intensity which produced a 

 feehng sensation. Intensities slightly higher than this result in pain 

 and in some instances serious injury to the ear. The dotted lines on 

 either side complete the enclosure and represent the upper and lower 

 limits of pitch that can be heard. It is obvious from this figure that 

 the upper or lower limit of pitch is greatly dependent upon the 

 intensity at which the sound is produced. It will be seen that near 

 the middle range of frequencies, the pressure range is one million to 

 one. The pitch range of pure tones is from about 16 to 25,000 cycles 

 per second. 



These results are for young adults, and it may be of interest to note 

 that as one becomes older the hearing acuity, at the higher frequencies 

 particularly, becomes less. In the table below is shown some measure- 

 ments to determine what the effect of age would be upon the hearing 

 acuity: 



TABLE III 

 Db Loss in Hearing with Age 



These are average values obtained from measurements on a large 

 number of persons. 



Another important measurement of average hearing is that con- 

 cerned with minimum perceptible differences in pitch and in intensity. 

 Careful measurements on large groups of people have given us reliable 

 data of this form. In Fig. 6 are shown the results of such measure- 

 ments. They are plotted on the auditory sensation area. The 

 ordinates are decibels above the reference pressure and the abscissas 

 are centi-octaves above or below a pitch of 16.35 cycles per second. 

 A frequency scale is also given for reference purposes. The numbers 

 within the area indicate the minimum changes in the intensity level 

 in db that the average ear is able to detect over that region of the 

 auditory area. It will be seen that near the threshold fairly large 

 changes are necessary to be perceptible, while at fairly high intensities 

 about 1/4 decibel is all that is necessary for the change to be perceived. 



In Fig. 7 are given similar data for minimum perceptible differences 

 in pitch. The numbers in the figure in this case are given in centi- 



