PHYSICAL MEASUREMENTS OF AUDITION 149 



Fig. 1, labelled the threshold of audibility, shows the results of such 

 measurements. The ordinates give the amplitude of the pressure 

 variation in dynes per square centimeter that is just sufficient to 

 cause an auditory sensation and the abscissae give the frequency of 

 vibration of the tone being perceived. Both are plotted on a loga- 

 rithmic scale. The experimental difficulties made it impossible to 

 make a very accurate determination for those parts of the curve 

 shown by dotted lines. More work needs to be done on these por- 

 tions of the curve. In the important speech range, namely, from 

 500 to 5,000 cycles, it requires approximately .001 of a dyne pressure 

 variation in the air to cause an auditory sensation. This corre- 

 sponds to a fractional change of about one-billionth in the atmospheric 

 pressure, which shows the extreme sensitiveness of the hearing mechan- 

 ism. 



In order to obtain an idea of the intensity range used in hearing, 

 an attempt was also made to obtain an upper limit for audible in- 

 tensities. When the intensity of a tone is continually increased, a 

 value is reached where the ear experiences a tickling sensation. Ex- 

 periments show that the intensity for this sensation is approximately 

 the same for various individuals and the results can be duplicated 

 as accurately as those for the minumum intensity value. It was 

 found that if this same intensity of sound is impressed against the 

 finger, it excites the tactile nerves, In other words, the sensation of 

 feeling for the ear is practically the same as for other parts of the 

 body. When the intensity goes slightly above this feeling point, 

 pain is experienced. Consequently, this intensity for the threshold 

 of feeling was considered to be the maximum intensity that could be 

 used in any practical way for hearing. The two points where these 

 two curves intersect have interesting interpretations. At these two 

 points, the ear both hears and feels the tone. At frequencies above 

 the upper intersecting point, the ear feels the sound before hearing 

 it, and in general would experience pain before exciting the sensation 

 of hearing. Consequently, the intersection point may be considered 

 as the upper limit in pitch which can be sensed. In a similar way, 

 the lower intersection point represents the lowest pitch than can be 

 sensed. 



There has been considerable work 32 " 51 in the past to determine 

 the upper frequency and lower frequency limits of audibility, but 

 it would appear that without the criterion just mentioned, such 

 limiting points apply only to the particular intensity used in the 

 determination. Not enough attention has been paid to the intensity 

 of the tones for such determinations. It is quite evident from this 



