6 
PHYSICS: FLETCHER AND WEGEL 
Proc. N. a. S. 
the ear cavity could be determined in dynes per square centimeter. This 
cahbration was checked by two methods employing entirely different 
apparatus. 
The measurements were made in a room which was especially constructed 
to eliminate all outside noises. The top, the bottom and the sides of this 
room were built of a number of alternate layers of loose felt and sheet 
iron, the final inside layer being felt covered with cheese cloth. It is 
extremely important that all noise interference be eliminated for this 
kind of work. 
To make a measurement the vacuum tube oscillator was set to give the 
desired frequency. The attenuator dial was then moved until the sound 
from the receiver reached the threshold of audibility. Measurements 
were made upon 93 normal ears. The average of all these measurements 
is shown in figure 1 by the heavy line. The results of previous observers 
are also shown on this figure. It was necessary to use the logarithmic 
scale for pressures due to the large range of values involved. In figure 2 
the curves show the variation of the female ears from the average. A 
similar set of curves for the male ears shows practically the same result. 
The average of the two sets was the same within the precision of the test. 
The probable error of observation was less than one-fifth the probable 
deviation of a single observer's results from the average. The peaks shown 
are real but vary greatly from one individual to another which make the 
average curve free from any noticeable peaks. 
Audiograms for people of various types of deafness show striking differ- 
ences in their relative frequency sensitivities. It is expected to make a 
complete report of work done along this line in the very near future. It 
is sufficient here to give from our general experience the amount of sound 
volume in the speech range that is required to make people of various 
degrees of deafness hear. Persons who have normal hearing require 
approximately 1/1000 dynes per square centimeter in order to hear sounds 
in this range. Persons who require a pressure variation of 1/10 dynes 
per square centimeter are called slightly deaf. Those who require one 
dyne are partially deaf but can usually follow ordinary conversation. 
Those who require 10 dynes belong to that class who use ear trumpets or 
deaf sets to amplify the speech waves. A pressure variation of approxi- 
mately 1000 dynes can be felt and produces a sensation of pain. It is 
practicable to assume that people who experience no auditory sensation 
at these pressures are totally deaf. 
This shows that among people who can follow ordinary conversation 
there is a range in ear sensitivity of more than 1000 and among people who 
are noticeably deaf there is another range of 1000 making a total range of 
more than a million for people who can hear or be made to hear by means 
of amplifying devices. 
