PIERCE. — A METHOD OF MEASURING THE INTENSITY OF SOUND. 393 



experiments the breaking of a chronograph circuit by an electric clock 

 in a distant room gave noticeable deflections. Most of these electric 

 disturbances may be easily tuned out by a change of the inductance 

 or capacity either in the disturbing circuit or in the rectifier circuit. 

 By wearing a head telephone connected in series with the galvan- 

 ometer during the observations, the observer may easily recognize any 

 foreign disturbances by their characteristic tones in the telephone. 



It was not the purpose of the present note to multiply observations 

 on the acoustic properties of a particular room. However, apart from 

 the interest attaching to the method of the experiment, the jesult that 

 for a sustained tone, even in a large room, there are practically all 

 over the room definite positions of sharp maxima and minima of 

 intensity is rather a striking fact when brought out objectively. The 

 results show that an auditor may sometimes greatly improve his 

 hearing of a discourse or a musical rendition by a slight motion of 

 his head so as to bring his ear into a position of maximum intensity. 

 Perhaps he already unconsciously does this, which may account for 

 the fixed attitude of an audience in close attention. 



The occurrence of these definite maxima and minima of intensity 

 of sound, due to reflection from the walls, should be borne in mind 

 when one attempts to interpret any experiment on sound performed 

 in a closed room. As Professor Sabine has repeatedly emphasized, the 

 mere fact that the walls are distant from the source of sound, while 

 the observer, or sound-receiving apparatus, is near to the source, is 

 not sufficient precaution against the influence of reflection, because 

 the reflecting surfaces are on all sides and act many times, and may 

 combine in their action in such a manner as to be a very considerable 

 factor in the resulting intensity. 



The curves of Figure 8 are plotted in terms of current in the 

 galvanometer. It was shown above, in Experiment II, how the indi- 

 cations of the galvanometer may be made independent of the rec- 

 tifier by substituting voltage from the curve of Figure G for the 

 corresponding current values. When this substitution is made, the 

 proportional differences between the maxima and minima, expressed in 

 voltage values, become somewhat smaller than these differences ex- 

 pressed in current values. However, on account of the intermedia- 

 tion of the telephone receiver between the sound vibrations and the 

 electrical indications, it is still not possible, without further calibration 

 of the apparatus, to obtain absolute or even relative values of the 

 sound intensity. Several methods of obtaining this calibration in 

 terms of sound intensity suggest themselves. One method is to 

 employ the distance law in connection with experiments performed 



