166 



To my knowledge Zernov was the fii-st to place the measuring 

 mirror in a space entirely free from resonance. To increase tlie 

 sensitiveness I took some years later, tbi- application to medical 

 problems, an airerent tube of the dimensions of the auditory canal 

 and the auricle. This enables us to perform an accurate measurement 

 even of whispered speechsounds. However, occasional currents of air 

 must be arrested by putting a very small plug of cotton-wool in 

 the artificial auditory canal. The mirror is placed at an angle of 

 45° close in front of the aperture of the tube, so that the sound- 

 wave, issuing from the auditory canal is di'iven against it as fully 

 as possible. The mirror is consequently tilted with maximum power 

 to a more transversal position. 



If weak sounds in the speechzone a, to e^ are to be measured, 

 it will be well to use large receiving funnels. Phonograph horns in 

 their various shapes ^viil be found to work very well. Small am- 

 plitudes are recorded more accurately, when the scale is placed at 

 a great distance. Then, however, a constant position of rest is ex- 

 pedient, which is hardly practicable, unless the streams of air in 

 funnel and auditory canal are removed through the insertion of an 

 india-rubber diaphragm of the size of a phonograph membrane. 

 Cover-glass or thin mica will do as well. Thus I was in a position 

 to establish the ratio of the average intensities of whispered and 

 spoken sounds. The experiment was made (together with Dr. Reutkr) 

 with 20 monosyllabic, aequisonorous and aequidistant words. The ratio 

 appeared to be 1 : 1 70. (The intensity is in the ratio of 1 : 170, the distance 

 at which sounds are heard of 1 : 13)"'). The modifying influence of 

 funnel and membrane may be controlled by going through the gamut 

 first with a simple physiological conducting tube and afterwards with 

 the same tube associated with a funnel and phonograph membrane. 



In the following pages I shall briefly state the rules which have 

 proved generally reliable in measuring sound. 



§ 1. Physiological measurement of sound. 



When the measurement of sounds witii regard to their audibility 

 is the subject under consideration, it is permissible to use an arti- 

 ficial auricle and an artificial auditory canal to direct the sound- 

 wave on to the measuring mirror. Provided the resonance of the 

 artificial conduit be e(|ual to that of the natural canal, nothing 

 foreign is added to the sound, for when perceived by the human 

 ear, it is transmitted througii a similar tube. The artificial canal 



') Proceedings of llie I4lfi Dutcli Gongiess foi- Phys. and Med. at Delft. 



