1Ö4 



Physiology. — "On reinforcement of sound and soundselection hy 

 inenns of microtelephone-apparatus" . By Prof. H. Zwaardemaker. 



(Communicated in the meeting of May 31, 1913). 



Of the innumerable sonorous vibrations with which the air and 

 the soil are pervaded, only those whose period is comprised between 

 10 and nearly 22000 per second, are lying within the limit of our 

 audition. Exactly midway between them lies the "a d'orchestre". 

 The pitch of musicians remains middle tone, even when age has 

 deprived us of a half octave at the extremities. This is no longer 

 so in pathological cases, when large pieces of various lengths are 

 generally cut off on the bass- and on the discant-side. 



Among the benefits man derives from his audible scale none 

 excels that of hearing speech-sounds. Compared with this the services 

 our hearing renders for our safety, for technical purposes, for 

 enjoying the pleasures of art etc. are relatively slight. Now there 

 is no part of the scale of such moment to the most important of 

 all functions of our sense of hearing as a small portion just beyond 

 the "a d'orchestre", stretching from />, to g^ (Bezold). Extending it 

 a little further, we get the zone to which local telephones are tuned, 

 viz. «1 to ('3. This portion of the human scale must be in good 

 condition, in order to secure a proper function of our sense 01 

 hearing speech-sounds. 



By confining myself to the speechzone a^ io a^ \ have been able 

 to simplify considerably the researches that have occupied me for a 

 couple of years and either concerned the sound of speech in buildings 

 or the use of hearing-apparatus. For this narrow field full knowledge 

 is .required of every single tone. 



Regardless of resonance an objective measurement of sound can 

 be performed alike for all pitches and impulses after Lord Rayleigh's 

 mode of arrangement. Devised in 1882 it had been w^orked out 

 theoretically by W. König in 1891 and put into practice by Zernow 

 in 1907. 



This method consists in placing a very small mirror (circular, 

 3 m.m. in diameter) obliquely to the plane of soundwaves. It is 

 hung by a fine quartz fibre and is kept stationary by a small 

 magnet attached to the back of it. As soon as the sound flow^s past 

 the miri'or in progressive waves or in often repeated pulses, the 

 disc takes up a position parallel to the wave-front, just like a sti'aw 

 on the surface of a river. This occurs with a force proportional to 

 the energy of the sound and ceteris paribus expressed in the torsion 

 of the fibre and in the magnetic force. 



