234 Intelligence and Miscellaneous Articles. 



ON A METHOD OF STUDYING THE REFLEXION OF SOUND-WAVES. 

 BY 0. N. ROOD, PROFESSOR OF PHYSICS IN COLUMBIA COLLEGE. 



It has been the custom for several years to introduce in certain 

 forms of the melodeon a revolving fan for the purpose of obtaining 

 rapid alternations in the intensity of the notes. This arrangement 

 is called a " tremolo ;" and its action was considered by its inventor 

 and by those interested in it to depend on the currents of air pro- 

 duced by the motion of the fan. An examination of the apparatus 

 soon convinced me that this idea was erroneous, and that the alterna- 

 tions in the loudness of the sound was due to reflexion or non-rer 

 flexion from the face of the revolving fan ; for I found that the 

 same effects could be produced by the aid of a circular disk consist- 

 ing of open and closed sectors and revolving in its own plane. A 

 disk of this kind constitutes a useful piece of apparatus for studying 

 the reflexion of sound-waves ; and some results obtained with it 

 were communicated by me to the National Academy of Sciences 

 as long ago as October, 1876. 



As no account of these experiments has ever been published, a 

 short description of them may not be without interest to those 

 engaged in experimental researches on sound, as with their aid the 

 following facts may be easily demonstrated. 



1st. At a perpendicular incidence the short sound-waves are more 

 copiously reflected than those that are longer ; and the regidar reflexion 

 is more copious from large than from small surfaces. 



The diameter of the zinc disk was, in the first set of experiments, 

 21 inches = 53*3 centimetres ; alternate quadrants were removed, 

 and the rate of rotation varied from two to four turns in a second. 

 The tuning-forks were mounted on their resonance-boxes, and 

 gradually removed away from the revolving disk till the alternations 

 could no longer be distinguished by the ear placed near the fork. 

 The results are given in the table below, in which " distance" indi- 

 cates that of the open end of the tuning-fork from the disk : — 



inches. 

 Diameter of disk 21 



Ut 3 fork, alternations heard at a distance of 13 

 *Jt 4 ,, ,, „ ,, 2S) 



Ut 5 ,, ,, ,, ,, Jo 



When the same experiments w^ere made with a disk having a 

 diameter of only S\ inches or 21*5 centimetres, it was found 

 necessary to bring the forks much nearer to the disk before the 

 alternations could be perceived. inches. 



Diameter of disk 8 J 



Ut 8 fork, alternations heard at a distance of 2 



V\ „ „ ,, 5 , O 



Ut 5 ,, ,, ,, ,, o 



2nd. When the sound-waves fall upon small flat surfaces at an 

 acute angle, the reflexion is most copious in the same direction as with 

 light, but the reflected and inflected waves can be traced all around the 

 semicircle. 



