74 Mr. V. Lough on the Beating 



importance of these wolf -notes in. the theory of bowed* 

 instruments, as demonstrated by C. V. Raman * in his 

 monograph on the subject, it was thought that an investi- 

 gation of the corresponding phenomenon in organ pipes 

 might lead to useful results. 



2. Experimental methods and Results. 



Having collected a number of wooden pipes of the 

 ordinary open-ended type, each pipe was tested separately 

 at various wind-pressures, and it was found that instead 

 of jumping suddenly to the octave as generally supposed, 

 in all or nearly all cases the transition is perfectly gradual 

 and usually includes an intermediate stage extending over 

 a considerable range of pressure in which regular beats 

 occur before settling down to the regular octave tone. In 

 one particular instance, selected on account of the promi- 

 nence of the* beats, the pipe having a pitch of approximately 

 300 vibs. per sec. and being voiced for a pressure of about 

 6-8 cm. of water, perfectly regular beats of a frequency of 

 8-10 per sec. were audible over a range of about 11-15 cm. 

 pressure. The frequency of the beats is nearly constant 

 throughout this range. From the results of analysis by 

 ear with the help of spherical resonators, it seems that the 

 beating or variation of intensity takes place mainly in 

 the first and second overtones, the fundamental remaining 

 steady or nearly so throughout its gradual decrease. The 

 character of the phenomena evidently depends on the 

 voicing of the pipe, but the relation between them has not 

 been established. 



In order to verify and enlarge these observations, a 

 series of photographic records of the vibrations was taken, 

 each under similar conditions but at different pressures, 

 and these records are reproduced on the accompanying- 

 Plate. The eight upper curves are records taken with 

 the particular pipe above referred to ; the lowest one is a 

 single record for another pipe selected at random. 



These records were taken by means of the special 

 phonodeik devised by Dr. P. H. Edwards and described 

 by Prof. Raman in a recent paper f . 



In view of the unavoidable effect of the horn and 

 membrane of the phonodeik and no doubt also of the 

 acoustie properties of the room in tending to distort 

 the form of the record obtained with the instrument, the 



* Bulletin No. 15, Indian Association for the Cultivation of Science,. 

 Calcutta (1918). 



t Phil. Mag. Jan. 1920, p. 145. 



