1921] on Absolute Measurements of Sound 411 



Either is then moved along at a constant height and the varying 

 deflections of the phonometer are read while the sound remains the 

 same. Interference sets in between the direct sound and its image 

 reflected in the ground, and the existence of a minimum is obvious 

 to the most naive observer by the ear alone. The reflection of either 

 grass or gravel was found to be about 95 per cent., while, with a 

 most carefully deadened room, the walls of which were covered with 

 thick felt, there was perhaps 20 per cent, reflection. The whole 

 measurement at both ends and the transmission checks up with an 

 accuracy of about 2 per cent. 



With this apparatus all sorts of acoustical experiments may be 

 performed. By attaching to the phonometer a long glass tube or 

 antenna, it has been possible to explore all sorts of places, such as 

 the field within a horn or tube lined with an absorbent substance. 

 The transmission of sound through fabrics, walls, and telephone 

 booths may also be quickly examined. The instrument is used by 

 psychologists and by telephone and acoustic engineers, and is of 

 interest to navigators. An interesting by-product is an instrument 

 for showing the direction of an acoustic signal in the fog. It has 

 been called a phonotrope, on the analogy of heliotrope, which turns 

 to the sun. It consists of two equal horns which bring the sound to 

 the opposite sides of the disc. When the whistle blows, the band of 

 light spreads out, and on turning the instrument it closes to zero 

 when the sound is directly ahead. Thus at several miles the direction 

 is given to within two or three degrees. 



Finally, let us consider that mystery of sound, the violin, which 

 has been studied by Prof. Barton, of Nottingham, and by Prof. 

 Raman at Calcutta. This may be described by the engineer as a box 

 of curious shape, made of a curious substance, wood, of variable 

 thickness, with two holes of strange figure to let the sound out of 

 the resonating box. The latter is actuated by a curious substance, 

 catgut, made of the intestines of a sheep, and set in vibration by 

 another curious substance, the tail of a horse. Yet from this 

 wonderful box we get the most ravishing sounds, which affect 

 profoundly the emotions of the most civilised. Yet the physicist 

 reduces all musical instruments to combinations of resonators with 

 strings, membranes, bars, plates and horns. The mathematical theory 

 of strings was given by Euler two hundred years ago, of bars and 

 plates less than a hundred years, of resonators by Helmholtz and 

 Rayleigh, and I have recently added a theory of horns which, while 

 only approximate, works well in practice, and investigations are now 

 being carried out by such methods on vowels and the violin. 



[a. a. w.] 



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