State of Experimental Acoustics, 299 



Sympathy and Drawing. 

 Under certain circumstances two sources of sound react on 

 each other, and affect either the pitch or intensity with which 

 they would speak separately. When two organ-pipes of the 

 same pitch weaken each other's intensity, there is said to be 

 " sympathy." When two of different pitch affect the pitch in 

 which they mutually speak, there is said to be " drawing." 

 Lord Rayleigh brought forward some cases of the latter (see 

 Proc. Musical Assoc. 1878-79, p. 26), in which the pipes 

 spoke the same note, lying above the pitch of either separately : 

 this was with open pipes. I have observed a case where two 

 stopped pipes "drew" together to a note below the pitch of 

 either separately. This mutual influence also occurs with har- 

 monium-reeds. With organ-pipes it presents an interesting 

 problem of atmospheric vibrations. With harmonium-reeds it 

 is practically important in connexion with the construction of 

 tonometers. This subject remains almost unworked. 



Loudness of Sound. Mechanical Equivalent of Sound. 



The subject of the measurement of the loudness of sound 

 will receive a new foundation in the admission of Fechner's 

 psycho-physical law, with respect to the perception of so and 

 by the ear*. This law is derived from the admission that 

 equal fractions of any existing mechanical intensity produce 

 equal impressions ; and it results in the statement that the im- 

 pression is the logarithm of the mechanical intensity. 



Under these circumstances impressions have to be classified 

 as to apparent ]oudness, in the same manner as stars are clas- 

 sified as to apparent brightness. The criterion of successive 

 stages is that they appear equally distinct from one another 

 when loudness only is considered. The following is a sketch 

 of such a classification. 



Audible sounds are divided into ten magnitudes. The first 

 magnitude includes the loudest sounds. The magnitudes down 

 to the fifth include lesser sounds that are still loud ; the sixth 

 to the tenth magnitude includes sounds that are not loud, the 

 tenth magnitude containing the softest sounds that can be 

 heard. The distinction between loud and not loud is very 

 definite to my ear ; it may not be equally so to others. Of 

 course the following list represents only my own impressions, 

 and it may probably require amendment when considered by 

 others. Each magnitude includes the sounds up to the next 

 on the loud side. 



* See Helmholtz, Phys. Optik, p. 312. l Nineteenth Century,' July 

 1879, p. 166, Galton (where the law is called Weber's law). 



