364 Tufts — Transmission of Sound, etc. 



For practical purposes, it is quite important to know the 

 comparative opacity of equal thicknesses of different substances 

 to sound waves, that is, the relative resistances they oppose to 

 the transmission of sound. Determinations of the relative 

 resistances offered by different materials showed considerable 

 variations, according to the conditions under which the mate- 

 rials were compared. For example : a comparison of the resist- 

 ances offered by the two sizes of shot, A and C, to the trans- 

 mission of sound, made in another set of experiments, showed 

 that when the materials were placed near the node of a station- 

 ary sound-wave the finer shot, C, opposed only twice as much 

 resistance to the to and fro motion of the air particles as the 

 coarser material A, while if the comparison was made near a 

 loop, C offered over ten times as much resistance as A. Experi- 

 ments are now in progress for the purpose of studying the varia- 

 tions in the relative resistances of the three materials, A, B and 

 C, to the transmission of sound. The resistances offered by equal 

 thicknesses of the different materials have been compared at 

 different positions, with respect to the nodes of a stationary 

 sound-wave. The results so far obtained seem to warrant the 

 conclusion that the relative resistance of C, as compared to 

 either A or B, varies according to the distance of the material 

 from the node of the stationary sound-wave. Diminishing the 

 distance from the node has the same effect upon the relative 

 resistances of the materials to the transmission of sound that 

 increasing the pressure-gradient has upon the relative resistances 

 of the same materials to the transmission of air currents. 



Physical Laboratory of Columbia University, 

 Nov. 17, 1900. 



