Tufts — Transmission of Sound, etc. 357 



Art. XXIX.— The Transmission of Sound through porous 

 materials ; by F. L. Tufts, Ph.D. 



A few years ago some experiments were made by the author 

 for the purpose of determining the relative conductivities of 

 different porous materials for sound waves. The methods 

 employed at that time were comparatively crude, and while 

 the data obtained could not be relied upon for great accuracy, 

 they showed unmistakably that when two porous materials dif- 

 fer noticeably in the resistance offered to the transmission of 

 sound, they also differ, in approximately the same way, in the 

 resistance offered to the flow of a current of air through them.* 

 "When I attempted to obtain numerical results expressing the 

 relative resistances offered by a number of different porous 

 materials to the transmission of sound, I found that deter- 

 minations made under different conditions gave results which 

 varied to some extent. Some rather rough measurements of 

 the relative resistances offered by these same materials to the 

 transmission of air through them under different pressures, 

 also showed variations greater than could be readily accounted 

 for. It therefore seemed desirable to make a more careful 

 comparison of the two phenomena. 



For this purpose three different porous materials were 

 selected, and in order not to introduce unnecessary complica- 

 tions, granular materials were chosen, the granules of which 

 consisted of ordinary lead shot : a very considerable uniformity 

 in the size and shape of the granules composing any one of the 

 materials was thus obtained, and it was found that the packing 

 of the material produced little or no difference in its conduc- 

 tivity. The coarsest material used consisted of a shot 4'37 mm 

 in diameter. This will be referred to as material A ; an inter- 

 mediate material composed of a shot 2*79 mm in diameter will 

 be designated by the letter B, and the material having the 

 finest granules, a shot l'22 mm in diameter, will be called C. 

 When a cylindrical vessel was filled with any one of these 

 materials, it was found that about 40 per cent of the volume 

 of the vessel was still occupied by air, and this percentage was 

 practically the same whether the coarsest material, A, or the 

 finest, C, was used. 



In the first part of this paper, I shall describe the experi- 

 ments for determining the variations in the resistances offered 

 by the three materials A, B, and C, to the flow of air through 

 them under different pressures. In the second part the 



* Science (K S.) ix, 219-220, 1899 



