452 Tufts — Transmission of Sound through Solid Walls. 



viz: *00008 cm . It was also found that they transmitted sound 

 equally well, although the cardboard disc was nearly thirty 

 times as thick as the brass disc. 



IY. A disc was built up of ten sheets of cardboard treated 

 with paraffine. The total thickness was '70 cm and the displace- 

 ment of the center for a pressure of one gram per square 

 centimeter of surface was *0002 cm . This disc was compared 

 with a single disc of cardboard *22 cm thick and which gave 

 about the same displacement. It was found that the two discs 

 transmitted sound equally well, although one consisted of many 

 layers while the other was of a single homogeneous material. 



Y. The braced brass disc used in experiment III, above, was 

 compared with a disc cut from the same piece of brass, and 

 which had a small mass of brass soldered to its center. The 

 total mass of the two discs was thus made the same. The 

 displacement of the center of the loaded brass disc, for a 

 pressure of one gram per square centimeter of surface, was 

 •0022 cm while that of the braced disc was -00008 cm . The loaded 

 disc transmitted sound very much better than the braced disc. 

 Even the noises from the street which entered the room could 

 be easily heard through the loaded disc, while nothing of this 

 kind could be heard through the braced disc. 



An examination of the above cases, I to IY inclusive, makes 

 it evident that the amount of sound transmitted through the 

 discs as an elastic wave in the material, must be negligibly 

 small as compared to that transmitted as a to and fro vibration 

 of the disc itself ; were this not so, then in case IY, where 

 the two discs possess the same rigidity, one would expect the 

 sound heard through the single homogeneous disc to be louder 

 than that heard through the built-up disc with its many 

 reflecting surfaces. In other experiments, where compara- 

 tively rigid discs of materials well suited to the transmission of 

 elastic waves, were compared with less rigid discs composed 

 of materials not so well suited to the transmission of such 

 waves, it was found in every case that the less rigid disc 

 transmitted sound better than the more rigid one in spite of its 

 unfavorable composition. The experiments, therefore, show 

 that when sound is transmitted from the air on one side of the 

 disc, through the disc, to the air on the opposite side, the 

 transmission takes place almost entirely as a to and fro 

 vibration of the disc. 



The experiment described under Y shows that the effect of 

 mass in a wall is of minor importance as compared to rigidity. 

 The lead disc, in I, had nearly six times the mass of the glass 

 disc, but even this great increase in mass was more than 

 compensated for by the fact that the lead disc gave a displace- 



