Tufts — Transmission of Sound through Solid Walls. 453 



ment twice as great as the displacement of the glass disc for 

 the same pressure. 



In order to investigate the effect of the mass of a wall on 

 its conductivity for sound, two cardboard discs of the same 

 dimensions were used. Both were treated with paraffin e, and 

 when clamped in the unions, both gave displacement's of 

 •0004 cm for a pressure of one gram per square centimeter of 

 surface. The intensity of the sound heard through the two 

 discs was, as far as could be judged, the same. The discs were 

 •22 cm thick, and each weighed it grams. To the center of one 

 of the discs was now cemented a mass of lead weighing 34 

 grams, and it was found that this cut down the intensity of 

 the sound transmitted through the disc by a very appreciable 

 amount. The effect due to the addition of a mass of five 

 grams could be readily detected if the mass was cemented to 

 the disc at its center, but not when the mass was cemented 

 about half way between the center and circumference. This 

 experiment shows that, other things being equal, the wall 

 possessing the greatest mass will be the poorest conductor for 

 sound. When the mass is uniformly distributed through the 

 disc, however, a very slight increase in rigidity will more than 

 compensate for a very considerable decrease in mass. For 

 example, a lead disc, weighing 145 grams, was compared with 

 a disc of red cedar, weighing only IT grams, the lead disc 

 gave a displacement of -00008 cin and the red cedar disc a 

 displacement of *00005 cm for a pressure of one gram per square 

 centimeter. It was found that the lead disc transmitted very 

 perceptibl}^ better than the red cedar disc, although it 

 contained over nine times the mass. 



It is a common practice in the construction of telephone 

 booths to make them of two, and sometimes of four walls, 

 separated by air spaces, and there seems to be an opinion that 

 such a form of construction is better adapted for the exclusion 

 of sound than one in which the same amount of material is 

 put into a single wall. In order to test the relative merits of 

 the two types of construction, six discs were cut out of card- 

 board and treated with paraffine in order to render them 

 impervious to air. In one of the unions three of the discs 

 were placed, and separated by cardboard washers, so that an 

 air space of two millimeters was left between the discs. In 

 the other union three discs were clamped in contact. It was 

 found that the discs separated by air spaces transmitted sound 

 better than the discs which were placed in contact. The same 

 experiment was repeated using brass discs and with a similar 

 result. The increased rigidity obtained by placing the discs 

 in contact more than balanced any advantage there might be in 

 having the intervening air spaces. 



