34 ILLINOIS STATE ACADEMY OF SCIENCE 



In this way, the study of a large number of materials 

 as well as of various structural units has been made. It 

 is important to bear in mind the different processes by 

 which the vibrations set up by any source of sound in one 

 room may result in aerial vibrations in an adjoining 

 room. If the vibrating source is in solid connection with 

 the floor or wall, these vibrations may be conducted di- 

 rectly along timbers or beams to the floors or walls of ad- 

 jacent rooms. Probably the greater proportion of the 

 sound from a piano, cello, or any stringed instrument 

 resting on the floor, as well as the hum of motors or ma- 

 chinery, is transmitted through buildings in this way. 

 Thus far, no attempt has been made to deal with this 

 aspect of the problem. In the case of sound of the voice 

 or from an organ pipe, the alternating pressures in the 

 aerial sound Avave in one room produce vibrations of 

 walls or partitions, which in turn communicate their mo- 

 tion to the air of the adjoining room. In the case of por- 

 ous materials, such as felts or fabrics, the motion of the 

 air may be communicated directly through the pores of 

 the material, with little or no motion of the partition 

 itself. The problems so far studied all come under the 

 last two heads. 



The case of solid impervious partitions is illustrated by 

 doors and windows. Here the partition acts as a heavy 

 plate, having its own natural frequencies of vibration. 

 Its response to tones having these frequencies will the- 

 oretically, at least, be much greater than to other tones. 

 Hence we should expect the sound transmitted to vary 

 widely with the pitch. The experiments amply justify 

 this conclusion, so that a complete study of the trans- 

 mission of sound by a single partition involves measure- 

 ments for a large number of tones covering the entire 

 musical range. The results here presented are for six 

 or seven tones, at octave intervals, covering the entire 

 musical range from 64 to 4096 vibrations per second. 



Figure 5 shows the effect of both stiffness and mass 

 upon the transmissioif of sound by glass windows. The 

 ordinates of the curves give the logarithms of the re- 

 duction of intensity of tones given on the horizontal scale. 



