308 ILLINOIS STATE ACADEMY OF SCIENCE 



drawn from different sources and apparently unrelated, 

 proved quite concordant and led to conclusions concern- 

 ing effective sound insulation. 



Tivo Types of Sound in Buildings. — Two types of 

 sound should be considered in the problem of insulation 

 in buildings. One type includes sounds that are gener- 

 ated in the air and that jDrogress through the air to the 

 boundaries of the room; the other is composed of com- 

 pressions generated in the building structure by motors, 

 elevators, and street traffic. 



Insulation of Sounds in Air. — Sounds of moderate in- 

 tensity such as those generated by the human voice or 

 a violin may be stopped with comparative ease if the 

 walls of the room are continuous and fairly rigid. The 

 more vigorous sounds of a cornet, trombone, etc., would 

 require especially heavy walls or else double partitions. 

 Any breaks in the w^alls for ventilators, pipes, or doors 

 should be guarded by effective insulation. 



Insulation of Building Fi&f<2f/o;?s.^— Compressional 

 waves generated in the building structure pass readily 

 along the continuity of solid materials, and, as they have 

 more paths for escape, are more difficult to insulate than 

 sounds in air. Moreover, they may create trouble when 

 they cause a wall or floor to vibrate. The insulation is 

 based on the same jjrocedure as that used for air sounds ; 

 namely, to interpose a new medium differing in elas- 

 ticity and density. An air space in masonry would be 

 effective if not bridged over by solid material, biit since 

 this is impossible for ordinary building constructions as 

 the weight of bodies necessitates contact for support, an 

 approximate insulation is sought by using air-filled sub- 

 stances like dry sand, ground cork, hairf elt, or flax, that 

 possess but little rigidity but are capable of sustaining a 

 floor or a partition that is not too heavy. 



Transmission of Sound. — Sound waves in air may be 

 transmitted through an obstructing medium in three 

 ways. First, they may pass through the air spaces of a 

 porous material. Second, they may be transmitted by 

 modified waves in the new medium. In this process 

 sound compressions and rarefactions progress rapidly 



