PAPERS OX CHEMISTRY AND PHYSICS 309 



througli tlie air. moving tlie molecules successively as 

 tliey pass in somewhat the same way as a gust of wind 

 blows the separate stalks in a wheat field. On reaching 

 a solid partition the foi'ward motion is hindered, par- 

 ticularly if the molecules of the new material are mas- 

 sive and resist compression. In this case most of the 

 energy is reflected and only a small proportion pro- 

 gresses through the wall. On meeting a further discon- 

 tinuity of material, such as wood or air, the waves are 

 again affected, until finally a part of the energy emerges. 

 Third, sound may be transmitted by setting a partition 

 as a whole in vibration. The partition then acts as an 

 independent source of waves, setting up compressions 

 and rarefactions on the farther side and giving a sort 

 of fictitious transmission. If the partition is rigid and 

 massive the vibrations are small and very little sound 

 is transmitted : if the partition is thin and flexible a con- 

 siderable amount of energy is transferred.* Usually in 

 building constructions the partitions are complex, as for 

 example plaster on wood lath and studding. In this case 

 the plaster areas between the studding act in a manner 

 similar to drum heads and transmit sound. Hard plaster 

 on wire lath presents a dift'erent surface with a modified 

 action on the incident sound. 



The transmission of sound involves a number of phe- 

 nomena and is not a simple matter. It depends essen- 

 tially on the character of the structure through which 

 sound is transmitted and can be calculated only for sim- 

 ple cases of homogenous materials of known constants. 



The systematic sui'vey of the subject of soundproof- 

 ing as given in the Bulletin leads to several practical 

 conclusions. 



Veufilatiou Sifstem. — Especial attention should be 

 paid to the ventilation system. All eft'ective soundproof 

 constructions either omit entirely a ventilation system 

 or else construct it in some special manner to avoid 

 transmission of sound. In some buildings air is sup- 



* Rayleigh. Lord. Theory of Sound". Vol. 2. Sections 2T0-272. see 

 also: 



Jager, G. "Zue Theories des Nachhalls", Sitzungber. der Kaisl. 



Akad. der Wissenschaften in Wien. Math. Xatur. Klasse, Bd. CXX. 

 Abt 2a. 1911. 



