ACOUSTIC PROPERTIES OE BALLS. \Vhen an organ-pipe or any elastic body is sounded in a room and then suddenly stopped, it is noticed that the sound does not instantly cease, but con tinues for several seconds. This is called rever beration; and the acoustic success of a room de pends largely upon its duration. It should not exceed two seconds by more than a few tenths of a second if the room is to he used as a music hall or opera house. It is found that the rever beration in a given room is practically inde pendent of the place where the vibrating body is situated, or of the position of the hearer; it de pend; upon the of the room, upon the material of the walls and floors, Upon the clash ion.", the audience. etc., and to it certain extent upon the intensity of the sound. The following approximate fi nimbi has been developed by Pro fessor Sabine of !larva rd University: (a b, X1+ .r,-f- ete.) 1 =0.164 Where a is a constant depending upon the ab• sorbing power of the walls of the room.
b is a coefficient of "absorption" for mu square meter of a definite material put anywhere in the room, the standard of comparison being the absorption of ono square Meter of open window.
cr is the number of square meters of the ma feria I.
f is the duration of reverberation.
is the volume of the room in cubic meters.
The absorption coefficients for some substances are as follows: Hard pine wood sheathing 0.001 Plaster on WI1o11 lath 0.034 Plaster on wire lath 0.033 Audience (per square meter) 0.96 Isolated an 0.54 isolated man 0.48 Carpet 0.20 House plants 0.11 Upholstered chair; 0.30 Bair cushions (per seat ) 0.21 The duration of in certain music halls and auditoriums is as follows: Old Music Hall, Boston, New' Alusie Hall, Boston, .3.1ass 2.31 Gewandhaus, Leipzig, Germany Sanders"I'lleatre, Cambridge, „Ilass :3.42 BIBLIOGRAPHY. Bayleigh, Theory of Sound, 2 Bibliography. Bayleigh, Theory of Sound, 2 volumes (London, ISM), a mathematical treat ment, brat with several d•seriptive chapters; Helmholtz. Stnsutions of 7'onc, translated by Ellis (London, 1895). the standard authority on harmony and music; Sabine. rehitretural Acoustics (Boston, 1900), which contains the only satisfactory treatment of this important question; Thomson and Poynting.,Sound (T,011 don, 1899), a text-book for schools and colleges, and a storehouse of accurate information.