HALL ACOUSTICS.

Putaruru Press, Volume II, Issue 28, 24 April 1924, Page 2

HALL ACOUSTICS.

AN INTERESTING LECTURE. Lecturing recently at University College, London, Mr. G. A. Sutherland said that if the acoustic properties of a hall or chamber were to be satisfactory they must have consideration in the original design. Among other requisites were uniform and adequate j loudness and absence of reveibatiou ! or echo. As to the first of these, curved walls produced concentrations of sound in some places and lack of sufficient intensity in others. Too many flat walls were also objectionable; but the conventional rectangular pattern might always be safely adopted in all but the largest halls. Rcverbation was the commonest fault in a modern auditorium; reverbation and echo were produced by hard surfaces which reflected sound. The architect; should so design that every auditor received in the first fifteenth of a second after the direct sound reached him not less sound than he would receive directly if he were situated fiCtv feet from the speaker. There was no scientific basis for choosing a distance of fifty feet, but the reason why onofiftecntli of a second was chosen was because sounds which struck the ear at intervals of time less than that were not distinguished as separate sounds. They might be considered as adding ui> to form the initial sound, but sounds arriving later had the nature of cither an echo or of a diffuse prolongation of the original sound. Echoes and roverbations could be reduced by the use of sound-absorbing materials in the building itself or in its furnishing. The best absorbent was an open window, for through this sound entirely escaped, but a full audience was almost equally effective. It was a matter of common experience that it was usually easier to hear in a room when full than in the same room when empty; but the shiallncss of an audience could in some degree be made up by paving heavilyupholstered seats. Taking the soundabsorbing efficiency of an open window as 1,000, that of an audience as ordinarily seated was 0.900; that of iut.e felt 6.6 cm. thick was 0.77, and that ol akoustolitli tile —a tile specially manufactured for use in auditoria —was 0.382. Brick, plaster, and glass were examples of substances that absorbed very litle sound; iD the case of a brick wall eighteen inches thick, set iu ocoil paint, the sound-absorbing efficiency was only 0.017. Of echo, the lecturer said that, besides reverbation and the unevenness of distribution caused by curved surfaces, echo was the commonest fault in large halls; it often came ffopi the back wall, apd ib that case could be relieved by covering that wall with some sound-absorbiny material. in me case or a uouucii Chamber it was necessary to aim at exceptional loudness and distinctness. The hall should be so arranged that a speaker, by merely turning in his place, could have an uninterrupted view of every hearer. This ruled out the possibility of overhanging galleries with potential speakers far underneath thepq Tbo ceiling was th§ natqr&l surface to use as a spupd reflector; it must not be more than thirty-five feet high, or it would produce reverbation or echo. The upper part of the walls should be of sound-absorbing material; otherwise reverbation would fip produced by tne refleeting of sound from wall to wall. Floors and gangways should be carpeted. All seats should be upholstered. Galleries not in use should be cut off by curtains. The seats: should preferably be arranged in some sort of hollow square slightly rising towards tho I walls .