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Page:Popular Science Monthly Volume 21.djvu/471

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457
ACOUSTIC ARCHITECTURE.

that hot and cold currents do not readily mix, but fill the whole auditorium with alternations, continually varying in temperature and therefore in density and relative motion.

The proper solution of the problem is to admit the air in large volume, and at the proper temperature, at one side of the auditorium, carry it bodily across the room in one large mass as nearly as may be without break of homogeneity, and exhaust it at the other side. Or it may be introduced through a perforated floor and rise to be exhausted at the roof. This plan has been tried with success; the air, being first brought to the proper temperature in the basement, passes up through myriads of gimlet-holes, and is exhausted at the ceiling by means of numerous openings connected with a high chimney or other means of producing an exhaust.

An example of the first method is shown in the Baltimore Academy of Music, where the author was able to make some experiments to determine how far acoustic properties were actually dependent upon the condition of the air. The ventilation of the house is arranged as follows: The whole supply of fresh air is admitted at the back of the stage, is then warmed, then crosses the stage horizontally, passes through the proscenium, and then, somewhat diagonally toward the roof, across the auditorium, in one grand volume and with gentle motion, so as to almost entirely prevent the formation of minor air-currents. It is exhausted partially by an outlet in the roof, and partly by numerous registers in the ceilings of the galleries. From this central outlet and from the large flues of the registers, the air passes into the ventilating tower over the great chandelier, which supplies, in its heat, a part of the motive power of the circulation. It is further expelled from the tower by means of properly constructed ventilators. The acoustic properties of this house are universally agreed to be very superior.

The experiment made by the author consisted in stationing observers in various parts of the house while the performance was going on, with directions to note, at intervals during the evening, the readiness with which they could hear what was said on the stage. The observers were ignorant of the experiment to be tried. Observers A and B were stationed in the first, and C and D in the second balcony, from 8 to 10 one evening, when Neilson was playing "Rosalind." At 8.50 the ventilators were closed, so as to interrupt the normal circulation of air; and the doors into the lobbies, and thence into the street, were thrown open, that counter-currents might be established. At 9.20 the doors were closed, and the ventilators set right. The testimony of the observers was:

A (first balcony).—8 to nearly 9, good; for about half an hour, bad; afterward much better.
B (first balcony).—8 to 9, good; 9 to 9.30, bad; 9.30 to 10, good. Strong current of air felt from door a little before 9.