The introduction of warm air through openings in or near the floor will not readily produce uniformity of temperature within a room. The simplest experiment in proof of this is constantly made by multitudes of people, who, in crowded assemblies, find their heads surrounded by warm and moist vapors, reeking with offensive odors, while their feet are chilled, though near the “hot-air register.”
A library, whose walls were 12 feet high, and whose floor—18 by 15—contained 270 square feet, was constantly warmed by a “Latrobe heater,” placed in the chimney at one end of the room. The pot holding the coal was raised one foot above the level of the floor, which was covered by a woollen carpet. Immediately under the library was a kitchen, whose temperature was kept at about 72° F. Three thermometers were placed thus: No. 1, standing on the carpet near the centre of the library floor; No. 2, three feet, and No. 3, six feet, above it. At the expiration of half an hour, No. 1 indicated 62°; No. 2, 66°; and No. 3, 72°. Numbers 1 and 3 were then placed side by side with No. 2, three feet above the floor. At the expiration of fifteen minutes, all three indicated the same temperature of 66°. The low temperature of the inferior stratum of air was certainly not due to that of the room beneath it, for that was above 70°. It was only the heavier, colder air of the room itself, and of adjacent apartments warmed in the same way, slightly affected by contact with the stratum of warmer air above it.
Such slight differences of temperature in small apartments could not greatly affect the transmission of “the pulse of sound.” But in larger and loftier rooms, like churches and public halls, corresponding differences of temperature would, and do, produce air strata widely different in density and elasticity, and occasion serious acoustic defects. But the acoustic requirement is not satisfied by uniformly elastic air alone; for its pulses are reflected, and unity—distinctness—of sound, is lost in echoes or reverberations, from windows, columns, floors, and ceilings.
To know the difficulties to be encountered is always a step towards their alleviation; and these are sufficiently apparent throughout the little volume before us. They are, First, inelastic air—which cannot transmit its pulses to the ear. Second, strata and amorphous volumes, of unequal densities, which transmit the air-pulses with unequal force; so that they produce distinct sounds and indefinite murmurs at equal distances from the initial pulse. Third, reflecting surfaces—the floor, the ceiling, walls, columns, and furniture of the auditorium; which variously reflect the waves caused by air-pulses, and produce effects analogous to the eddies and whirlpools made by conflicting currents of running water.
The first and second of these difficulties are clearly within the province of “heat and ventilation;” and any means by which a constant tidal flow—not a current—of wholesome air, from floor to ceiling, may be produced, and by which the unwholesome, inelastic, heavier gases generated in crowded assemblies shall be prevented from accumulating but be forced to give place to the purer air, will practically solve the problem which they present.
The third difficulty is purely architectural. While surfaces reflect what are called pulses of sound, and so multiply their effects, they also create conflicting waves, which partially neutralize each other, or else strike the ear in irregular succession, to destroy the unity and harmony of sound. We cannot have buildings free from the inconveniences of walls, floors, and ceilings; but we can regulate and utilize surfaces to give aid in the transmission of air-pulses in one direction, and greatly diminish the reflecting power of those that would give back conflicting waves of air. A sounding-board or arch, whose lower surface should be a semi-paraboloid, so placed that a line drawn from its highest points, and parallel to its axis, would pierce the opposite wall four feet above the floor, while the axis itself should attain the same height at a distance of forty feet from the focus, would be an example of what we mean by utilizing surfaces to transmit air-pulses in one direction. The employment of an inelastic substance, like coarse felt, between the furring of a wall and the lathing, would undoubtedly tend to destroy its ability to reflect the “pulse of sound.” And hollow cast-iron columns, filled with clay, would hardly vibrate from a pulse of air.
In one of the Protestant churches of our city, we were shown a sounding-board, whose authors seemed to have halted between the acoustic merits of the paraboloid and the graceful shape of the pilgrim’s scallop-shell. We were told that “it helps the voice of the preacher.” There seemed to be too much of it for ornament, if its principle be wrong or inefficient, and too little for usefulness if right. Many attempts to improve the acoustic properties of halls designed for public lectures are failures through faulty execution of correct designs.
We once saw the working-plans of a lecture-room, where the line of intersection of the end wall with the floor of the stage or platform was a parabola, the arch above and behind the lecturer’s desk being a semi-paraboloid, springing from the wall at the height of the speaker’s voice. Thus, it was supposed that the pulses reflected from the walls and arch would proceed in parallel lines or “waves of sound,” because the initial pulse would always be given at the focus of the reflector.
The place of every joist in the cylindrical wall was carefully marked, and the dimensions and place of each rib of the paraboloidal arch accurately given. But in executing the design, the builder discovered a mistake!—“the floor of the stage would not be a true circular segment!” So he “corrected it”—with stunning effect upon the lecturer, and to the utter confusion of his audience. And the design was pronounced a failure.
In looking through the work before us, we almost unconsciously began to say: “This is nothing new; we have seen this, and more than this, before.” And in the same sense, we suppose it might as well be said that nothing is essentially new.