ACOUSTICS AND VENTILATION. [52]

ACOUSTICS AND VENTILATION. [52]

Every effort to elucidate what is obscure, or to provide a remedy for acknowledged evils, is a just title to that friendly acknowledgment which the writer of this little book bespeaks. It is a step in the direction of progress. But it is of the highest importance in the attempt to impart clear ideas upon any subject, that they should be so distinctly expressed as to leave no doubt concerning the identity of their subject. Thus, in treating of sound, it seems to us that the question first presented is this: What is sound? Our author says that it “receives its vitality or its life through the air, and without air sound loses it and becomes extinct.”

We object to this statement of the origin of sound, as both unsatisfactory and indistinct. It implies that sound is something born and floating in the air, and external to the mind perceiving. We fancy that, without an ear to hear, sound would not become extinct, but have no existence; and that the vitality of which our writer treats is not in or on the air, but in the mind itself. This exception to the supposed origin of the life of sound may not seem to affect the discussion of acoustics as far as the practical purpose of the architect is concerned; but we insist that neither the drumsticks nor the drum, nor the air within it or without, nor even all these at work, are sound, more than the telegraph wire and the electric current are the message sent from one operator to another.

That inaccuracy which we discover in our author’s use of terms, we find also in his quotations from others. For example: “The intensity of sound depends on the density of the air in which the sound is generated, and not on that of the air in which it is heard. A feeble sound becomes instantly louder as soon as the air becomes more dense. So you will always find, on great elevations in the atmosphere, the sound sensibly diminished in loudness. If two cannon are equally charged, and one fired at [from] the top of a high mountain, and the other in a valley, the one fired below, in the heavy air, may be heard above, while the one fired in the higher air will not be heard below; owing to its origin, the sound generated in the denser air is louder than that generated in the rarer. Peals of thunder are unable to penetrate the air to a distance commensurate with their intensity on account of the non-homogeneous character of the atmosphere which accompanies them; from the same cause, battles have raged and have been lost within a short distance of the reserves of the defeated army, while they were waiting for the sound of artillery to call them to the scene of action.”

It seems to us that the truth here expressed is not unmixed with error. In the very first sentence, we think that accuracy would require the suppression of the word not. The intensity of sound depends not only upon the density and elasticity of the air whose pulsation is an antecedent condition, but also upon the density and elasticity of the air through which the pulse is transmitted. While it is true that a pulse given to the denser column or stratum of air may be transmitted through a rarer medium with greater resultant force than if its origin and direction were reversed, it by no means follows that the intensity of sound is unaffected by the density of the air in which it is heard. We apprehend the truth to be that the pulse given to highly rarefied air is very feeble; and its secondary effect upon a denser and more elastic fluid, correspondingly slight; while the pulse from the denser air would be transmitted with greater—but still diminished—force, through the rarer atmosphere in which it reaches the ear. An absolute vacuum could not transmit the pulse given through a column or stratum of elastic fluid. A rarefied atmosphere could but transmit it with a force always varying with its own elasticity. And were it possible to preserve one’s consciousness within the exhausted receiver of an air-pump, we doubt if the most sensitive ear could be made to hear the roar of a cataract without.

“A feeble sound becomes instantly louder as soon as the air becomes more dense;” but not as loud as if the same initial pulse were immediately given to the denser air. In the case of two cannon equally charged, one of which is fired on the top of a mountain, and the other in a valley below it, to say that “owing to its origin, the sound generated in the denser air is louder than that generated in the rarer,” sounds much like saying it is because it is. If it be more than this, it is wrong. It is a clear case of non causa pro causa. The origin [of the pulse] of sound is in either case the same: the explosion of equal charges of gunpowder, in guns supposed to be of like material and equal size. The effects are not the same, because the effect of a force depends upon its transmission as well as upon its origin.

Does the atmosphere “accompany” peals of thunder? Or does this expression convey a distinct idea of the office of the atmosphere in the production of sound? We understand that the atmosphere receives the pulse or blow, and that its transmission to the ear is due to the elastic force of the intermediate air. It is not the homogeneousness of air, but its elasticity which transmits the pulse. And though, in architecture, the object sought is a uniformly elastic air throughout the auditorium, it does not follow, nor is it even desirable, that the maximum effect at a given point should be obtained by it.

“Science,” says our author, “teaches us that, whenever a shock or pressure of any sort is suddenly applied to material of any nature, whether metal, wood, gas, water, air, etc., it is immediately affected in all its parts, from the point of contact to the whole extent of the material, in displacing and replacing the particles of a determinate volume; and the velocity of the movement of the particles of the mass, created by the concussion of shocks or pressure, depends solely (?) upon its elasticity and density. Sound likewise causes motions (?) with every particle of the air, and as far as the motion reaches; so that each particle, with regard to that which lies immediately beyond it, is in a progress of rarefaction during return.”

What is meant by affecting a mass of matter “in all its parts,” by “displacing and replacing the particles of a determinate volume,” we do not precisely understand. That whatever causes motion does it “as far as the motion reaches,” is as unquestionable as any other identical proposition. But that the velocity of the movement of the particles, created by the concussion of shocks, pressure, upon an unconfined elastic fluid, depends solely upon its elasticity and density, we dispute. That pulses “are propagated from a trembling body all around in a spherical manner” may be true, if the air is on all sides equally elastic. Such might be the case with those produced by the vibrations of a bell, when the surrounding air is undisturbed by other causes, and is uniformly elastic at equal distances from it. It would not be strictly true if the initial pulse were made only in a certain direction. “Every impression made on a fluid is propagated every way throughout the fluid, whatever be the direction wherein it is made;” but it is not true that the impressions are equal at equal distances from the initial pulse, irrespective of its direction. This result would presuppose a fluid perfectly elastic; which we never have—and then we might, with equal truth, say that the impressions would be equal at all distances.

Everybody is familiar with the fact that the “transmission of sound,” the pulse which strikes upon the ear to produce the sensation, is affected by currents of air—the direction, force, and velocity of the wind—between the initial pulse and the hearer. How? and how much? directly or indirectly? are questions distinct from the fact itself. The distance through which guns are heard, as well as the loudness of their report, varies with the direction, force, and velocity of the wind; and, in very still air, with the aim of the gun itself, the direction of the initial pulse. For short distances, these differences may be so minute as to escape notice; just as the false proportions of a miniature picture are unobserved until the magnifier displays them. And for longer ranges, they are so small, in contrast with the magnitudes compared, as to seem rather like accidental than legitimate differences. But the difference is not the less real because the reality is less. Words spoken in a faint whisper are clearly heard by a listener immediately before the speaker, when quite inaudible or indistinct to one at an equal distance behind him.

The actual velocities of wind and sound differ so widely that the small fraction by which their relative velocity is denoted is held as proof that the propagation of sound—the pulse—through distances of a few yards or feet, is not affected by currents of air: that there are no differences in the “velocity of sound.” Yet the ear detects them as one of the small differences between discord and harmony in music; distinctness and confusion of speech. In music these differences may be blended by the prolonged intonation of vowel sounds; but in speech, whose distinct significance is due to consonants, “which cannot be sounded without the aid of a vowel,” these differences are fatally evident. The sharp edges of the vocal pulses, which give shape and meaning to vowel sounds, are destroyed alike by a husky voice and a puff of air. What remains is vox et præterea nihil.

It seems to us that some of the many failures in practical acoustics come from considering the air—the material involved—as perfectly elastic. From this it is inferred that sound is not affected by the direction of the initial pulse: that the direction and velocity of the effective pulse are not varied by currents and blasts of air. In short, that the slight inaccuracy of these assumptions will be the actual measurement of resultant error.

Were the purpose only to ascertain the acoustic properties of unadulterated air, varied experiments might eliminate the errors of anomalous results. But when the process is reversed, and we deduce effects from only one among concurrent and conflicting causes, theory is confounded by discordant facts. Theories of sound in purely elastic air might give results approximately realized in practice, if the actual pulses with which we are concerned were given by a flail; but are pregnant of error when the atmosphere is mixed with vicious vapors, and the pulse is a breath of air. Then, the assumption that “pulses of sound” proceed equally in all directions from the initial point, is simply false; and theories based upon it can only complicate the problems to be solved.

Water, as well as air, is a highly elastic fluid, and, if confined and subjected to pressure, the force applied is exerted on all sides of the confined volume. But the effect of a pulse or blow upon a surface of large extent varies with the direction of the force as well as with its power and velocity. We have seen fish swimming near the surface killed or paralyzed by a blow upon the water immediately over them. And we have seen the blow fail of its intended effect solely because it was misdirected. Perhaps the water in the latter case was not perfectly elastic! Neither is the air of churches and public halls, when their atmosphere has yielded a portion of its oxygen, and, in return, is charged with carbonic acid and moist vapors from the breath of crowded assemblies. Carbonic acid gas is heavier by one-half than atmospheric air. It does not, then, always rise toward the ceiling or roof, but remains in solution with impure exhalations; or else, condensed by contact with the colder walls, descends to poison the lower air and impair its elastic force—its power of transmitting the “pulse of sound” to the ear.

We have just come from one of our city churches, where we have had a striking example of this result. The church in question will accommodate (?) about two thousand people. Twenty-five hundred may be crowded into it. At the commencement of the sermon, the preacher’s voice was distinctly audible at points fifty or sixty feet from the pulpit, in spite of reflections of sound—air pulses—from galleries, wooden columns, and the arched ceiling and side-walls, of lath and plaster. Before it was ended, the exhalations of the breathing crowd had so filled the lower half of the “auditorium” that only vowel sounds could be distinguished; and the peroration seemed to consist of spasmodic utterances—scarcely sounds—of a, e, i, o, u. W and y had lost their affinity to vowels, and the rest of the alphabet were no longer consonants, for they were not heard at all.

The acoustic and sanitary problems are here identical—to find a method of preventing an accumulation of foul and inelastic vapors around the breathing and listening congregation, and to give, instead, wholesome air to their lungs, while enabling their ears to hear. And since these poisonous and inelastic gases are specifically heavier than atmospheric air, and must fall to the floor by their own weight, the problem is reduced to providing a practicable way for their escape, and guarding it against counter-currents which might obstruct the passage.

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.

We have lately seen a notice of an invention for tracing patterns on glass by means of a jet of sand. Of course, it is nothing new. The wind has been doing the same trick with the sand of the sea-shore for ages. We have seen it long ago, and often. Doubtless, the same effect has been noticed by many others. A thought of the possible utility of a process whose result was seen may have flitted through many minds, and, like the outline of a passing cloud, have been forgotten as it passed. But honestly, we never thought of tracing lace patterns on glass by any such process. And while new combinations of well-known truths give new and useful results, we hope they may never cease to be made.

Mr. Saeltzer’s book is full of good hints. But that is not its chief merit. It recognizes the inseparable connection of sound and ventilation, and insists upon observance of the laws which govern them. As he is so evidently alive to the sanitary and acoustic defects in public buildings, we shall be disappointed if his little volume does not prove to be the preface to more specific, practical directions for their removal. He has put his finger upon the principal cause of failures. The laws of light, and heat, and sound are sufficiently understood to render their phenomena as controllable as time, space, and velocity in mechanics. The more intelligent efforts are therefore directed not to the discovery of new principles involved, but to utilize what knowledge we possess. And when the effort is made at the right point and in the right direction, we can heartily say, Go on and conquer. The world is full of wonderful monuments signalizing defeat. Let us see just one crowned with victory.

As yet, modern ecclesiastical architecture, especially, is but the imperfect reproduction of ancient and mediæval models. It is the heathen temple or the Gothic minster, or, more recently, an attempt to vary the monotony with Byzantine forms of old basilicas, without their grandeur. In decoration, we have crude, unmeaning imitations of Moorish tracery, weak in imagery of form and symbolism, without those glowing contrasts and harmonies of colors which are to architecture as rhythm to poetry of sound. We know the cause and history of this poverty in constructive and decorative art. History tells us how men became so spiritual, in their own conceit, that symbolism was held to be a sin; and how, by losing the sign, the thing signified was forgotten or denied. But it seems almost unaccountable that the world should be teeming with philosophers, to whom the laws of nature, even their least tangible phenomena, seem familiar as things of daily use, while great temples are so constructed that they who have ears to hear cannot hear.