Popular Science Monthly/Volume 21/August 1882/Acoustic Architecture
LATE FELLOW IN PHYSICS OF THE JOHNS HOPKINS UNIVERSITY.
IN the construction of a building in which large numbers of people are to be gathered together to listen to music or speaking, it is highly important to consider the conditions which shall best allow the sound to be carried from the musician or speaker to all of the hearers. It is the aim of the present article to place before the reader an outline of the art of so constructing buildings, and to present certain general principles upon which acoustic success depends.
The subject divides itself naturally into three parts. In the first is considered the effect of the condition of the air within an auditorium upon its acoustic qualities, and it will be shown, as would naturally be expected, that the condition of the medium which conveys the sound exercises a very considerable influence on the facility and accuracy with which the sound is conveyed. In the second part is considered the effect of the arrangement of the walls which inclose the auditorium, and of the materials of which these walls are composed. In the third and last part there are discussed several minor points, attention to which will aid in the securing of a building which shall be good and not bad for sound. These three headings, it is believed, will cover the whole ground.
It seems almost self-evident that the condition of the air, which is the medium by which sound is conveyed from one part of an auditorium to another, will exercise a considerable effect upon its acoustic properties. An experimental inquiry shows us that such is the case. What peculiar condition of the air is it that affects the transmission of sound? Whether the air is hot or cold, wet or dry, whether it contains a larger or smaller percentage of oxygen, nitrogen, or carbonic acid, seems to have no effect on its acoustic properties. But whether the air is quiet and mechanically homogeneous, or whether there are mingled draughts of hot and cold air moving in various directions, does seem to have a considerable effect. In other words, the motion of the air within an auditorium does have a very perceptible effect on its acoustic qualities.
Probably most readers have noticed, or in any event they have seen recorded, instances in which sounds of very ordinary intensity have been heard, and heard distinctly, at a very considerable distance from the source. In particular, the author remembers an instance which came to his notice one summer afternoon, while resting half-way up the side of one of the hills near the Green Mountains. The hill sloped gently to a meadow at the foot, and its sides curved somewhat like the walls of an amphitheatre, of which the meadow was the floor. Nearly a mile away, across the meadow, a man was mowing grass with a mowing-machine drawn by horses. The day was slightly cloudy, and from the mower, up the side of the hill to the observer, was moving a slight, hardly perceptible breeze. The air was optically very clear, and appeared to be rather dry. The click of the mowing-machine was heard with wonderful distinctness, but the author was not a little surprised when, the machine having stopped for the moment, the "Go along!" of the driver was plainly heard as he urged his horses on. Not only were the words plainly intelligible, but the provincial twang peculiar to the country-folk of that region was distinctly distinguishable. Here the human voice, raised probably very little above the ordinary tone, and not at all above that of a preacher in his pulpit or an actor on the stage, was distinctly heard, and with all its peculiarities of quality, nearly a mile away.
Another instance is that mentioned by Sir John Ross in his account of his voyage to the polar regions. He says, "I found no difficulty, in that cold and quiet air, in conversing with a man a mile away." It will be noticed that in both of these cases the air was mechanically homogeneous; that is, there were no alternating currents of hot and cold air.
In striking contrast with these may be mentioned the condition of the air as a vehicle for sound in the burned district of Boston, just after the fire had swept over it. There were many places, where there was a mixture of hot air, smoke, steam, and currents of cold air, in which the shouts of two people hardly a hundred feet apart, although audible, were so confused and indistinct as to make communication entirely impossible, and this too in quiet parts quite remote from the scene of conflagration. This effect was noticed when there were mingled currents of hot and cold air. That is, the air was mechanically heterogeneous. Humboldt speaks of the great difference in transmissive power for sound of the tropical air during the day and at night, and attributes this difference to the homogeneous condition of the air at night as compared to its heterogeneous condition in the day-time, due to convection-currents rising from the heated sands.
A large number of instances might be cited, and we should find that a clear homogeneous air transmits sounds readily, while an atmosphere broken by alternating convection-currents of warm and cold air is a very poor vehicle for sound. The explanation of this is not difficult. The original ray of sound, striking upon the first current of air, is partially reflected and partially transmitted.
The loss of the reflected portion causes a decrease in the intensity of sound. The transmitted portion, striking upon a second current, is likewise divided, and its transmitted portion continues to be so divided as many times as there are variations in the density of the air. Its reflected portion, as well as that of all the succeeding reflections, instead of being wholly lost, is interrupted in its backward course by the first current of air, and reflected along the path of the primary wave, but following it at an interval of time, depending upon the thickness of the current of air. Each reflection being thus again and again divided and reflected, we have, following close upon the primary wave, a multitude of secondary waves, which, falling later and later upon the ear, greatly mask the distinctness of the original sound, and give rise to indistinctness and confusion.
It is evident, then, that in order to procure the proper propagation of sound, one must do away with these air-currents. It must be remembered, however, that, when large numbers of people are crowded into halls, the air within is usually subjected to very considerable disturbances in order to obtain even indifferent ventilation. Registers, sprinkled here and there over the floor, send up their currents of hot and cold air. Opened windows or other cold-air ventilators send in their currents of cold air. If these currents could be made visible to the eye, as they can be detected by proper apparatus, we should see 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.
C (second balcony).—8 to 8.50, good; 8.50 to 9.20, bad; 9.20 to 10, good.
D (second balcony).—8 to about 9, good; 9 to 9.20, bad; 9.20 to 10, good.
An examination of the foregoing data can leave very little doubt that the condition of the air within an auditorium exercises a very considerable influence upon the facility and accuracy with which sound is conveyed.
The Academy of Music, in Baltimore, is an example of how a desirable condition of the air may be obtained. It must not be supposed, however, that acoustic success depends entirely upon the condition of the air. In fact, the condition of the air is a matter quite secondary to that which next comes up for discussion—the material and arrangement of the walls.
It is not uncommon to find churches or halls built nearly square, with a speaker's desk at one end, and a bare wall of stone covered with plaster opposite. If one goes into such a room when it is empty and speaks from the desk, he notices a loud and disagreeable reverberation following each syllable, which, if the room be large enough, comes back to him as a distinct echo. When the room is filled with people, this resonance or echo will considerably decrease; but, in such a room as we have described, it will not by any means disappear.
If now we choose a similar room, but with walls sheathed up with thin boarding or a thin layer of plaster laid on to light laths and having a free air-space behind, we shall, in all probability, find that, when this room is filled with people, the echo or reverberation will have almost if not quite disappeared. This is a comparison that the author has often made, and it gives the cue to the whole art of choosing the materials of which the walls are to be built. They must be built of such material and arranged in such way that they shall absorb and not reflect sound-waves falling upon them.
When the speaker utters his first syllable, the sound goes out in straight lines from his mouth to all parts of the house. So much of it as goes directly to the ears of the audience is effective, but all the other rays of sound ought to be as completely as possible absorbed and destroyed, else they will be reflected from the walls and ceiling back to the audience; and, arriving at their ears somewhat later than the direct sound, will give rise to the confusion or echo whose apparent effect is to bridge each syllable of the speaker over into the next, and so cause apparent indistinctness of articulation.
The presence of an audience in a room causes the absorption of such words as would otherwise be reflected from the floor, and thence to the walls, and so back and forth. But it is out of the question to cover the walls and ceiling with an audience. The absorption of the sound is, however, sometimes effected by draping heavily with cloths; but it has been found by experiment that there are other materials, more conveniently handled, which answer the purpose much better. A sheathing of thin pine-wood, lightly suspended, particularly if there be a large and free air-space behind it, will absorb sound very completely.
There is a very great difference in the absorptive power of different wall materials for sound. Walls of stone and brick absorb hardly any of the sound that falls upon them, but reflect it nearly all; while walls of thin and dry pine-wood absorb a very much larger proportion and reflect comparatively very little. In order to determine the absorptive power of different wall materials for sound, the author has made bold to extend the general principle in radiant energy, that "bodies which give out rays most readily when excited absorb them most readily when exposed to their action," to such rays as we have in sound. Suspecting from analogy that this principle might be true for sound-waves, he has proved by experiment that this is, at least in a general way, the case, and has then devised the following method of measurement:
If a tuning-fork be set in vibration and held against a wall, it will communicate its vibrations to the wall and the wall will give out a sound, which sound will be feeble or intense just in the same proportion that its capacity to absorb sounds falling upon it is feeble or intense. In this way the absorptive power of different wall materials has been measured, and a few of the results are arranged in the order of this power in the following table:
1. Thin and dry pine sheathing, lightly supported or in panels.
2. Corrugated iron.
3. Heavy wood paneling.
4. Thin, dry, and hard plaster on light laths lightly suspended.
5. Heavy plastering on laths lightly supported or attached to wooden Avails.
6. Heavy plaster on laths closely fitted to brick or stone walls.
7. Thin brick walls.
8. Thin brick wails covered with plaster directly laid on.
9. Thick brick walls.
10. Marble and other stone walls.
A comparison of this table with results actually obtained in churches in' which these different wall materials are used has amply proved their correctness.
There is one fact of considerable practical value that seems especially Worthy of attention. Corrugated iron, we see, has a very good absorptive power for sound, while it is durable, safe from fire, and can be easily worked into any ornamental forms desired. It seems, therefore, peculiarly fitted for the lining up of an auditorium. Bethany Church, on Franklin Square, Baltimore, is built entirely of this material, and is a decided acoustic success. We need, however, not only to attend to the material of the walls, but to their arrangement as well. While a plain blank wall opposite the speaker will throwback a strong reverberation, if this wall be broken up by recesses, or spaced with pilasters, or if a gallery be extended across it, the reverberation will be much less.
The lining of the walls should, if possible, be placed at a considerable distance from the main wall, and supported by it so as to allow as free vibration as possible. Thus when other considerations require that the walls of a building shall be of stone or brick, the acoustic qualities may be recovered by lining up with thin pine, corrugated iron, or a thin coating of plaster on light laths, and suspending this lining as lightly as possible and at a considerable distance from the solid wall. The arrangement of walls in panels is a very advantageous one, as each panel may be so constructed as to be easily set in vibration. Perhaps the ideal arrangement would be an auditorium with brick walls, within which is a shell made up of thin wooden panels, and placed at such a distance from the solid walls that the passage-ways to the entrances on the floor and various galleries maybe placed between.
In this connection it becomes necessary to discuss the proper shape to be given to an auditorium. A good way to arrive at this is to consider first an audience in the open air on a calm day. The open air on a quiet day, when the atmosphere is not disturbed by convection currents, is probably the best possible place for speaking to a large number of people. Wesley is said to have preached successfully to twenty thousand people gathered together in a natural amphitheatre formed by the hills, and on a day when the atmosphere was at rest.
Let us take a small platform arranged for an open-air speaker, and notice how an audience will form itself about it. If the audience is large and each person anxious to hear, we shall find that the outline of the crowd will be that of a section cut through an egg, with the speaker placed at the focus of the smaller end. As in an auditorium we trust to the natural diffusion of sound to absorb the stray rays, we should evidently adopt this same ovate section for the outline of the floor.
But, when we come to consider further that it is desirable to place each member of the audience so that he can see the speaker, and so that the speaker's voice may come directly to him, we see that we conform still further to the egg-shape, for, in order that we may do this, the floor must curve upward as it recedes from the speaker, and the galleries form only a continuation of this curve. To that we may say that the proper shape of an auditorium is in general that of an eggshell, the speaker being at the focus of the smaller end and the audience being seated over the lower half, while the upper half forms the vaulted roof. Like an egg-shell, we have seen that the walls should be thin and capable of absorbing, as fully as possible, all the stray rays of sound. While the egg-shape is the ideal, other considerations frequently require considerable departures, but it will generally be found that rooms constructed on this plan, if the materials of the walls and the condition of the contained air be secured, are acoustically good.
The Greek and Roman amphitheatres, having the audiences arranged in semicircles, each circle rising above the one in front of it, and having the auditorium either open at the top or covered with awnings so that the waste sound might easily escape at the top or be absorbed by the audience around the sides, were crude approaches to this plan. The modern theatres, in which the floor slopes upward as it recedes from the stage, and in which the balconies are placed one above the other and are of horseshoe form, conform still more closely to it.
Other considerations frequently demand that music-halls and churches shall be square or oblong in shape, and it must not be supposed that acoustic success can not here be obtained. But, in these forms, great care must be taken to avoid large reflecting surfaces, and, by means of paneling or other devices, to absorb fully the waste sound.
We have now examined the two important features on which acoustic success depends. The atmosphere must be in such condition as to best allow the natural diffusion of sound; and, further, the walls must be of such material, and so arranged, as to absorb as fully as possible the waste sound. It now remains for us to look at some of the minor points which contribute to acoustic success.
There is one danger to which buildings having a vaulted roof are peculiarly liable, and that is, that the roof, if constructed of proper curvature and of non-absorbing material, is apt to act as a great concave mirror to gather up waste rays of sound and reflect them back to a focus somewhere in the audience, and so produces a loud and disagreeable echo. The architect can not exercise too great care in selecting absorbing materials, and in so arranging them as to prevent this possibility. A change in curvature, or breaking by transverse arches, will often do this. This focus would be a small area in case of a dome; but, in case of an arched roof running from front to rear, it would be a straight line.
It often happens that churches constructed without regard to acoustic principles are found, when completed, to possess this fault in a striking degree. The only complete remedy in such cases is to entirely replace the ceiling. It may, however, often be largely alleviated by placing the pulpit in a different position, as near one corner or against the wall half-way. down one side. Sometimes the fault may be largely remedied by using a reflector to throw the sound out toward the audience and prevent its going up toward the roof.
Reflectors, or sounding-boards, should be used only with judicious care. Their object is not, like a concave mirror, to gather the rays of sound and throw them out to a focus in the audience. When so constructed they have been found to do more harm than good, especially when the focus is so distant that a strong echo is returned from the opposite wall. Their object is rather to control such rays of sound as would otherwise go up to the roof and be retained as a disagreeable echo. A perfectly flat surface placed over the pulpit and not too far from the speaker's head, will often do this; but it is better to have the wall back of the pulpit gradually curve forward until it completely overhangs the speaker's head. The speaker must be near to this wall, so that the direct and reflected sounds may be as nearly as possible identical.
There is a belief, prevalent among some architects, that a hall, in order to be acoustically good, must have its length, breadth, and height in harmonic proportion. There seems to be no good foundation for this belief, and the author, after careful experimental inquiry, has failed to find that this is the case.
It frequently happens that, in a building in which there is considerable resonance, the speaker, by timing his syllables so that the resonance of one shall have disappeared before the next is uttered, may make himself understood to a large audience with comparative ease. This is recognized by most public speakers, and it is not uncommon to hear them speak of the "key-note" of any particular hall.
The act of striking the "key-note" consists not so much in pitching the voice at any particular key as in carefully timing the rate at which the syllables succeed each other.