Page:Catholic Encyclopedia, volume 17.djvu/22

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of reverberation which wiU produce a sufficient average audience conditions should be not more

volume of sound. than 1.9 seconds, nor less than 1^ seconds. For

That reflection, with its consequent reverberation, most forms of music it is desirable to have the is necessary, is readily understood if one considers reverberation exceed 2.1 seconds. When a room is the difference in volume of sound produced indoors to be used for both speaking and music, as in a and out of doors. Reflection is essential; too much church, it is usual to compromise, having the re- reverberation is detrimental. Just how much re- verberation slightly excessive for ideal speaking con- verberation will produce the proper effects is not ditions, and slightly less than that demand^ for entirely a matter for scientific demonstration, 1but ideal musical conditions. The solution can be is rather a decision to be reached by those who varied, of course, to suit the special conditions pre- have the correct taste. Inasmuch as the objection sented by each case.

to excess reverberation is the resultant confusion. Besides the duration of audibility, or time of

the eleipent of rapidity of succession in the sounds reverberation as it is often called, refraction of

to be differentiated enters into the question. Here sound waves, diffusion, interference, a lack of reso-

we have a hint that the requirements of an audi- nance, concentration of sound energy due to curved

torium depend upon its intended use. surfaces, comers, pockets, etc., must be considered.

Nearly all the investigators on the subject have However, it usually happens that if the duration of arrived at the conclusion that the time of reverbera- audibility can be reduc^ to a proper time all the tion in an auditorium depends upon the volume of other defects will automatically cease, since they the room and upon the absorbing power of the sur- are to a more or less extent its functions, faces. This will seem reasonable enoiigh when one As in most instances a long time of audibility is observes that if sound loses a part of its energy by directly due to the non-absorbing qualities of the reflection, then the fewer the number of reflections interior surfaces of the room, and our present con- per second, the less the loss per second and the struetion is tending toward harder and more rigid longer the soimd will last. An increase in the interior surfaces, wnich have a greater capacity for volume of the room of course increases the length reflecting sound; it is necessary in order to oDtain of the paths between reflections, thus decreasing good acoustical results to replace or surmount some the number of reflections and extending the time of of the reflecting surfaces with surfaces which have reverberation. Again, the amount lost by a reflec- the property of absorbing a great amount of the tion will depend upon the absorbing aualitv of the souna striking upon them. The surfaces to be re- surface. Consequently the greater the absorbing placed with absorbing materials must be scien- power of the surfaces, the less will be the time of tifically chosen, otherwise the effect will be nullified reverberation. Professor Sabine gave a numerical or discoimted; as the distribution of the absorbing value for the absorbing power of various materials, material in the structure is of greater importance

In certain known instances the duration of audi- than the quantity. Various combinations of mate- bility has been computed to be as long as twelve rials have been used as absorbing surfaces, but pos- seconds, which means that the ear is capable of sibly the best of these is matted hair felt about one hearing the same sound twelve seconds after the inch thick covered with a tightly stretched mem- source has become quiet. You can readily see how brane of light canvas, which latter, is secured to such a condition produces great confusion, indis- well braced and rigid frames, built before erection tinctness and discomfort. A deliberate speaker will and applied to the surface requiring the absorbing utter about four averaee lullabies per second, element. When properly decorated or painted, these Nature has pr<^vided a "factor of safety" of about materials serve a purpose equally as good as plaster, twice as many in the ear. That is, the ear can hear wood or other interior building materials, and are without confusion about ten syllables per second, architecturally practical.

Therefore, it can readily be understood that if an It is very important to have an air space of about

auditorium has a duration of audibility of more one-quarter of an inch between the tightly stretched

than two and one-half seconds, it is very close to memorane and the hair felt, since it has been found

the time where confusion of hearing will result. that it is not enough to have absorbing surfaces

The ideal duration of audibility in an auditorium alone to prevent reverberation, but the absorbing

varies greatly. It depends upon the uses to which surfaces should possess the quality of multiple ab-

the auditorium is put, and its size. The ideal time sorption, by being able to vibrate and thus nave a

of audibility is slightly less for speech than it is certain resonance to respond to the overtones of

for vocal music, and slight^' different for piano the sound waves, and give that quality to the audi-

music from orchestral, etc. Oftentimes it is found torium which corresponds to the resonance quality

that an auditorium, when empty, has a long time of a violin body or the sounding board in a piano,

of audibility which diminidies very rapidly as the It is the advance made in the study of how to ob-

size of the audience increases, and becomes prac- tain in the wall and ceiling covering, this important

tically normal when an average attendance is quality that makes it possible to predetemune the

reached. acoustical success of a church and to correct the bad

As the volume increases it is necessary to increase conditions which now exist in so many of our

the duration of reverberation. Unfortunately, a churches. Based upon the data which have been

reduction in the reverberation produces a cor- obtained in connection with hundreds of installa-

responding reduction in the intensity. For this tions throughout the United States and Canada of

reason, in a room having a volume of say 400,000 "acoustile" pre-built panels on the' walls and ceil-

cubic feet, it would not be advisable to reduce thp ings of churches and auditoriums, the number of

reverberation below 2.7 seconds. This duration of square feet of treatment required in the average

reverberation is slightly excessive for an untrained auditorium can be roughly estimated at from 2%

speaker but necessary to insure sufficient intensity to 3% of the number of cubic feet in volume, de-

in the furthermost parts of the room. In a small penoing on the sound-absorbing value of other

room there is less need to augment the sound, for materials in the auditorium.

the auditors are situated near the speaker and also It can thus be seen that regardless of the shape,

near to reflecting surfaces. In the average room it is so far possible to obtain good acoustical results

used only for speaking, when the volume is approxi- in a church, that the architectural design may in-

mately 150,000 cubic feet, the reverberation under elude domes, high groined vaults and other features