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1948-49 Theatre Catalog, 7th Edition, Page 338 (325)

1948-49 Theatre Catalog, 7th Edition
1948-49 Theatre Catalog
1948-49 Theatre Catalog, 7th Edition, Page 338
Page 338

1948-49 Theatre Catalog, 7th Edition, Page 338

sorb only a part of the sound spectrum, or an excess of material may produce too much over-all absorption.

A room treated so as to produce see lective absorption will give a very unnatural result. If the high frequencies are heavily absorbed the room will be boomy and the voice and instruments hardly distinguishable because of the loss of overtones. The degree of absorption should be about the same at all audible frequencies to maintain naturalness.

Many studios have been treated with material intended mainly for high-frequency typewriter or knife-and-fork noise, and the musicians have complained that these spaces are too dead, or too boomy. These studios have been corrected by taking out some of the high-frequency absorbent until a balance was secured.

The second excess is too much overall absorption, making the room approach zero-reverberations or ufreefield" conditions. Here we go back to outdoor conditions for which our music is not suited. An acceptable balance must be secured based on study of old halls, long used and liked, or new ones thought to be better because of public acceptance of the results obtained in them. Knowing the upper and lower limits has been a great help in setting up proper reverberation times for vari ous spaces. When a room is to be used for recorded sound, both the originating

studio and the hall should be treated slightly more than the rule calls for because of the additive effect of their reverberations. This is true of sound stages, motion picture theatres, and recording and broadcast studios.

Another fault commonly found in older theatres was echoes, or delayed repeats, of the original sound. These were generally caused by large domes, curves, and large flat surfaces. If a curved ceiling had focuses at the floor level or even multiples of this distance, disagreeable echoes were more than likely to spoil the seating down the center of the room. Most modern designers avoid these shapes and the domes of many of the old auditoriums have been heavily treated or new ceilings suspended under them with curves to kill the echo.

The modern theatre is now generally constructed of splayed surfaces which tend to straighten the path of reflected sounds passing to the rear. The back wall is not curved to focus near the stage unless it is very heavily treated, and upholstered seats are used to act as compensation absorption with small audiences. This last item keeps the reverberation from varying too much between full and empty conditions and does not require such a wide swing in gain to operate the horns.

The recording studio is a special problem in acoustics. First the sound should be so well distributed as to require only one microphone, or a very few in any case. If possible the reverberation should be quite fiat over the frequency range except for the high end where it should rise.

The scratch and circuit noises are still intense enough to make very wide range recording a little unpleasing. If the high


frequencies, that is, the overtones, timbre, and brilliance, can be built up excessively by reverberation and attenuated electrically back to normal level, a high signal-to-noise ratio will be maintained and the scratch considerably subdued. Such a treatment must be done carefully with as much attention to reflective surfaces as to the location and kind of absorbing surfaces.

Reflective materials are, generally, the floor which must be hard enough for its trafiic, plastered surfaces, plywood, and transite. Plaster, plywood, and transite may be formed into polycylindrical and faceted or prismatic surfaces. These scatter sounds for better mixing. The deeper the offsets, the lower the frequencies which are affected. The soundabsorbing materials generally have a base of rock wool Which will be covered by membranes of thin plywood, or perforated transite, to avoid too much highfrenquency absorption. For the same reason, great care should be taken to use sparingly such materials as carpeting and draperies.

The problem is virtually the same for the frequencyemodulation studio, but it it not quite so rigorous for the more general amplitude-modulation studio because of the millions of limited-range receiving sets already owned by the public. Most of these run almost entirely on bass notes and almost twenty-four hours a day. However, as equipment is improved, all of these amplitude-modulation studios will have to be adjusted for high-fidelity sound.

There is another problem connected with all studios and places where we listen to sound. This is background noise. Practically all theatres are in urban areas with plenty of traflic noise all around, and their problem has been solved by omitting windows, using heavy walls, and deep vestibules, heavily carpeted, and lobbies with double doors. We must expect to endure some objectionable background noise when we sit in the middle of a thousand or so people. Fortunately, we become psychologically deaf to their presence when the show is interesting. This is not true of a real studio. A recording or broadcast that carries with it traffic and fire-engine noise, or the sound from other spaces in the building such as presses, elevators, and plumbing is not interesting. The background noise in a studio should be such that the intended silent moments are really silent and not distracting.

Such conditions can only be attained by sound isolation. This work bears no relation to the acoustics of the room itself and is a matter of construction and location. A single studio may be built far enough in the country to avoid all noise except thunder and airplanes, yet it still must be isolated from its control room and recording room to prevent feedback and pickup of machine noise. Most studios are built in the cities to be accessible to talent, service, and other requisites. Here they must contend with other building noises and sometimes subways, heavy traffic, and near-by manufacturing.

There is no such thing as absolute soundproofness; at least not on earth. A body at absolute zero floating in an

absolute vacuum might be said to be absolutely soundproofed. Under special conditions we are able to hear sounds at some pitches down to a point a little below zero decibels, which is a sound intensity of 10 to 16 watts per square centimeter. However, as a person's body makes a noise of about 14 decibels just doing its regular work of pumping, digesting, and breathing we do not often experience quite below that.

Ordinarily a studio is considered good if the background level can be held to the twenties of decibels. Naturally, more soundproofing will be needed in a,noisy than in a quiet location. Suppose we build a wall which will attenuate sound 40 decibels. If the level around it is 70 decibels, the level inside the walls will be 30 decibels; if the level outside rises to 90 decibels when a truck goes by, the inside level goes up to,50 decibels. In such a location more soundproofing will be required.

The foregoing indicates clearly that a proposed studio location should be examined before construction is contemplated in order to determine the problems to be expected from extraneous sounds. This survey should include observations of the background noise level, the amount of vibration in the structure, its mass and rigidity, and its allowable floor loading.

A building on Third Avenue in New York was recently examined in order to determine if it could be used for recording studios. Because of the scarcity of space a thorough examination was made of it. It was an old building with lightweight Wood joist floors and ceilings which could not stand the weight of studio construction. The ceilings were so low that if we had used the necessary space for isolation and treatment, there would hardly have been room left for a bass viol. And besides, every time a Third Avenue elevated train passed by the whole building shook and the noise level rose to a point which made conversation impossible. We had to advise against signing the lease.

The weight of the structure is very important as the heavier the walls and slab construction, the better it resists vibration and passage of sound. A 12inch brick wall is much more sound proof than a stud partition, as everyone knows.

The majority of the better studios are constructed of the itroom-within-a-room" principle. The interior surfaces of the studio are iffloated" away from the actual building floor, ceiling slabs, and structural partitions. This "fioating" is accomplished by means of felt or spring isolators which carry or support the inner surfaces of plaster and floor slab.

Entrances to the studio are accomplished through soundproof doors. The ventilating system is fiexibly connected to the room and the ducts are lined with sound-absorbing material. The electrical conduits must be flexible, too, where they enter. Heavy double glass is used for observation windows. Each glass is of different thickness or tilted out of parallel with the other to reduce transmission.

After all of this is done the isolated studio is ready for acoustical correction of the interior which means the control
1948-49 Theatre Catalog, 7th Edition, Page 338