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1950-51 Theatre Catalog, 9th Edition, Page 35 (15)

1950-51 Theatre Catalog, 9th Edition
1950-51 Theatre Catalog
1950-51 Theatre Catalog, 9th Edition, Page 35
Page 35

1950-51 Theatre Catalog, 9th Edition, Page 35

cycles per second and its octaves up to and including 4,096 cycles per second. By custom, acoustic treatment for auditoriums and theatres is specified in terms of the sound absorption coefficient at 512 cycles per second, but there is a trend toward specification at additional frequencies.

An important point is that the absorption coefficient depends not only on the physical properties of the material, but also on how it is mounted. As a general rule, an air space between the absorbent and the rigid wall on which it is mounted enhances the absorption.

Measuring Techniques

Physicists and engineers have long struggled with the problems of how to define better and how to measure the absorption coefficient of a material. In Fig. 2 is shown a technique for measuring absorption when sound is incident normally, i.e., perpendicularly, to the surface. The source is a vibrating diaphragm located to the right in the diagram. The acoustic material is to the left. By measuring the sound pressure at the center of the surface, we can deduce, from a knowledge of the amplitude of vibration of the diaphragm, what the absorption coefiicient is for sound at normal incidence.

Variations of the above technique have been used. Sometimes the standing-wave pattern in the tube is explored with a probe tube and microphone. The important thing is that the general idea is always the same, namely, we measure the absorption coefficient when sound is incident perpendicularly on the surface. The coefficient obtained in this way is called the ffnormal-incidence absorption coefiicient." l

The basis idea of the other technique which is extensively used for measuring absorption is to have the sound incident on the material from all possible directions. To achieve this, the material is placed on the wall or door of a large, highly reverberant room. Sound of the desired frequency is introduced into the room, and the distribution of- sound energy is "randomized" by any one of several ingenious-methods. This means that sound rays strike the surface equally from all directions. When the sound field has become thoroughly randomized, the source is turned off, and the rate of decay of the sound energy is measured with microphones and a recorder. The absorption can be deduced from the measured rate of decay. The coefficient obtained with this technique is called the tirandom-incidence absorption coefiicient."

In Fig. 3 is shown the 15,000-cubic-foot reverberation room at the National Bureau of Standards. The sample, which is usually 72 square feet in area, is on the fioor. The loudspeakers which supply the sound are on the vanes. The vanes rotate while measurements are being made, and help to randomize the sound field. The microphones which pick up the sound are not shown.

IPrac'l'ical Applications

The important question is, how significant are the normal-incidence and ran 1950-51 THEATRE CATALOG

dom-incidence absorption coefficients in practice? As was pointed out earlier, the principal use of an acoustic material in an auditorium is for control of the reverberation time. Many years of experience seem to show that the reverberation time can be computed correctly if the random-incidence coefficient for the acoustic treatment in the auditorium is used in the calculations.

On the other hand, it is not really necessary to design a motion picture theatre for optimum reverberation time. The loudspeaker system can supply ample acoustic power, and hence large amounts of absorption can be, and usually are, installed in a motion picture theatre. Even though the normal-incidence absorption is more easily determined in the laboratory, it is still difficult, and in some cases it is impossible, to deduce the random-incidence behavior from laboratory measurements with normally incident sound. On the Whole, the conclusion at the present time is that the random-incidence absorption coefficient is more useful in auditorium design.

Measuring Difficulties

Some of the difficulties involved in deciding how to measure sound absorption can be appreciated by referring to Fig. 4. The sketch shows that the listener receives sound which is nonnormally reflected from the acoustically treated ceiling. If one wishes to compute the intensity of the refiected sound, neither the normal incidence nor the random incidence coefficients can be used! To make matters worse, the angle Wm of the refiected sound is different in different parts of the auditorium. The object in pointing out these things is to indicate the limitations on the absorption coefficients when an acoustic material is being chosen for a theatre.

FIGURE PThe effect of three coats of brushapplied paint on a typical acoustic plaster. The paint has caused a serious adverse effect on the absorption at high frequencies.


Since the majority of commercially available acoustic materials depends on porosity for sound absorption, it is clear that painting will present a problem. There is always the possibility that excessive painting will clog the pores and prevent absorption of sound.

Perforated or Fissured Types

In the case of porous materials having a mechanically perforated or fissured facing, there is no serious difficulty. Paint can be applied so long as the perforations and fissures remain open.

Non-Perforated Kinds

The painting of a porous material without large holes or fissures is more difficult. The paint must be applied as thinly as possible, preferably with a spray gun. If it is brush-applied, care must be taken to thin the paint and to get it on the surface without closing the pores.

Non-Porous Types

The non-porous, rubber-like materials can be painted, provided the paint does not substantially increase the weight of the facing. Too much paint will reduce the absorption of sound at high frequencies.

Examples of Painting Effects

The effect of painting on some typical porous materials can be seen from the following data obtained from a paper by Chrisler.2 Fig. 5 shows a fissured material which had a noise coefficient of 0.55 before painting. The noise coefficient is here defined as the average of the random-incidence absorption coefiicients measured at frequencies of 256, 512, 1024, and 2048 cycles per second.

Fig. 6 shows the same material after , it was brush-painted with four coats. The noise coefficient fell to 0.45 after painting, which is not a serious reduction. The reason for the success of the brush painting is that the material was fissured. The fissures which lead down into the porous material were not covered over, and a considerable amount of sound absorption remained after painting.

Fig. 7 shows a porous material consisting of organic and inorganic granules held together with a binder, and having a granulated surface. Before painting, the noise coefficient was 0.60.

Fig. 8 shows the same absorbent after five coats of brush-applied paint, when the noise coefficient fell to 0.25. This is a too-familiar horrible example of bad treatment of an acoustic material. The method of painting in this case should have been either with a spray gun, or with a thinned paint carefully brushed on so as not to close the pores.

Fig. 9 brings out the fact that excessive painting of a material has the worst effect on the sound absorption at high frequencies.


(1) The results of tests made at the National Bureau of Standards have been published in its Letter Circular III-870, "SoundpAbsorptlon Coefficients of the More Common Acoustic Materials."

(2) V. L. Chrisler, "Effect of paint on the sound absoyption of acoustic materials," I. Rm. Nut. Bur. Sum ., vol. 24, p. 547; RP 1298, 1940.
1950-51 Theatre Catalog, 9th Edition, Page 35