The motion picture projectionist (Nov 1931-Jan 1933)

November, 1932 Motion Picture Projectionist Sound Absorption and Materials In the discussion of sound absorption and its relation to theaters I have limited myself to the concepts and materials in commercial use today. When absorption is spoken of we mean the change of sound energy into other less objectional forms. This change of energy might be into the form of heat or mechanical work but which ever it is we are rid of the annoyance caused by the sound energy that is excessive reverberation, troublesome reflections, etc. Sound impinging on a diaphragm of a microphone is transformed into mechanical work while sound impinging on a finely porous material is transformed largely into heat by friction. The use of these acoustical facts especially the latter, enable us to correct and to build acoustical correct rooms in which all the auditors can hear to a perfection. Having a transforation of energy it is only natural that the scientific mind should turn to a means of measurement and expression so that the engineer and acousticians can use it in their work. Wallace Sabine, one of the pioneers in the study of acoustics, devised a unit in which absorption could be expressed. He compared all absorbing materials to an open window, since all the sound impinging on an open window passed through. He gave the open window a coefficient of one or one hundred per cent absorption, based on an area of one square foot. Now anything which absorbs only part of the incident sound energy will have a coefficient less than unity and will mean that per cent absorption as compared to a square foot of open window. Methods of Measurement There are several methods of measuring the coefficient of absorbing materials. The reverberation method in which the reverberation time of the bare room is obtained, the material to be tested is then placed in the room and the reverberation time is again obtained. From the two time values one can calculate the value of the absorption. This is the method used at the various laboratories throughout the country. It has several disadvantages in that it requires a very large sample of the material and a considerable number of measurements. Another method makes use of the well known law of optics, in which the angle of reflection equals the angle of incidence. Sound is allowed to impinge, from an angle, on the material to be tested. The ratio of the reflected intensity of the material to the reflected intensity of a non-absorbing blank, is considered a measure of the absorp *U. S. Gypsum Company. From a paper cad before the Chicago Section S. M. P. E. By GEORGE W. BAKER tion of the material. The apparatus must be calibrated against samples of known coefficients. This method requires small samples consequently it can be used to study the variation of absorption coefficients between individual small tile. Another method is known as the tube measurement in which the sample is placed at one end of a small diameter tube and a source at the other end. Standing waves are set up in the tube and the points of maximum and minimum pressure are found by means of an exploring microphone. From the two pressure values and their respective distances from some known point on the tube, one can get the absorption coefficient. This apparatus must be calibrated by samples of known coefficients. This method is not very favorable because of mechanical difficulties which present themselves to the technician. The mineral wood sample, manufactured from a slag blown into cold water, has a high coefficient of absorption, i.e., about .82. Due to its dustiness it must be enclosed in a cloth sack. However, this sack does not have an appreciable effect on the coefficient since the sound pulse passes through the cloth, as if it were not there. The pad formed is placed behind a perforated metal pan and the whole suspended from the ceiline. Sound Absorbing Materials The sample of acoustone is made from mineral wool and other substances mixed with a binder and cast into molds. The material can be made into various shapes and by the addition of various dyes which can be made into a very decorative product. The absorption coeicient is about .61. Due to its surface and the various shapes and colors into which the tile can be made, it is used as a ceiling as well as a wall decoration for sound treatment. The material may be spray painted, several times before the absorption coefficient begins to decrease appreciably. There are many forms of sound absorbing products such as hairfelt, flaxolinum, cork board, masonite, insulite, pyrocell, celotex, etc. Celotex is a shredded wood product drilled with holes at a definite spacing. These holes form an access into the interior of the material more readily and thus the higher value of sound absorption is obtained, (coefficient between .63 and .70). The material has the advantage in that it can be brush painted as many times as necessary without suffering a decrease in absorbing power. The holes must not be filled with paint. Weatherwood, masonite and presswood come under this form of absorbing material. The coefficient of the undrilled material ranges from .18 to .80 depending upon the porosity and thickness of the material. Reverberation Time I have been talking about absorbing material and the various coefficients; but what about how to use them and what are the most efficient uses for them. If we take a medium untreated room (volume about 10,000 cubic feet) with hardwood floors, plaster walls and ceiling, we will find that when we speak or play music, the various individual syllables or notes can be heard for several seconds after the sound has stopped. There will be interference between the notes as they follow one another. In talking to another person in this room we will find that we have difficulty in being clearly understood. This is due to the various reflections from the walls, floors and ceiling; in other words when the path of the direct sound from the source is considerably shorter than the path of the reflected ray, we hear the note as many times as it is reflected. Since the difference in path is related to time we have only to find the ideal hearing time as it were and use this time in the construction of further rooms. Again we are interested in the time necessary for the sound of an initial intensity of 60 db to decay to inaudibility. This time is known as the reverberation time of the room. When this time and the time difference in paths are correct in a room, ideal hearing is assured. Wallace Sabine, has given us a formula which is almost universally used in the calculation of reverberation time, but it does not apply to dead rooms, i.e., highly treated rooms. For our purpose Sabine's formula will suffice for the room we have under consideration. Though a great many observations on various individuals it has been found that if the time difference in paths of the direct and reflected sound is less than about .06 seconds, we will be unable to notice the phase difference between the two rays. By the same process of experimentation on rooms of various volumes it was found that an optimum curve or reverberation time could be drawn and from this curve the time for any room could be found. Use of Sabine's Formula In the room we have under consideration the optimum reverberation time is about 1 second. Substituting this time and the volume of 10,000 cubic feet in the formula we can solve for the amount of absorption. This absorption consists of the produce of various areas and their corresponding coefficients. In order to get the amount of absorption we need to have the room corrected, we must subtract the (Continued on Page 32)