Projection engineering (Sept 1929-Nov 1930)

Projection Engineering, May, 1930 Page 19 The calculation of the total absorption of the empty room is made as follows, the computations being carried to the nearest integer : Wood floor 4,550X0.03=137 Plaster ceiling 4,500 X .03=137 Plaster walls 5.320X .03=160 Stage opening (no furniture, bare walls).. 600X .25=150 Wooden seats 500X .1 = 50 Total absorption of empty room =634 In the case of a half audience we must add 250X4.7=1,175 and subtract the absorption of 250 seats at 0.1, giving the net addition of 1,150 absorption units, and bringing the total absorption of the empty room up to 1,784. The reverberation time for half audience is then found by the formula 0.05X100,000 =2.8 seconds 1,784 which is considerably too large, the acceptable range for this size of room being (by Table 1) 1.5 to 1.8 seconds for a half audience. For full audience we add to the absorption of the empty room 500X4.7= 2,350 and subtract 500X0.1=50, making a net addition of 2,300, giving for the total absorption of the room 2,934, with a reverberation time of 1.7, a little in excess of the upper limit of 1.5 in Table 1, but not seriously so. However, as a maximum audience can not always be relied upon, it is well to add absorbing material to the walls to reduce the reverberation time in the case of the half audience at least to the upper limit, 1.8 seconds. This would require a total absorption given by the formula 0.05F 5,000 •4 = = =2,777 absorp t 1.8 tion units. The value of A for half audience has been previously found to be 1,784; hence 993 units of additional absorption are required. The choice of absorbing material is a question of price and appearance. Suppose it is decided to use a hypothetical material of coefficient 0.25. The coefficient of the plaster which this covers is 0.03, hence the net coefficient of added absorption is 0.22. To obtain a total absorption of 993 units would require the application of 993 = 4514 square feet of material. 0.22 This is slightly in excess of the ceiling area and much greater than the available wall space. The best practical solution would be to distribute the material as uniformly as possible, filling wall panels only (if such exist) and placing the remainder on the ceiling in some acceptable pattern which shall cover the whole ceiling. Distributing the absorbing material in strips or patches has the added ad vantage of reducing somewhat any echo that may exist, as the reflected sound is thereby broken up. The application of this absorbing material will reduce the reverberation time for full audience to 0.05X100,000 = 1.3 seconds 2,860+993 within the allowable range, though near its lower limit. In the above example it will be noticed that all of the added absorption was placed in the auditorium and none on the stage. Experiments conducted by Watson indicate that both speakers and musicians prefer reflecting surfaces about them to intensify the sound while the listeners prefer absorbent material in their neighborhood. It is generally considered that the reverberation time is independent of the positions of the absorbing material and of the source of sound. In general, this statement is true, but there are some important exceptions. For instance, if the absorbing material is placed back under a balcony or in some place where the sound intensity is low the absorbing material will not be as effective as if it were placed where the intensity is greater. There are also some cases where between large parallel nonabsorbent surface the sound may be reflected back and forth for a longer time than is to be expected from Sabine's formula. The sound in this case does not resemble ordinary reverberation but may be described as a "flutter," and is heard only when the listener is between the parallel surfaces. Exceptions of this kind are rare, but must occasionally be dealt with. Planning an Auditorium In planning an auditorium we must consider three factors — shape, size, and interior finish. As stated in discussing echo, the design of an auditorium should avoid curved walls or ceilings. An attempt to introduce such features for their artistic effect is almost certain to be detrimental to the acoustic quality of the room. Auditoriums of a rectangular shape have been the most uniformly satisfactory. Prior to Sabine's work there was current an idea that there should be a certain ratio existing in the dimensions of the room; just what ratio no one seemed to know certainly. Sabine quotes several different recommendations. Modern opinion regards such a ratio as immaterial unless, of course, it be carried to an absurd extreme, such as a very long and narrow room. The question of size must be determined principally by the purpose for which the room is to be used and not by considerations of space available or seating capacity desired. True, modern amplifying practice makes it possible to use a very large auditorium for speaking, but the present discussion is limited to the consideration of natural features and characteristics. The alteration of quality and the noise introduced by amplifiers are such that they will require much improvement before they will be acceptable for the rendition of anything in which artistic quality is a prime requisite, and for this purpose unassisted auditoriums will for a long time, perhaps always, be the rule. Generally speaking, a theatre must be moderate in size, while an auditorium for musical numbers, such as orchestral or choral performances, may be much larger. Such performances usually include several vocal solo numbers and this rather limits the size of the room. Experience with existing auditoriums leads to an empirical rule connecting the volume of the room with the maximum number of orchestral instruments suitable. This rule is expressed in Table 4. No distinction is here made between wind and string instruments, which are supposed to be present in balanced quantity. In case the orchestra is reinforced by the organ due allowance must be made. The new music room at the Library of Congress is a case in point. Its volume is about 100,000 cubic feet. At the opening concert there was present an orchestra of 26 pieces, which, with the organ, produced an excessive reverberation perceptibly spoiling the effect of sudden pauses after a loud chord. The indicated limit for this room is, perhaps, 12 or 15 pieces with the organ. As to interior finish, this should be planned with both echo and reverberation in mind. A liberal use of coffering on ceiling and sloping upper walls should effectually prevent echo from this source, and the interior finish should be calculated to give a reverberation time as indicated by the average range in Table 1, using panels of absorbing material in such quantity as may be necessary to reduce the reverberation time to a suitable value. Such materials, of several kinds, are now available commercially. Table 4 Number of Volume of room instruments 50,000 10 100,000 20 200,000 30 500,000 60 800,000 90 Bibliography 1. Sabine, Collected Papers on Acoustics, Harvard University Press; 1922. 2. Watson, Acoustics of Buildings, John Wiley & Sons, New York ; 1923. 3. Eckhardt, The Acoustics of Rooms, J. Franklin Inst. ; June, 1923. 4. Swan, Architectural Acoustics, J. Am. Inst. Architects ; December, 1919.