Projection engineering (Sept 1929-Nov 1930)

Page 18 Projection Engineering, May, 1930 Balconies may require special consideration, especially if low and deep. As a rule, this type of balcony should be avoided, as the sound intensity is sure to be diminished at the rear under the balcony, and may be so low that hearing will be difficult. Reverberation A sound produced in a room is reflected back and forth from walls, floor, and ceiling, a portion being absorbed at each reflection until its intensity is so reduced that it becomes inaudible. Owing to the high speed of sound there may be many such reflections in the course of a single second in a room of ordinary size; and the greater the dimensions of the hall the more prolonged will be the reverberation. If the walls of the room are covered with sound-absorbent material, such as hair-felt or acoustic plaster, a few reflections may suffice to destroy the sound. Such a room is acoustically "dead" and undesirable. A little reverberation is necessary to satisfy our established tastes and auditory habit, and the desired amount of reverberation is found empirically to increase with the size of the auditorium. It is customary, since the pioneer work of Sabine, to define the "reverberation time" of a room (perhaps, somewhat arbitrarily and artificially) as the time taken for a sound of specified intensity to die away to inaudibility. This standard intensity is a sound somewhat difficult of reproduction. Fortunately, its use is not necessary in ordinary practice, for in most cases the "reverberation time" can be calculated with sufficient accuracy. The method of making this calculation will be explained later. Excessive reverberation is an evil because it prolongs unduly each syllable or note of music, causing it to interfere with the next. The ideal conditions for intelligibility of sound are two : Each syllable should die away before the next arrives, which in ordinary speech may be, perhaps, one-tenth of a second ; and the sound must always be loud enough to be heard. The first of these conditions can always be secured by providing enough sound-absorbing material in the room. For a small auditorium which can easily be filled by the speaker's voice, this is the most important consideration. For a very large room it may be that the amount of absorption dictated by the first condition is so great that the speaker can noot be heard in the back of the room. Since the intensity of the human voice can not be much increased it is necessary to compromise between these two conditions and to permit longer reverberation in larger rooms. In the case of theatres used for sound pictures this compromise is not necessary, as the acoustic output of the loudspeaker is by no means as limited as that of the voice. For such auditoriums there may be employed to advantage a somewhat shorter reverberation time than is desirable for rooms of the same size used for speaking or musical performances. Broadcasting studios may be equipped with variable absorption, consisting of smooth plaster walls covered with heavy curtains which may be pushed back exposing the wall when more reverberation is desired. Experience with a number of existing auditoriums of acceptable acoustic quality makes possible the formulation of the following table, in which the acceptable limits of the standard reverberation time are expressed for rooms of different volume, used for speaking or musical performances. Table 1 Volume of (in cubic 10,000 ... room feet) Acceptable limits of reverberation time (in seconds) Half audience 0.9-1.2 1.0-1.3 1.2-1.5 1.5-1.8 1.8-2.0 2.1-2.3 2.3-2.6 2.5-2.8 2.6-2 9 Maximum audience 0.6-0.8 25,000 . . . .8-1.1 50,000 . . . .9-1.3 100,000 . . 1.2-1.5 200,000 .. 1.4-1.7 400,000 .. 1.7-2.0 600,000 . . 1.8-2.2 800,000 . . 1.9-2.3 1,000,000 . 2.1-2.5 The limits given in the table are not to be regarded as rigid. Auditoriums are known which exceed these limits in either direction by several tenths of a second and yet are of fairly satisfactory quality. And, as mentioned above, large auditoriums used for sound pictures may advantageously be designed for a figure somewhat less than the minimum given in the table. Calculation of the Reverberation Time As a result of Professor Sabine's investigations we have a formula giving the reverberation time of a room. Let t = reverberation time in seconds, V — volume of room in cubic feet, A = "total absorption" of the room (to be explained later). Then the following relation holds 0.05F The only point that needs explanation in this formula is the quantity A. Different materials differ considerably in their absorbing powers for sound. The most complete absorber known is an open window. It is theoretically possible that a small amount of sound may be sent back by diffraction from the edges of the window, but this quantity is so small that it is permissible to say that an open window is a perfect absorber. A good absorber of sound may absorb, perhaps, half as much sound as an equal area of open window. In other words, if it may be said that an open window absorbs (or transmits) all the sound that falls upon it, its coefficient of absorption is unity, while that of the material above mentioned would be 0.50. In like manner, every substance may be said to have its own absorption coefficient. This constant was measured by Sabine for a number of common materials, and later workers have extended the list. Table 2 gives the absorption coefficient for several substances. Strictly speaking, these coefficients will vary somewhat with the frequency of the incident sound, and in Table 2 the values given are for a frequency of 512 (Watson). Table 2 Sound-absorption coefficients Concrete 0.015 Glass, single thickness.... .03 Marble .01 Open window 1 .00 Plaster .03 Stage opening (depending on furnishing) 0.25-.40 Ventilators (50 per cent open space) .50 Wood, varnished .03 Absorption coefficients for a variety of materials have been measured at the Bureau of Standards, and the results, at frequencies ranging from 128 to 4,096 cycles per second, are available upon request. Sound-absorbing materials are frequently changed by the manufacturers, and a printed list is consequently soon out of date. In Table 3 there are given values of the total absorption of individual objects as determined by Sabine. The unit of absorption in which these values are expressed is the absorption of 1 square foot of open window. Table 3 Total absorption by individual objects Audience, per person 4. 7 Church pews, Per seat . 2 House plants, per cubic foot. . . . 003 Seats, upholstered, depending on material and lining, per seat 1. 0-2. 5 Seat cushions, cotton, covered with corduroy, per seat 2. 2 Seat cushions, hair covered with canvas and light damask, per seat 2. 3 Settees, upholstered in hair and leather, seat and back, per seat 3. 0 Wood seats, for auditoriums per seat .1 As an example of the use of these coefficients let us take an auditorium of 100,000 cubic feet capacity, including the stage opening. There is a wooden floor of 4,550 square feet, a plastered ceiling with the same area, 5,320 square feet of plastered walls, a stage opening of 600 square feet, and 500 plain wooden seats. The coefficients for plaster, wood, and glass being the same to the accuracy requisite for this calculation, no special allowance is necessary for closed doors and windows.