Theatre Catalog (1947-48)

the above procedure will give the correct amount of acoustical correction, it will be necessary to acoustically treat any surfaces on which the sound waves may impinge, such as the rear walls of the orchestra and balcony, the face of the projection booth, the balcony or stadium facia, and in some cases the soffit of the mezzanine or balcony. If the horns are properly flared it should not be necessary to use any acoustical treatment on the auditorium ceiling as the horns can be set so that the sound will not reflect from the ceiling. When a glass screen is employed at the standee rail, velour drapes on the orchestra side should be provided to prevent the reflection of sound. These should be hung on tracks so they can be opened for standees if required. Too much sound absorption is almost as objectionable as too little, since a “dead” house results and the amplification will have to be increased with consequent distortion. It is very important that this problem be solved early in the period when the interior is being designed as the required sound absorption treatment will very definitely affect the architectural result. Decoration Employing any one of the materials mentioned for acoustical treatment, the general decorative design can be developed with the use of run plaster or wood mouldings between panels, using curves or straight lines, and so forth. In the case of fabric materials, their own design may be used with a variation of shades in the different panels or may be used as an all-over pattern like wall paper. With the front portion of the auditorium and the proscenium arch area free of any restrictions so far as materials are concerned, the designer is not limited as much as it might first appear. However, it must be remembered that this room will be seen, for the most part, only under subdued lighting. Specal features included in the decorative scheme such as murals, panels, and so forth, which require lighting to bring out their full value, are for the most part a waste of money and at the same time may distract the audience’s attention from the real reason for their being present and for the room itself, namely, to view and enjoy the program being shown on the screen or stage, in a pleasing, restful atmosphere and bodily comfort. Wall Treatment The finished wall coverings and therefore the decorative schemes for the theatre auditorium walls are limted to materials or a combination of materials which have definite sound absorbing characteristics. This is true of the entire area of all rear walls and that portion of the side walls which are in the direct path of the sound waves produced by the horn assembly at the rear of the screen. These horns are flared to throw the sound from the center of the screen through an are varying from 8712 degrees to 105 degrees, depending on the width of the auditorium. This leaves certain areas of the side walls at either side of the proscenium arch and for a 1947-48 THEATRE CATALOG certain distance in front of the curtain line, on which the sound waves do not impinge. These areas, which are always within normal sight range of the seated persons, lend themselves to a different treatment using a material such as ornamental plaster, which is not necessarily sound absorbent. The outline of these areas should, however, be free of concave surfaces which might tend to concentrate stray sound waves and cause a flutter or dead spot in the seating area. If plaster is used in this location, it should be well broken up with ornament or fluting which will trap the sound waves. Concave surfaces in any location should be avoided in the design of the auditorium, such as coved surfaces joining the walls and ceiling, or domes in the ceiling, unless they are covered with a sound absorbent material. The entire side and rear wall areas except those portions as described above, and a wainscot about 5 feet high, require some acoustical treatment in order to eliminate side wall flutter caused by the sound waves bouncing away from these areas, even though the amount of acoustical treatment required to produce the correct optimal revérberation time may be less. The materials which are suitable for acoustical correction all have the same fundamental characteristics, namely, a porous composition with varying amount of air spaces between the fibres. Materials which have an average of 60 per cent coefficient of absorption within the range of 128 to 4,096 cycles have been found to be the most satisfactory. These ratings are made by actual tests by the National Bureau of Standards and can be obtained for the asking. There are many different types of materials available for this purpose, such as acoustical plasters, acoustical tiles, pierced metal or other fire-proof materials with the acoustical material behind, rockwool, cotton or similar loose material made up in blankets of different thicknesses which are attached to wall grounds and covered with a fabric. Every material has its advantages and disadvantages, and limitations of design. Acoustical plasters are not adaptable to surfaces which are liable to wear or abrasion, and can only be TABLE 1—YValues of absorption from room volumes, based on the formula where absorption is equal to .29 times the cube root of the volume squared. (Interpclation for all values not given will give sufficiently accurate results.) Volume Absorption Volume 14,000 15,000 26,000 30,000 34,000 AAG none 40,000 42,000 44,000 ba SCaouNnw ona reals ouceo | 48,000 60,000 70,000 75,000 85,000 10,000 ......... 11,000 ......... 12,000 . 18.000 16,000 . 17,000 18,000 .......... 19,000 . | 20,000 . 22,000 onan 24,000 ............ 28,000 . 32,000 ........ 36,000... | 38,000 .... | 46,000 ....... : 50,000 .... 55,000 .... 65,000 ....... 80,000 90,000 .......... 95,000) 100,000 ......... Absorption Volume Absorption 134. 100,000 00. 624. 143. j 110,000 .......... 665. . 152. 120,000 hUOE 160. 130,000 ........... 743. —_ 168. 140,000 ....... 782. sans 176. 150,000 ..... 817. . 184. 160,000 W.. 854. 192. 170,000 ...... 890. 199. 180,000 925. . 206. 190,000 .. 959. . 214. 200,000 ......... 990. 228. 220,000 ...........1057. 241. 240,000 ........... 1120. 260,000 ......... 1180. 280,000 ........ 1240. 300,000 .......... 1300. 2.42903 320,000 ........1350. 340,000 ..........1412. 360,000 ... 1478. 380,000 .... ....1520. nals 339. 400,000 ..... 1570. : 420,000 .........1626. 36 440,000 1676. 460,000 .....1728. ae OOOs 480,000 .......1780. . 500,000 ..........1825. 550,000 ......1945. 600,000 .. ...2063. 469. 650,000 ...... 2176. = 492. 700,000 ..... 2287. . 516. 750,000 .......2598, 5387. 800,000 ...... 2499, mee. 560. 850,000 ......2600, 581. 900,000 ....... 2700. 603. 950,000 ....... 2800. 624. 1,000,000 2900, eee