Document jNg49RbGdBN4pEx4mzr1Kyo4N

American Society of Heating and Ventilating Engineers Guide, 1934 a = the total amount of absorption supplied by the boundaries and contents of the room. = i5i + a tSt + atSi +.........., where Si, St, St.............are the areas of the boundary materials for the room. i. "i,.......... are the corresponding coefficients of absorption. Hence, by in creasing tenfold the absorptivity of the boundaries of a room it is possible to reduce tenfold the average intensity of sound in the room; that is, the intensity level would be reduced 10 db. Thus it is possible to compute the noise level in the room if the intensity of noises entering the room or generated in it are known. It will be seen that the noise intensity reduction is dependent upon the amount of sound absorption in the room, and that the first units of absorp tion are more effective than succeeding units. In general, the room noise level will be from 10 to 20 db lower than the air inlet or outlet noise intensity, the 10 db being in the case of bare rooms having large venti lating or air conditioning openings in relation to their size, and the 20 db in the case of rooms having large amounts of absorptive material with small openings. In some cases, the noise level reduction may run up to as much as 30 db, but then the higher sound intensity adjacent to the openings tends to nullify the effects of the extra reduction. Where these openings are large, the local effect on the noise intensity extends some distance from the opening; for instance, a four square foot opening might Table 2. Coefficients of Sound Absorption3 Material Thickness Coefficients or Sound Absorption (Inches) 12S 512 2048 - Cycles Cycles Cycles Acoustex 60, spray painted. Acousti-Celotex, Single B........................................ Acousti-Celotex, Triple B....................................... Acoustic Flexfelt. ..................................................... Acoustone................................................ Akoustolith piaster.... ............................................... Akoustolith A, Tile................................................... Brick wall, unpainted............................................... Calicel................................. ...................... ............ Corkoustic, Type C._........................... filaQQ Insulite Acoustile, Type 44._ ................................ Kalite, with three coats lacquer.......................... Macoustic Plaster, stippled to depth of in.... Masonite............................. .................... Plaster, gvpsum on hollow rile Plaster, gypsum, scratch and brown coats on metal lath on wood studs Plaster, lime, sand finish, on metal lath............... Poured concrete, unpainted . Rockoustile Sabinite........... . Sanacoustic Tile Stuccoustic Plaster, Type XB.......................... Transite Tile Trutone Tile.......................................... ............. Wood sheathing, pine Wood, varnished.... i A IK 1 K l 18 1 m m K K Ke X l K IK K l 1M X 0.16 0.11 0.20 0.27 0.21 0.14 0.024 0.23 0.08 0.035 0.26 0.35 0.13 0.18 0.013 0.020 0.038 0.010 0.18 0.19 0.29 0.19 0.31 0.098 0.05 0.51 0.45 0.75 0.56 0.66 0.29 0.48 0.031 0.72 0.61 0.027 0.50 0.43 0.31 0.32 0.020 0.040 0.060 0.016 0.57 0.34 0.79 0.59 0.81 0.57 0.10 0.03 0.72 0.68 0.67. 0.68 0.69 0.37 0.83 0.049 0.71 0.64 0.020 0.61 0.45 0.58 0.33 0.040 0.058 0.043 0.023 0.72 0.49 0.74 0.72 0.72 0.64 0.082 0.03 Architectural Acoustics, by V. O. Knudsen, pp. 219, 220, 240-251, 246 Chapter 18--Sound Control have a local effect within ten feet, while a one-half square foot opening would have a local effect within only five feet. The coefficients of sound-absorption for a number of standard absorp tive materials used, or suitable for use, in equipment rooms are given in Table 2. Coefficients are given for frequencies of 128, 512 and 2048 cycles. Where the frequency of the noise is not known, the values for 512 or 128 cycles are usually used. INSULATION OF AIR-BORNE SOUND The transmission of air-borne sounds through rigid partitions is accom plished primarily by the diaphragm-like vibrations of the partition. The weight per square foot of the wall is the determining factor, and the insulation value of a wall, in terms of the transmission loss in decibels, is proportional to the logarithm of the weight per square foot. Other factors, such as size, stiffness, composition, manner of mounting, and the use of multiple structures separated by air spaces of flexible connectors, contribute to the effective insulation. If the coefficients of sound trans mission of different types of structures and the noise intensity in the space adjoining a room are known, it is possible to calculate the noise intensity in a room by the use of formula (1) and the following formulas: where P = 7"t (3) r> = Noise intensity in space adjacent to room. t = Coefficient of sound transmission. Coefficients of sound transmission for some common walls are shown in Table 3. Example 1. Suppose the brick wall between an equipment room and an adjacent auditorium has an area of 200 sq ft and a coefficient of sound of 0.00001 (see Table 3) ; that the auditorium contains 2000 sabines' of absorption; and that the noise level in the equipment room is 70 db above zero level. p\ 70 -- 0 = 10 logio (from Formula 1) lo -j- = 10' X 0.00001 = 100 (from Formula 3) Jo -J- = 100 X = 10 (from Formula 2) Room loudness = 10 logio 10 = 10 db If the sound absorption in the auditorium had been as small as 200 sabines, the sound intensity in the auditorium would have been 10 times as great and the noise level in the auditorium would have been 20 db. If the rest of the auditorium has an area' of 20,000 sq ft with a surrounding noise intensity of 50 db (I" = 10s) the noise level due to all of the noise entering through the wall would be found as follows -- = 10s X 0.00001 = 1 Jo -- *A sabine is 1 sq ft of totally absorptive surface. 247