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HEATING VENTILATING. AIR CONDITIONING CUIDE 1943 Table 6. End Reflection of Plate Absorbers Percentage Free Area of Absorber 50 40 30 25 20 Attenuation, db i 2 4 5 6 rock wool blanket or 1 in. sound absorbing board preferably nailed to wood strips on the inside of the plerium. With such a lining the plenum is particularly effective in reducing low frequency fan noise. The absorp tion of the plenum in sabines is the sum of the products of each interior area of the plenum measured in square feet multiplied by its corresponding absorption coefficient. Plate Cells One of the most economical methods of applying sound absorbent material from the standpoint of both labor and material is the plate cell. The plate cell consists of or 1 in. sound absorbent board, spaced on 2,3 or 4 in. centers. The attenuation given in Table 6 depends on the spacing, depth, and the absorption of the mateijal. At each end of the cellfurther attenuation results from the reflection of sound from the face of the cell. An important objection to the plate cell is the increase in duct cross sectional area required. Often on the fan discharge, particularly with Unitary equipment, where a number of branch ducts take off, the plate cell may be installed with little or no difficulty. The attenuation per foot of length for 1 in. board neglecting the end effect is given approxi mately by Equation. 6. where R = attenuation, decibels. . ,. L = linear length of duct, feet. S = spacing between plates up to 3 inches. a = absorption coefficient of plates. For value of a see Table 7. Outlet Sound Absorbers Outlet sound absorbers are rectangular or plate cells installed directly behind an outlet or they may be the lining of a pan or plaque outlet. Table 7. Decibel Attenuation Formulae for Typical Duct Lining Material Frequency 256 512 1024 2048 Absorption Coefficient 0.37 0.69 0.78 0.78 Attenuation Reduction, db 3.0 L P/A 7.5 L P/A 9.5 L P/A 9.5 L P/A 634 CHAPTER 33. SOUND CONTROL They are particularly effective in the elimination of high frequency whistles which are generated by air flow in the ducts. They are also employed in large systems with long runs where only a few outlets near the fan require treatment. Frequently outlet cells are the only means of correcting existing noisy installations, as the duct sections directly behind the outlets may be the only sections accessible for treatment. (See Fig. 2.) Duct Lining or Rectangular Cells One series of experiments6 made on a commonly used type of duct lining .material (1 in. rock wool sheet) has shown that, subject to certain restrictions, the attenuation of single-frequency sounds may be expressed by the approximate Equation 7. This equation is accurate within plus or minus 10 per cent for duct sizes ranging from 9x9 in. to 18 x 18 in., for cross-sectional dimension ratios of 1:1 to 2:1, for frequencies between 256 and 2048 cycles, and for absorption coefficients between 0.20 and 0.80. ' R = 12.6 L -~ a14 (7) where R = attenuation, decibels. L = length of lined duct, feet. P = perimeter of. duct, inches. A = cross-sectional area of duct, square inches. a .= absorption coefficient of lining. _ In Table 7, the absorption coefficients at different frequencies of a material of the previously mentioned type are listed, together with the corresponding values for Equation 7- Results of other experiments indicate, however, that Equation 7 may be in error when applied to other types of duct lining material and to duct The Absorption of Noise in Ventilating Ducts, by Hale J. Sabine (Journal Acoustical Society of America, Vol. 12, p. S3, 1940). ..... 635