Document JN5oJkKqzxd50rRqqpzg3kgqO

HEATINC VENTILATING AIR CONDITIONING. CUIDE 1944 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 due to the plate cell may be divided into two parts. There is reflection at each end due to the change in area and the absorption at the ends. Values for this attenuation are given in Table 6 which depend on the spacing. There is also attenuation due to absorption of sound within the passages of the cell, which depends Fig. 2. Outlet Cells for Pan Outlets or Grilles on the length and the spacing. The attenuation per foot of length for 1 in. board neglecting the end effect is given approximately by Equation 5. where 10Ia*4 R= 5 (5) R = attenuation, decibels. ___,,_ L -- linear length of duct, feet. S = spacing between plates up to 3 inches. a = absorption coefficient for the full thickness of the cell material. value of a see Table 7. For typical 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. 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. 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 lorig 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.) 628 CHAPTER 33. SOUND CONTROL Table 6. End Reflection of Plate Absorbers Percentage Free Area of Absorber 60 40 30 25 20 Attenuation, db 1 2 4 5 6 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 6. This equation is accurate within plus or minus 10 pier cent for duct sizes ranging from 9 x 9 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 -j- * (6) 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 6. Results of other experiments indicate, however, that Equation 6 may be in error when applied to other types of duct lining material and to duct sizes and shapes outside of the range specified. An empirically derived chart7 representing the average experimental data oh a number of different types of materials including the rock wool sheet mentioned as applicable to Equation 6 is shown in Fig. 3. Since individual materials vary, the curves in Fig. 3 are given only as representing the best available averages for duct sizes of square cross-sections from 6 x 6 in. to 48 x 48 in. As an Table 7. Attenuation Formulae for 1 In. Thick Typical Duct Lining Board 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 The Absorption of Noise in Ventilating Ducts, by Hale J. Sabine (Journal Acoustical Society of America, Vol. 12. p. 63. 1940). ,. 7The Prediction of Noise Levels from Mechanical Equipment, by J. S. Parkinson (Heating and Venti lating, March. 1939.; pp. 23-26). Methods of Rating the Noise from Air Conditioning Equipment, by J. S. Parkinson (A.S:H.V.E. Journal Section, Heating, Piping and Air Conditioning, July, 1940, p. 447). 629