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842 CHAPTER 42 1949 Guide Table 2. Attenuation in Straight Sheet Metal Duct Runs Duct Size, In. 24x24 72x72 Attenuation per Ft, db 0.10 0.05 0.01 taken as 3 db higher than a similar single width fan operating under the same conditions of speed and pressure. NATURAL ATTENUATION OF DUCT SYSTEM Straight Sheet Metal Ducts. The attenuation of sound in straight sheet metal ducts is a function of the length, shape, and size of the duct4. Attentuation values are given in Table 2. In general this attenuation is so negligible except for long runs that it may be disregarded for all practical purposes. Elbows and Transformations. Due to reflective interference, attenuation will take place at elbows and transformations. The magnitude of the attenuation will depend on the size and abruptness of the elbow or trans formation as shown in Table 3. When the area of. a duct increases abruptly, an attenuation of noise level takes place, in the duct. In duct design practice the total area of the branch ducts is greater than the supply duct. Similarly with outlets, the area of the outlet plus the area of the duct after the outlet is greater than the duct area before the outlet. Therefore in an outlet run, attenua tion occurs in the duct as it passes each outlet. Table 4 gives the db reduction for various ratios of total branch duct and outlet area to supply duct area. Grilles to Room. The large abrupt change in area between the grilles and the surfaces within a room results in an appreciable noise attenuation. This attenuation is a function of the total grille area (supply and return) and the total sound absorption of the room in sdbins. (The sound absorp tion of a room in sabins is the summation of the products of each surface of the room measured in square feet multiplied by its corresponding absorption coefficient. The sabin is a unit of sound \absorption equivalent to the Table 3. Attenuation of Elbows* Elbow Size, iN.b Very small Small______ ................................................... Medium... . _ __ __________ ____ _ -- _. Large-------------------- . ---------- ... 2 wide 3 to 15 15 to 36 36 plus Attenuation per Elbow, db 3 2 1.6 1 "The attenuation in vaned elbows should be considered the same as in elbows having the same dimen sions as the radius of curvature of the vanes. - If the vanes are lined for the purpose of damping any vibra tions in them, one third may be added to the attenuation values listed. bThese attenuation values are based on elbows having a center line radios 1-5 to 2 times the diameter or width of the duct. The attenuation will be greater if the ratio is less than 1.5 and less when the ratio is greater than 2. Sound Control 843 Table 4. Attenuation at Duct Branches or Outlets Ratio Branch Duct + Outlet Area Supply Duct Area 1.00 1.20 1.35 1.50 1.75 2.00 Sum of Branch Areas Supply Duct Area Attenuation per Transformation, db 0.0 0.8 1.3 1.8 2.5 3.0 absorption of one square foot of a totally sound-absorbent surface). attenuation is given in Equation 3 as: f Attenuation between\ . . Total Room Absorption in Sabins \ grilles and room / - &0 Total Grille Area The - . Values in Table 5 approximate the attenuation for various rates of air change, and general types of room surfaces. DUCT SOUND ABSORBERS The difference between the required sound attenuation and the natural attenuation is that which must be supplied by the proper sound treatment of the ducts. - .... Selection of the Absorptive Material When a sound wave impinges on the surface of a porous material, a vibrating motion is set up within the small pores of the material by the Table 5. Approximate Attenuation Between Grilles and Room Outlet Velocity FPU 600 750 1000 1250 Ant Change Min, 5 10 15 20 5 10 15 20 5 10 15 20 6 10 16 20 Live Room** e> ~ 0.05 db ii 14 16 17 13 16 18 19 14 17 19 20 45 18 20 21 Medium Room" a - 0.15 db 16 19 . 21 22 18 21 23 24 19 22 24 25 20 23 25 26 Dead Room* a - 0.25 . db 18 21 23 24 20 23 25 26 21 24 26 28 . 22 25 27 28 "Average absorption coefficient for the room. bUve room-average absorption coefficient 0.05. Bare wood or concrete floor--hard plaster walls and celling--minimum of furniture. / - "Medium room-average absorption coefficient 0.15. '* Carpeted floor, upholstered furniture, hard piaster walla and ceiling or bre room with acoustically treated ceiling. . - . dDead room-average absorption coefficient 035. Heavy carpeted floor.. Walls and ceiling acoustically treated. Upholstered furniture.