Document 2jxkXzGmNdR7yb7LmLmKOKjY5

HEATING VENTILATING AIR CONDITIONING GUIDE 1942 at other values of tip speed, total pressure, and size may be approximated by the following relationships. 1. With total pressure and size constant. [SfsSt]6 db (change) = 1 Log.o (1) 2. With total pressure and tip speed constant. db (change) = 10 Log,, (|g^)2 (2) 3. With size and tip speed constant. db (change) = / (Total Pressures -- Total Pressurenew) (3) The factor / is a function of the fan type. For a centrifugal fan with backward curved blades / = 9.6. For a single inlet single width type of ventilating fan with backward curved blades, the noise level at the fan discharge or intake operating at 4000 fmp tip speed and total pressure of 1 in. may be approximately 65 db. The noise level of a double inlet width fan may be 3 db higher than a single width fan at similar conditions Table 2. Attenuation in Straight Sheet Metal Duct Runs Duct Size, In. 24 x 24 72x72 Attenuation per Ft, db 0.10 0.05 0.01 Table 3. Attenuation of Elbows3 Elbow Si2e, In. 3 to 15 15 to 36 36 + Attenuation per Elbow, db 3 2 1.5 1 aThe attenuation in vaned elbows should be considered the same as in elbows having the same dimen sions a9 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 above attenuation values. Table 4. Attenuation at Duct Branches or Outlets Ratio Branch Duct -f Outlet Area _ _ Supply Duct Area Sum of Branch Areas Supply Duct Area 1.00 1.20 1.35 1.50 1.75 2.00 Attenuation per Transformation, db 0.0 0.8 1.3 1.8 2.5 3.0 634 CHAPTER 33. SOUND CONTROL Table 5. Approximate Attenuation Between Grilles and Room -- Outlet Velocity FPM -- 500 Min. 5 10 15 20 Live RooMb aa = 0.05 db ii 14 16 17 Medium Roomc a = 0.15 db 16 19 21 22 . Dead RooMd a = 0.25 db 18 21 23 24 750 ' 1000 5 10 15 20 5 10 15 20 13 18 20 16 21 23 18 23 25 19 24 26 14 19 21 . 17 22 24 19 24 26 20 25 28 1250 5 10 15 20 15 20 22 18 23 25 20 25 27 21 26 28 aAlpha (a) is the average absorption coefficient for the room. bLive room average absorption coefficient 0.05. Bare wood or concrete floor--hard piaster walls and ceiling--minimum of furniture. cMedium room average absorption coefficient 0.15. Carpeted floor, upholstered furniture, hard plaster walls arid ceiling or bare room with acoustically treated ceiling. dDead room average absorption coefficient 0.25. Heavy carpeted floor. Walls and ceiling acoustically treated. Upholstered furniture. of tip speed, total pressure, and size: For the same tip speed and size the noise level of a fan with forward curved blades is higher than for one with backward curved blades, however, the capacity of the fan with the forward curved.blades will be greater. NATURAL ATTENUATION IN THE DUCT SYSTEM The natural attenuation existing in a duct system consists of three component parts which are described herewith. Straight Sheet Metal Ducts The attenuation of sound in straight sheet metal ducts is a function of the length, shape, and size of the duct. Attenuation values are given in Table 2. In general, this attenuation is so negligible except for long rUns that it may be neglected 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 transformation as shown in Table 3. When the area of a duct increases, 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 635