Document LKa8d3yj14j1V4m71r2K5exdg

HEATINC VENTILATING AIR CONDITIONING GUIDE 1944 2. For constant RPM and pressure rating, the noise level of a given type of fan will increase with increasing fan size. db (change) = 20 logio ( size*) (2) Fan size refers to wheel diameter, housing height or some dimension that is directly proportional to lineal units. Fan sizes based on arbitrary systems or systems of preferred numbers have no significance. The noise of a given fan is not constant at constant speed if the air delivery changes due to change of resistance. In general, a backward curved blade fan is lowest in noise at or near the. point of maximum efficiency; a forward curved blade fan at or between the point of maximum efficiency and shut-off; an axial flow fan at or between the point of maxi- Table 2. Attenuation in Straight Sheet Metal Duct Runs Duct Size, In. Attenuation per Ft, db 24 x 24 72x72 0.10 0.05 0.01 Table 3, Attenuation of Elbows* Elbow Size, iN.b ' 3 to 15 15 to 36 36 plus Attenuation per Elbow, db 3 2 1.5 i 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 radius.J..5 to 2 times the diameter or width of the duct. The attenuation will be greater if the ratio is less than 1.5 andless when the ratio is greater than 2. murn efficiency and free delivery. The noise level of a double width fan may be 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 duct*. Attenuation 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, attenua tion 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. *A.S.H.V.E. Research Report No. 1205--Determining Sound * Attenuation in Air Conditioning Systems, by D. A. Wilbur and R. F. Simons (A.S.H.V.E. Transactions. Vol. 48,1942, p. 267). 624 CHAPTER 33. SOUND CONTROL 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, attenu ation 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 sabines. (The sound absorp tion of a room in sabines is the summation of the products of each surface 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 op Branch Areas Supply Duct Area Attenuation per Transformation, db 0.0 0.8 1.3 1.8 2.5 3.0 of the room measured in square feet multiplied by its corresponding absorption coefficient). The attenuation is given in Equation 3 as: ,, (Attenuation between\ _ 1n . Total Room Absorption in Sabines \ grilles and room / *ul Total Grille Area (3) 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 treat ment 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 alternating sound waves. As the ratio of the cross sectional area of the pores to their interior surface is small, the resistance to the movement of air in the pores is large. This viscous resistance within the pores of the material converts a portion of the sound energy into heat. The decimal fraction representing the absorbed portion of the incident sound wave is called the absorption coefficient. Considerable absorption may also result, particularly in the low frequency range, from the flexural vibra-- 625