Document X7EXkjj85V6VqLrzxgLmmjNZG

774 CHAPTER 42 1948 Guide. Table 3. Attenuation of Elbows3 Elbow Size, In b Very small............. Small....j................. Medium Large....... 2 wide 3 to 15 15 to 36 36 plus 'Attenuation per Elbow, db 3 2 1.5 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 radius 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 Ires when the ratio is greater than 2. 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 duct4. 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. 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 of the room measured in. square feet multiplied by its corresponding absorption coefficient). The attenuation is given in.Equation 3 as: Table 4. Attenuation at Duct Branches or Outlets Ratio Branch Duct + Outlet Area _ _ Supply Duct Area Sum op 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 Sound Control 775 IAttenuation betweenY \ grilles and room / Total Room Absorption in Sabines 10 logio 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 Table 5; Approximate Attenuation Between Grilles and Room Outlet Velocity PPM 500 750 1000 1250 Air Change Min. 5 10 15 20 5 10 15 20 - 5 10 15 20 5 10 15 20 Live RooMb a* a 0.05 db ii 14 16 17 13 16 18 19 14 17 19 20 15. 18 20 21 Medium * .. Roomo a = 0.15 db 16 19 21 22 18 21 23 24 .... 19 22 24 25 20 23 25 26 to 00 Dead' 'Room*. ' a - 0.25 db 18 21 23 24 ..; 20 23 25 26 21 24 26 28 22 25 27 Average absorption coefficient for the room. bLive room-average absorption coefficient 0.05. Bare wood or concrete floorr-hard plaster walls ftnd ceiling--minimum of furniture.. , ... Medium room-average absorption coefficient 0.15. Carpeted floor, upholstered furniture, hard plaster walls and ceiling or bare room with acoustically treated ceiling.: dDead room-average absorption coefficient 0.26. Heavy carpeted floor. Walls and celling acoustically treated. Upholstered furniture. 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 tions of the duct. In the selection and application of the absorptive material, several points should be considered. -1. For the absorption of the low frequencies below 500`cycies per second the material should be at least 1 to 2 in. thick. Thm materials, particularly when mounted on hard solid surfaces, will absorb the high frequencies and reflect the low. . ... 2. In order to provide as much low frequency noise absorption as possible by. means