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W 91:4` CHAPTER 41 '1954 Guide cf Table 5. Attenuation op Elbows* * / Elbow Sira lN.b > Attenuation per Elbow, db Small.................................. ..... .......... ..... Medium............ .........----------------------Large................ .............. ..................... 2 wide 3 to 15 15 to 36 36 plus 3 2 1.5 .1 * The attenuationin vaned elbows should-be considered the same as in elbows having me sameaimeueions as the radius of curvature of the vanes. If the vases are lined for the purpcee of draping any vibra tions in them, one third may bfe added to the attenuation values listed.- b These attenuation values are based on elbows having a center line radius 1.6 ;to 3 times the diameter or width of the duct.; The attenuation will be greater if the ratio is less than 1.6 and less when the ratio is greater than 2. ' This attenuation is a function of the total grille area (supply and return) and the total sound absorption of the room in safaris.' (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 absorption of one square foot of a totally sound-absorbent surface). The attenuation is given in Equation 5 as: ' ,, / Attenuation between \ ... Total Room'Absorption in Sabins )grilles and room " 10 log"------- Total Grille Area ' V . (S)! Values in Table 7 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 must'be supplied by the proper sound treatment of the'ducts;1 Selection of the Absorptive Material . , ;.g When a sound wave impinges on the surface of a porous material, ai 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 thepores 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 vibrations; of the duct, in the selection and application of the absorptive; material,'- the following points should be considered: ' ;.t - Table 6. Attenuation at Duct Branches or Outlets Ratio Branch Duct -f Outlet Area Supply Duct Area 1.00 1.20 1.35 1.50 1.75 2.00 Sum op Branch Arras Svppv~Dvcz~Arba~ Attenuation Transformation, db' 0.0 0.8 1.3 1.8 2.5 Sound Control 915 1. For the absorption of. the low frequencies below .500 cycles per second the material should be at least 1 to 2 in. thick. Thin 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 of flexural vibration, it is desirable to,fasten the absorptive;panels discontinuously. This result may be attained to some extent by spot cementing, but better results are obtained when it is possible to fasten the absorptive panels to fur ring strips, leaving an air space behind. However, the'exact resonance character istics of the panels, and thus their absorption, are so unpredictable .that flexural vibration cannot;be relied upon for a specific value of attenuation. Requirements for a good sound absorption material are: (1) High absorption at low frequencies;5 (2) adequate strength; to avoid breakage; (3) fire resistance and compliance with national and local code require ments; (4) low'moisture absorption; (5) freedom from attack by bacteria Table 7. Approximate Attenuation Between Grilles and Room v Outlet Velocity ppm 500 750 1000 1250 Am Change Mm. -. 5 10 16 20 5 10 15 20 6 10 15 20 5 10 15 20 Live Room* aa a 0.05 db u 14 16 17 13 16 18 19 14 17 19 20 45 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 Dead Room* a 0.25 db 18 21 23 24 20 23 25 26 21 24 26 28 22 25 27 28 -_ . bimui^iuuu uueiuuieub iur me room. Live room-average absorption coefficient 0.05. Bare wood or concrete floor--hard plaster walls and --minimum of furniture. * Medium room-average absorption coefficient 0.15. Carpeted floor, upholstered furniture, bard plaster walls and ceiling or bare room with acoustically treated ceiling. Dead room-average absorption coefficient 0.25. Heavy carpeted floor. Walls and ceiling aooustically treated. Upholstered furniture. and algae; (6) low surface coefficient of friction; (7) particles should not fray off at the higher design velocities; and (8) freedom from odor when either dry or wet. With every application, the use of sound absorptive material should be considered in the dual function of insulation and sound absorption. It has been shown theoretically* that the reduction (in decibels per linear foot) of sound transmitted through a duct lined witH sound absorbing material, 18 related in a rather complicated maimer to the size and shape of the duct, to the frequency of the sound, and to the sound absorbing char acteristics of the lining. Experimental evidence likewise indicates that there is no simple formula involving the variables which will apply accu rately to all cases. However, it may be stated generally that the attenua tion in decibels at a given frequency is directly proportional to the length of