Document RJ2a01M4zydxm4MRqyQvM3K1V

, 880 CHAPTER 40 1952 Guide Table 5. Attenuation op Elbows* Elbow Size lN.b. Attenuation peb Elbow, db Verv small. . .. Large............... . ' 3 to 15 15 to 36 36 plus 3 2 1.5 1 * The attenuation in vaned elbowB should be considered the same aa 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. b These attenuation values are based on elbowB 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 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 sabins: (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 cofresponding 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: n / Attenuation between \ ,,,, . Total Room Absorption in Sabins' ,,,N * V grilles and room 10 lo&0 Total Grille Area . " (5) ' Values in Table 7 approximate the attenuation for various 'fates 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. Selection of the Absorptive Material When a sound wave impinges on the surface of a porpus 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 vibrations of the duct. In the selection and application of the absorptive material, the following points should be considered: Table 6. Attenuation at Duct Branches or Outlets Ratio Branch Duct + Outlet Abba Supply Duct Area 1.00 1.20 1.35 1.50 1.75 Sum op Branch Areas Supply Duct Abba Attenuation per Transformation, db 0.0 0.8 1.3 1.8 2.5 Sound Control 881 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 an 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 RooU: Outlet VBLOCtTT PPM 500 760 1000 , 1250 An Changb Min. 510 16 20 '5 10 15 20 5 10 16 20 6' 10 15 . 20 Live Room* ot - 0.05 db ii 14 16 17 13 16 18 19 14 17 19 20 ,16 18 20 21 Medium Room o ~ 0.15 db 16 19 21 22 18 21 23 24 19 22 24 25 20 23 25 26 Dbad .Room* a - 025 ' db 18 : 21 23 ...... 24 20 23 26 26 . 21 24 . ,28 ; 28 22 25 27 28 * Average absorption coefficient for the room. b Live room-average absorption coefficient 0.05. Bare wood or concrete floor--hard plaster walls and ceiling--minimum of furniture. 0 Medium room-average absorption coefficient 0.15. Carpeted floor, upholstered furniture, hard plaster walls aud ceiling or bare room with acoustically treated ceiling. d Dead room-average absorption coefficient 0.25. Heavy carpeted floor. Walls and tiling 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, is related in a.father complicated manner 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