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356 CHAPTER 25 1960 Guide C SpGrter plats typ* absorber 0. Cefl-typ* absorber fee open area b A* same for eecb cow sbown if J in. thkk absorption material is ostuned. Fig. TO .... Acoustic Treatment of Ducts absorption, are so unpredictable that flexural vibration cannot be relied upon for a specific value of attenuation. 3. It has been shown" that increased attenuation at fre quencies below 700 cps may be realised by using a perforated feeing in which the area of the perforations is from 3 to 10 per cent of the surface area. Requirements for a good sound absorption material are: (1) high absorption at low frequencies;1* (2) adequate strength to avoid breakage; (3) fire resistance and compliance with national and local code requirements; (4) low moisture ab sorption; (5) freedom from attack by bacteria 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. Rectangular Ceils (Plate or Cell Absorbers) If the available length of duct between the main duct (or the fan) to the grille is shorter than the length of lining indi cated as necessary by Equation 21, the duct may be sub divided into smaller ducts as shown in Fig. 10, or it can be even more subdivided by an egg-crate construction. In such a construction in which all the subdivided ducts are the same size, sound will be equally absorbed down each channel. It is, therefore, only necessary to calculate the sound attenua tion of an individual channel. For this, Equation 21 is ade quate, assuming that the 0f the material is halved if it forms a common splitter between two cells. When the number of splitter plates or cell partitions is large, the percentage free area of the gross duct may be mate rially reduced. This leads to a further sound attenuation. Values of the attenuation possible, due to this cause, arc given in Table 7. Table 7.... End Reflection of Plate or Cell Absorbers Serencttagn Frtc Area of Absorber 50 40 30 25 20 Attenuation db 1 2456 Package Units Package units for attenuating sound in ducts are coming more and more into common use. These units have the ad vantage of providing, in a short length, a known amount of noise reduction with a minimum opportunity for errors in TAKEOFF Absorption Plenums With and Without Sound Cells Sound Control Table 8 .... Typical Values of Sound Attenuation tn Decibels for Sound Traps" ffaff-Opeomg (/nth Fo0 Opening Units Octavo Bead, cps length, ft Length, ft 23 5 4 68 37.5-75 75-150 150-300 300-600 600-1200 1200-2400 2400-4800 4800-9600 9 10 11 13 7 9 11 10 11 12 14 9 11 15 12 16 20 26 11 17 24 15 19 26 35 18 26 33 25 31 45 59 30 `45 56 36 46 56 63 40 56 62 29 39 49 59 30 46 55 22 30 40 51 27 42" 52 Console the msonfftQturer far dots on aousd refeneration dm to *ir 357 installation and for erosion of materials. The cost of inspec tion of the completed job is eliminated and the outside di mensions of the duct can be reduced as compared to a lined duct. These units are available in sizes to fit 6 x 12 in. ducts up to 80 x 84 in. ducts. Charts are supplied by the manufacturers giving the attenuations and pressure drops for different types. In Fig. 11, the static pressure drops for several types of package units are shown. Typical attenuation ratings are shown in Table 8. Manufacturers' literature -should be con sulted for exact values. Plenum Absorption In systems where individual ducts are directed to a number of rooms, and sound treatment is required in every duct, a sound absorption plenum on the fan discharge as shown in Fig. 12 will often prove the. most economical arrangement. Based on both experiment and ray acoustics theory, the following approximate expression has been derived for acoustic plenum attenuation.* 1 R 10 iogu & cos 0 1 -- a 2*tf* + S,, (22) where R = attenuation, decibels. a " absorption coefficient of the lining, dimensionless. 5, = plenum exit area, square feet. Sw -- plenum wall area, square feet. d -- distance between entrance and exit, feet. 0 = the angle of incidence at the exit, i.e., the angle which the direction d makes with the normal to the exit opening, degrees. For frequencies sufficiently high so (hat the dimensions of the plenum exceed about one wavelength. Equation 22 will predict the sound attenuation within a few decibels. At low frequencies, somewhat more attenuation is realized than will be computed by Equation 22, due to the existence of a reso nant muffler action in the duct-plenum system. Square Comer Bends Square comer bends can be quite effective in noise reduction provided they are widely separated in the system. The noise Fig. 13 .... Attenuation in Decibels Provided by (A) Unlined Bend, and (B) Lined Bend in a Duct with the Lateral Dimension D reduction for a single square corner bend with no turning vanes, unlined, is shown in Fig. 13 Part A. Turning vanes are usually employed in square comer ducts. When the width of the turning vanes is small, say less than one-fourth wavelength of the sound, they do not affect the noise reduction. At higher frequencies, they decrease the noise reduction substantially below that shown in Fig. 13 Part A. When a square comer bend is lined with sound absorbing material, the lining should be extended several duct widths beyond the comer in the direction of sound transmission. For square bends lined in this manner the attenuation may be estimated from Fig. 13 Part B. Regeneration of Sound Air flow generates sound energy in elbows, vanes, sound traps, or any other element of the duct system where turbu lence may be introduced. This sound is generally broad band in character. The final sound power level is calculated first on the basis of no regeneration and then the generated sound power level is added logarithmically to it. X Branches At a junction where a duct divides or has side branches or outlets, the sound power traveling down the duct divides. The decibel reduction in sound power from the main duct to any branch may be calculated as 10 logit of the ratio of the branch area to the total area of all branches leaving the junc-