Document mZ0w8Om7DQ3q6jr9owdZ3MjO

1008 . CHAPTER 40 1957 Guide area of all branches leaving the junction. Generally the total area of the branches leaving a junction is greater than that of the duct entering a junc tion; therefore, the area of the branch should not be divided by the area of the supply duct. In a complicated branched-system, errors incurred by such a practice may accummulate to several db. Table 4 gives the db reduction for various ratios of branch duct area to total branch duct area. - Square Comer Bends Square comer bends can be quite effective in noise reduction provided they are widely separated in the system. The noise reduction for a single square comer bend with no turning vanes, unlined is shown in Fig. 8. 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 frequen cies, they decrease the noise reduction substantially below that shown in Fig. 8 Part A. When a square comer bend is lined with sound absorbing material, the lining should be extended several duct widths beyond the corner in the Table 4. Power Level Attenuation at Duct Branches or Outlets Ratio Branch Duct Abba Sum op Branch Abba8 Attenuation DB 1.0 0.8 0.63 0.5 0.4 0.32 0.25 0.2 0.16 0.13 0.1 0 1 2 3 4 5 6 7 8 9 10 direction of sound transmission. For square bends lined in this manner the attenuation may be estimated from Fig. 8 Part B. End Reflection Losses All of the noise power that is produced in a duct by a fan (or by turbu lence) does not radiate from the end of a duct into the room. Instead, part of the power is reflected back from the end toward the fan. The amount of noise power reflected is related to the product of the frequency of the sound and the length of a duct side L, assuming that the duct is square. If the duct is not square, the value of L is taken to be approximately equal to VLxLy. The end reflection loss is shown in Fig. 9 for two cases: (1) a duct ending in free space and (2) a duct ending flush with the wall. Duct Lining Procedure In the past, the most common method of obtaining sound absorption in ventilating systems has been to line the duct with absorbing material. It is usually more convenient to line all four sides of the duct, but a lining on one side over a longer length of the duct will, in general* give the same effect for the same area of applied acoustical material. Subject to cer Sound Control 1009 tain restrictions, the attenuation of a fully lined duct to single-frequency sounds may be expressed by the approximate Equation 22.9 R = 12.61 ~ a'-4 db A where R = attenuation, decibels. (22) Fig. 8. Attenuation in Decibels Provided by (A) Unlined Bend, and (B) Lined Bend in a Duct with the Lateral Dimension D The attenuation is given for eight octave frequency bands with D in inches as the parameter. p = iengtb of lined duct, feet. "s " perimeter of duct, inches. A = cross-sectional area of duct, square inches. a -- absorption coefficient of lining (a function of frequency) ,10 This formula was empirically developed for a set of duct sizes ranging irom 9 x 9 in. to 18X 18 in.; for cross-sectional dimension ratios of 1:1 to .4 > fr frequencies between 256 and 2048 cycles; and for absorption coeffi cients between 0,20 and 0.80. The duct lining material used was 1 in.