Document DvdZN021GmZO8zp0ZQLnojOR4

1016 CHAPTER 40 1957 Guide For the four duct positions of Fig. 15, approximate values of Q are given in Fig. 16 for 6 = 0 deg. and for 6 -- 45 deg. The abscissa is-the product of frequency times L assuming that the duct is approximately square with an area of L2 sq in. If the duct-is rectangular, the value of Q should be computed for L equal to the smaller dimension. The room constant R is defined by the equation R = aS/Cl - a) (25) Sound Control 1017 being considered. (Values of a for many materials are published by the Acoustical Materials Association."1) For estimating purposes, approximate values of the room constant R may be obtained from Fig. 17 as a function of room size for five classes of rooms. The value of the second and third terms of Equation 24 (including 0.5 db) is plotted in Fig. 18 with distance r from the source as the abscissa, 17.Fig. Value of the Room Constant ft as a Function of Room Volume fob Rooms with Pbopobtions of About 1:1.5:2 These proportions give S =. 6.25 V*/'. The parameter is the average sound-absorption coefficient for the room. The subjective ratings "(lead," "live," etc., are the author's11 and are not necessarily in standard use. From Reference 13. Used by permission. where s = average sound absorption coefficient (dimensionless) for the room at the mid frequency of the band of. noise being considered. jS = total area of the boundary surfaces of the room, sq ft. Where a room contains several surfaces each with different absorption coefficients the average absorption coefficient may be determined by the equation S,ai -f- Stas -f- Si + St + . (26) where Si , St = the areas of each type of absorbing material, on , as etc., = the absorp tion coefficients of each type of material at the mid-frequency of the band of noise . So : =ROOM CONSTANT IN SO.FT b TOTAL AREA OF BOUNDARIES OF AOOM IN SO-FT 0= AVERAGE ENERGY ABSORPTION COEFFICIENT OF THE SURFACE OF THE ROOM THE ABSOLUTE SPL (08 RE 0.0002 MICROBAR) EQUALS THE SUM OF THE ORDINATE AND THE PWl ( DB RE lO-'3 WATT) OF THE NOISE SOURCE AT NORMAL ROOM TEMPERATURE AND PRESSURE CONDITIONS Fig. 18. Curves for Determining the Sound Pressure Level in the Room Relative to the Sound Power Level / The listener's position is at a distance r and an angle 0 with respect to the duct opening. Q is determined tbL!i *?njMroia the duct dimensions (See Fig. 16). To use the graph proceed as for the example shown by oftli j- hue for r = 7 ft 0 = 2, and R ~ 2000 sq ft. The sound pressure level in the room equals the sum tne ordinate and the sound power level. Normal room temperature and pressure conditions are assumed. with the sound pressure level in decibels as the ordinate and the room con stant R in sq ft as the parameter. -The directivity factor Q is given at the bottom left-hand side of the graph. Ihe procedure for using the graph is as follows: Hi ^e*:ermine the room constant R and the directivity factor Q for the particular jrection e of interest. Enter Fig. 18 at the bottom at the distance r. Then move agonally to the left until the value of Q is reached. Then move vertically until j.va*ue of R is reached. Then read the relative sound pressure level from' the R = example is shown by the dotted line in Fig. 18 for r = 7 ft, Q = 2 and ~ /00 sq ft. The relative sound pressure level is added algebraically to the sound