Document M0GpJxYVg4yY2J10XvZzzg4M

198 CHAPTER 14 1965 Guide And Data Bode 'Sound 'Control 199 flow velocity over the vanes. Doubling the velocity will in crease die sound level by approximately 16 db. Although some generalised data have been published on the sound generated by a few styles of diffusers0 and grilles,0 selections should be based on ratings for the particular make. When comparing different manufacturers' ratings, corrections may. have to be applied for any differences in the acoustical environment on which the ratings are based.*-*1 The maximum air flow velocity, and hence the sound level, depend not only, on the air quantity, size, and design of -the air outlet, but also upon die air approach configuration.. When a volume control damper is installed close to an air outlet, the air.flow velocity in some parts of the outlet.will increase when the damper is throttled to build up resistance;, Table 6 can be used to estimate the increase in sound level from the increase in resistance of'the outlet-damper combina tion, onceboth* are a function of the degree of damper opening, i To avpid.the increase in.noise level shown in Table 6,;ivok iirna control dampers should be located several duct -widths upstream from the air outlet,' so that' air streams- from* the damper will not impinge on die vanes or cones of die outlet.. A-eound level, or NC level) which they produce in a typical room at a typical listener's position (see Fig. 4). This short cut method is accurate within- 2 db, provided that the amount of room absorption on which the ratings are based is within 40 percent of the amount of room absorption avail able per outlet in the proposed installations. 1 Sound Power Ratings Whenever there are' several different sources of sound, it is more convenient to use ratings in terms of the sound power generated in each octave band, or at least in each of die most important bands. Sound power ratings have the advantage of being absolute, i.e., independent of room absorption. For ductconnected central station equipment, octave band sound power ratings are the only practical way to express acoustic performance. Unless design specifications are also expressed in terms of power level, it is necessary to convert them to that form before equipment power level ratings can be applied. Direct Ratings . r Since* there is no path attenuation involved, it may be posP able to save time by'selecting air outlets from ratings ex pressed directly in terms of the noise level '(loudness level. Table 6 .... Effect of Damper Throttling oh; Noise Generated by an Air Outlet '. (A o Ghrsa Air Quantity]'. - , ' ~ t] fffocffco- - - Retisfaaco of ' Dooptf Opening -- - - Ootfef-Ocnpor - - (Reference On/jd' CoBbfeetien , , 100% ! 82% 70% 50%. . 100% " * 400%..-;, ' *" For dampen located dos* to outlet. Notts tmP 0 db -! 4K db 8 .db . , , u 16 db Room Absorption Effect The computation of the sound power level that corresponds to a specified room sound level is similar to a heat loa^Tcalcu lation and involves the rate at which 'sound energy is dissi pated by absorption at the walls, ceiling, and furnishings of the room, as well as the direct radiation of sound from the source to the listener: Lw=L,+10 logJ IL 4im Vri* r* -j-10.5 (10) where Lw "'sound power level, db re 107" watt. L, = sound pressure level, db re 0.0002 microbar. Q = directivity factor, dimensionless. - n " number of sound sources of equal sound power. ru f> etc. -- distance from sound sources,-feet. R " room constant, square feet. Equation 10 b presented graphically in Fig. 6 for a single sound source. It should be noted that (Lw -- Lp) becomes a function of R only when the distance from the source is suffi ciently great The same is true if the number of sound sources," u, b sufficiently great The room constant R b therefore the most important vari able in Equation 10. It can be calculated from Equation 11: tchere 8 " total area of the ceiling, wall, and floor surfaces, square feet . " average sound absorption coefficient for the surfaces, S, dimensionless. -1 , The room absorption Sa needed for using Equation 11 can be determined in several ways: 1. For existing rooms, Sa can be calculated from: - -- - a. The measured sound pressure level produced by a refer ence sound source0 of known sound power output b. The measured reverberation time, T, in seconds. This is -- the time it takes for the sound pressure level from an intermittent source (such as a shot) to drop 60 db. 0.049V Sa T- (12). tofureV = volume of the room, cubic feet. Reverberation time b a good criterion of acoustical .quality of a room and therefore b closely related'to the type^of-airotfty Ion which the room b.suitable. A short reverberation time means that sounds die out quickly. Dead rooms are good for conversa tion at close range and for studios for sound recording. Live rooms, on the other hand, have considerable echo and, therefore; are not suited for speech communication because words and syllable^ will blur into each other. However, live rooms are desirable for "certain types of music, especially music "composed for large churches!" 2. During the design-stage, the room absorption-can" be com; puted as the sum of the absorption value for each surface and object in the. room: ... ; .. Sa " Sien + Sift] + -f- Ai + At -f- 4mV (13) Si, St, etc. = area of each type of room surface,* square feet <n, 04, etc. = absorption coefficients" of each type of "surface at a frequency typical for the band of.sound being considered. " ' A,, At, etc. " absorption units of occupants, furnishings,'etc!, sabines or square feet. -` "'''** m factor for absorption of sound in air. ' This b the method which the architect or acoustical designs uses to determine the amount and kind of absorbihg`mstenal needed to provide the proper acoustical quality for the particular space. For the mechanical engineer this computation is-seldom