Document b5v390r7o5GvmnBLQLKp693RZ

596 ______;Chapter 32 ___________ 1945 Guide The resultant room noise level can be approximated by Equation 10. r^uv,. . [*" ''**]-'" Grille Selection In practice the allowable total souhd and the required air flow are usually known,, and it is desired to determine the maximum allowable velocity. In comparing sound ratings of various grilles, several factors must be known if the information is to be properly applied : / 1. The threshold intensity on which the decibel ratings are based. 2. The distance from the grille at which data were taken. 3. If stated as loudness level versus velocity for a given grille, the core area (not nominal area) must be known. 4. The sound absorbing characteristics of the test room. 5. Whether or not corrected for test room loudness level: if not, the room level ' (without grille noise) must be known. 6. Methods used for recording data. (Characteristics of sound meter). Sound Control 597 rection must be made for area to secure the total sound level of supply openings of more or less than one square foot area from Equation 11. where Decibel Addition = 10 logio-4, A -- core ara, square feet. (ll) With Fig. 5 it is possible to find directly the velocity.in feet per minute which will give a predetermined total loudness at a predetermined rate of flow expressed in cubic feet per minute. The values used are arbitrarily chosen for the purpose of discussion and do not necessarily represent data Since total loudness and air flow are both functions of velocity and area, the solution of the problem implies a trial and error method. It has been ' found possible to present these data with sufficient practical accuracy as a family of uniform curves, as illustrated in Fig. 5, which are based on these assumptions: . 1. Threshold intensity = 10-IS watts per square centimeter9. 2. Microphone location 5 ft from lower edge of supply opening on a line downward at . 45 deg and in a plane bisecting the supply opening perpendicularly. 3. Where data are given as loudness level versus velocity, the rating is per square foot of core areai 4. The room is assumed to have 100 sabines absorption. 5. Plotted data are. loudness levels of supply openings only, correction having been made for test room level. 6. Data taken with q direct reading sound-level meter with frequency weighingnetwork intended to approximate the response of the human ear. If the published ratings are in terms of decibels per square foot, cor- Loc. Cit. Note 1. Fig. 6. Room Absorption Correction Chart referring to any particular design of air supply opening. A correction chart is shown in Fig. 6 for a room having a sound absorption other than 100 sabines. Emmple S. Determine the core area (see Chapter 30) of an air supply grille which Will maintain a noise level of not more than 40 db m a room having 100 sabines of sound absorption, if an air volume of 2400 cfm is required to maintain the proper air con ditioning. . Solution. Assuming a grille noise rating of at. least 5 db below the noise level of the room. Fig. 5 shows that the limiting grille velocity for a total loudness of 35 db is about 7-25 fpm and the core area becomes fixed at 2400 H- 725 or 3.31 sq ft. ffi room absorption had been greater, the previously selected velocity of 725 fpm would be safe, since the loudness reduces. If the room absorption had been 200 sabines acorrection of plus 1.3 should be made by reference to Fig. 6, and the permissible velocity becomes that corresponding to a total loudness of 36.3 or approximately 800 fpm. If the room had been highly reflective with an absorption of less than 100, the cor rection would be much more important. For instance, for a room of 35 sabines a cor. ooCVl n ` P?'nus 3 db should be made and the maximum velocity corresponding to the at db total loudness would be approximately 600 fpm.