Document jgREO8n2ZY4K0JNgapbRme7MR

934 CHAPTER 41 1955 Guide^ i- r preciable velocity has not yet been mastered, although it is a subject of ' current investigation. Development tests indicate that the sound level ' at the fan outlet is in the order of 10 to 15 db higher than the average value determined by the seven-station traverse around the fan. Where sound treatment of a distribution duct is required, the initial level should be taken as approximately 12 decibels higher than the reported Code rating sound level. KINDS OF NOISE In solving a sound problem, it is desirable to consider, separately, the several means by which noise reaches the room. This avoids to some ex tent the necessity of knowing the noise level at the source, and instead, places the emphasis on ascertaining the level at the point where the sound enters the room. The noise introduced into a room or building by ventilating or air conditioning equipment may be divided into two general kinds, depending on how it reaches the room : 1. Noise transmitted through the ducts. . From equipment such as fans, motors, pumps, sprays, etc. . From outside, and transmitted through duct walls into air stream. e. From duct wall vibrations, transmitted into air stream. d. From air currents, including eddying noises. e. Cross talk and cross noises between rooms connected by the same duct sys tem. /. Noise produced by the grilles. 2. Noise transmitted through the building construction. . From machine mountings as vibration. . From equipment through room wall surfaces. The next step in the solution of this problem is to present data and discuss methods whereby solutions of the noise problem can be obtained when the allowable room noise level, and the path through which the noise reaches the room, are known. NOISE TRANSMITTED BY AIR THROUGH DUCTS Operation of an air distribution system results in the generation of noise which may be transmitted by air through the ducts to the ventilated or conditioned room. The transmission of this noise may be controlled by the proper application of sound absorptive material within the ducts.. The application of the absorptive material is a problem in balancing the room noise level requirements against the intensity of the noise generated. The four steps in the problem are: 1. Determination of acceptable room noise level resulting from the operation of the equipment. 2. Determination of noise level generated by the equipment. Add 5 decibels to the difference between items 1 and 2 to obtain the overall noise reduction required between the equipment and the room. In the discussioD wbn follows, reduction of noise will be referred to as attenuation of noise. 3. Determination of the natural attenuation of the duct system. 4. Selection of the proper sound treatment for the duct system. The difference in decibels between the overall attenuation required and the nat ural attenuation (3) is the additional sound attenuation to be provided by absorp tive materials installed in the duct system, or by special constructions designed w absorb sound. Experience has shown, for example, that where ventilating require ments permit, introduction of an expansion chamber or a change in area in the due will frequently provide further reduction in low frequency noise. Sound Control 935 Table 2. Typical Sound Levels" Weighted Network Response Rooms Sound Film Studios....................................................... Radio Broadcasting Studios;........................................ Planetarium..................................................................... Residence, Apartments, etc........... ............................... Theaters, Legitimate...................................................... Theaters, Motion Picture............................................... Auditoriums, Concert Halls, etc................................... Churches........................................................................... Executive Offices, Acoustically Treated Private Offices Private Offices, Acoustically Untreated....................... General Offices................................................................. Hospitals................................................................................. Class Rooms.................................................................... Libraries, Museums, Art Galleries................................ Public Buildings, Post Offices, etc................................ Court Rooms................................................................... Small Stores.................................................................... Upper Floors Department Stores.................................. Stores, General, Including Main Floor Dept. Stores... Hotel Dining Rooms....................................................... Restaurants and Cafeterias........................................... Banking Rooms............................................................... Factories........................................................................... Office Machine Rooms.................................................... Sound Level in Decibels to be Anticipated Min. Represent ative Max. 10 14 20 10 14 20 15 20 25 33 40 48 25 30 35 30 35 40 25 30 40 25 30 35 30 38 45 35 43 50 50 60 70 25 40 55 30 35 45 30 40 45 45 55 60 30 35 45 40 50 60 40 50 55 50 60 70 40 50 60 50 60 70 50 55 60 65 77 90 60 70 80 Vehicles Railroad Coach Pullman Car........ Automobile......... Vehicular Tunnel Airplane....... 60** 70 55b 65 50 65 75 85 75 80 80 75 80 95 90 * These v&lues are tentative. More detailed measurements by D. F. Seacord, Bell Telephone Labora tories (Journal Acoustical Society of America, Vol. 12, pp. 183-187, 1940) give overage values and standard dtvjations of room noise in residences, offices, stores, factories, etc., in large American cities. For train standing in station, a level of about 45 db is the maximum which can ordinarily be tolerated. DESIGN ROOM NOISE LEVEL Measurements of sound levels in various types of rooms and locations nave been observed by numerous investigators. However, close agree ment upon these values has not been realized, and more detailed measure ments are needed to accurately establish the normal sound levels in occupied spaces and enclosures subject to sound analysis and control. Typi cal sound levels, of a tentative nature, based upon earlier determinations, arejisted in Table 2. The levels listed are weighted levels by the 40 db or /0 db network, depending upon the range of level existing. Levels aken upon the flat response network may be from 5 db to 20 db higher, * governed by the predominating frequencies which may influence the weighting level. anTa^e ^ sound levels based upon more recent surveys than Table 2, *Pon the basis of the flat response network. The flat response net- i 0 i offers a more logical correlation of space sound level to fan sound ve* which, under present practice, is reported upon basis of the flat espouse reading. rpi oe values listed were determined with the air conditioning or ventila-