Document 2jVy9EKMZN1pro9K9xnL21KbL

HEATING VENTILATING AIR CONDITIONING CUIDE 1944 KINDS OF NOISE To solve a sound problem of this type it is desirable to consider sepa rately the several means by which noise reaches the room. This avoids to some extent the1necessity 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 with various sub-divisions: 1. Noise- transmitted through the ducts. a. From equipment such as sprays, fans, etc. . b. From outside, and transmitted through duct walls into air stream. c. From air current, including eddying noises. d. Cross talk and cross noises between rooms connected by the same duct system. . e. Noise produced by the grilles. 2. Noise transmitted through the building construction. a. From machine mountings as vibration. b. From equipment through room wall surfaces. The next step in the solution of this problem is to present data and discuss methods whereby solutions to 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 THROUGH DUCTS- Operation of an air distribution system results in the generation of noise which may be transmitted 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 problenv in balancing the room noise level requirements against the intensity of the noise generated. The four steps in the problem are: X. Determination of acceptable room noise level resulting from the operation of the equipment. 2. Determination of noise level generated by the equipment. -The difference between steps 1 and 2 in decibels is theoverallnoise reduction required between the equipment and the room. In the discussion which 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 natural attenuation (3) is the additional sound attenuation to be obtained by absorptive ma terials installed in'the duct system. .- DESIGN ROOM NOISE LEVEL Measurements of noise ievels have been observed-by several investi gators in various rooms and locations and are listed in Table I. The 622 CHAPTER 33. SOUND CONTROL values given were determined with the air conditioning or ventilation equipment not in operation, and with all windows and doors closed simulating the conditions of an actual installation. This is an important consideration, for in offices or stores adjacent to busy thoroughfares the difference between the .typical noise level in the space with the windows and doors open and' closed may be as high as 10 db. Minimum, representative, and maximum levels are given for each type of space. The values are intended to give the variation with respect to location and not to time, and may be roughly classified by the following: Minimum loudness refers to: Spaces of expensive construction, typified by double windows, carpeted floors, heavy upholstered furniture, or accoustically treated walls and ceilings. Representative loudness refers to: Spaces of average construction and furnishings which are exposed to external noises typical of the locality in which the space is usually found. Maximum loudness refers to: (i) Any space of inexpensive construction, and bare furnishings where noise is not an important factor. (2) Spaces in close proximity to very intense street traffic or to intense factory noise. In general, if the noise level in the space resulting only from the opera tion of the air conditioning equipment is equivalent to or less than the typical level given in Table 1, the installation will prove satisfactory. If the typical level and the equipment level are heard together the resultant level will be 3 db higher than either of them. In some cases it is desirable to keep the equipment noise level in the' ventilated or conditioned room at such a value that it actually will not increase the noise level in the room to any measureable degree. This can be accomplished if the equipment noise at the room can. be. kept. 10 db below the noise levels shown in Table 1. "> NOISE GENERATED BY FANS Noise generated by fan wheels may be divided into two classifications, rotational noise, and vortex noise. In ventilation and air conditioning work, where the maximum ratio between the fan .tip speed and the velocity of sound is not greater than 0.12, vortex noise is by far the most important. The. rotational nqise may be described as. that due to the thrust and torque applied-to the air. Vortex noise'is that due to the shedding of vortices from the blade and is dependent on the,angle of. attack, velocity, air turbulence, and blade shapei .:;Vprtex.noise is due to pressure variations on the blade as a result of variations of air,circulation. Given the noise level at the.outlet or inlet of onetypeof fan. construction under specific conditions of size, tip speed, and total pressure, noise-levels at other values of tip speed,, total pressure, and size may be approximated by the relationships: , 1. For constant size and pressure rating, the noise level of 'a fan will' increase with increasing speed. ' - - .:. - db (change) = 55 login ti) 623