Document XRg9rxJLwvgQY7oXknK3d47KJ

772 , CHAPTER 42 1943 Guide DESIGN ROOM NOISE LEVEL Measurements of noise levels have been observed by several investi gators in various rooms and locations and are listed in Table 1. The 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 Table 1. Typical Noise Levels3 Rooms Noibs Lsvel m Decqku to bbAnticipatbd 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 RoomsI----------------------------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.-------------------------------------------- Railroad Coach__ ___ Pullman Car________ Automobile Vehicular Tunnel Airplane Vkhiclss Min. 10 10 . 15 33 25 30 25 25 30 35 50 25 30 30 45 30 40 40 50 40 50 50 65 60 60b 55b 50 75 75 Representative - 14 14 20 40 30 35 30 30 38 43 60 40 35 40 55 35 50 50 60 50 60 55 77 70 Max. 20 20 25 48 35 40 40 35 45 50 70 55 45 45 60 45 . 60 55 70 60 70 60 90 80 70 80 65 75 65 80 85 95 80 90 These values are tentative. More detailed measurements by D. F. Seacord, Bell Telephone Labora tories (Journal Acoustical Society cf America. Vol. 12, pp. 183-187. 1940) give average values and stand ard deviations of room noise in residences, offices, stores, factories, etc., in large American cities. hFor train standing in station a level of about 45 db is the maximum which can ordinarily be tolerated. space with the windows and doors open arid 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 acoustically 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. Sound Control 773 Maximum loudness refers to: (1) 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 equal and heard together the resultant level will be 3 db higher than either of them alone. 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 noise 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 shape. Vortex 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 one type of 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 point of rating, the noise level of a fan will increase with increasing speed. db (change) = 55 log,,, (1) 2. For constant pressure and tip speed, the noise level of a given type of fan will increase with increasing fan size. db (change) = 20 logi (f^) (2) Fan size refers to wheel diameter, housing height or some dimension that is directly proportional to lineal units. Fan sizes based on arbitrary systems or systems of preferred numbers have no significance. The noise of a given fan is not constant at constant speed if the air delivery changes due to change of resistance. In general, a backward curved blade fan is lowest in noise at or near the point of maximum efficiency; a forward curved blade fan at or between the point of maximum efficiency and shut-off; an axial flow fan at or between the point of maxi mum efficiency and free delivery. The noise level of a double width fan Table 2, Attenuation in Straight Sheet Metal Duct Runs Duct Size, In. Attenuation pee Ft. db 6x6 24x24 . 72 x 72 0.10 0.05 0.01