Document J3pJqy63R7b5XKbpZwMQbXe42

HEATING VENTILATINC AIR CONDITIONING GUIDE 1943 other intensities in terms of decibels above that level. For this purpose a reference intensity of 10~16 watts per square centimeter has been selected. This intensity is slightly less than the threshold of audibility for the average ear at a frequency of 1,000 cycles per second. This reference level also corresponds to a pressure of 0.0002 dynes per square centimeter. A stated sound level in decibels, unless otherwise defined, will thus be related to a threshold of 10"16 watts. For example, a level of 60 db above this reference threshold is 10~10 watts. In a similar manner, when sound measurements are given in actual intensity or energy units, they can be converted to decibels by this relation. Since the decibel is a ratio, it can only be employed when related to a reference threshold level as' given. Noise levels, which vary with fre quency as well as intensity, must not only be related to this reference threshold level, but also to a reference frequency, which is taken as 1000 cycles. These terms and procedures may be found in Tentative Standards1 published by the American Standards Association. APPARATUS FOR MEASURING NOISE Since the relative loudness to the ear, rather than the actual physical intensity, is the quantity in which engineers are usually interested, it has been found necessary to allow for the varying sensitivity of the ear at different frequencies in designing noise measuring equipment. The most satisfactory method of measuring noise is by means of a sound level meter which usually consists of a microphone, a high gain audio-amplifier, and a rectifying milliammeter which will read directly in decibels. This meter is calibrated to give readings above the standard reference level and usually contains a weighing network to make it less sensitive at those frequencies where the ear is less sensitive. For complete specifications relative to the approved type of sound level , meters refer to the infor mation2 published by the American Standards Association. GENERAL PROBLEM OF SOUND CONTROL As previously stated, .the problem confronting the air conditioning engineer is that of designing a system which will operate without in creasing the noise level in the conditioned space. To be sure that this is accomplished, it is necessary:' .\ . a. To determine the noise level existing without the equipment. b. To ascertain the noise level which would exist if the equipment were installed without sound control. c. To provide as a part of the installation sufficient sound control appliances to reduce the noise level substantially to that found in (a).. To accomplish this the engineer should have information of three kinds: 1. A.knowledge of the noise levels currently considered acceptable in various rooms in order that he may have a basis on which to proceed. American.Teqtatlve Standards for Noise Measurement, American Standards Association. .'American Tentative Standards, for Sound Level Meters for Measurement of Noise and Other Sounds, 'American Standards Association. 626 CHAPTER 33. SOUND CONTROL 2. A knowledge of the nature and intensity of the noise created by the various parts of the equipment. ' 3. A knowledge of how, when necessary, to vary and control the noise level between the equipment and the conditioned space. In addition, the engineer should have information to analyze noises which may be transmitted by the duct system from one conditioned space to another, or from an outside space to the conditioned space. While the general problem may be logically outlined and the items of knowledge necessary to its solution can be listed, the available infor mation at present is lacking in certain respects. However, attention may be directed to that information which is currently available, and a solution of the noise problem, based on these data, may be outlined. Information concerning the noise levels created by ventilating and air conditioning equipment such as fans, motors, air washers, and similar items is not yet on a basis which permits tabular presentation, although certain manufacturers are prepared to offer such data and do state the noise producing properties of their products. Absence of this information makes it necessary to resort to indirect means in solving certain problems arid also prevents a direct, logical solution. . - KINDS OF NOISE To solve a sound problem of this type if is desirable to consider sepa rately the several means by which noise reaches the room. This avoids to some extent 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 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 preserit data and discuss methods whereby solutions to the rioise 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 627