Document JvDBMR304YLQqapkRd1Yo8RaZ

CHAPTER 32 Sound (Control Unit of Noise Measurement, Apparatus for Measuring Noise,General Problem, Kinds of Noise, Noise Transmitted Through Ducts, Design Room Noise Level, Noise Generated by Fan, Natural Attenuation of Duct System, Duct Sound Absorbers, Air Supply Noises, Grille Selection, Cross Transmission Be tween Rooms, Controlling Vibration from Machine Mountings IN ventilating and air conditioning a, building or a-room, the effect of the. mechanical system employed must be considered on the acoustics of the space conditioned. It is important to consider also that the use of air conditioning often permits keeping the windows closed, thus giving relief from certain external noises, but at the same time increasing the necessity of providing adequate sound control. It is assumed that in a given space the architect and acoustical engineer have produced a room or rooms which are satisfactory for speech, music, or other uses. The ventilating engineer's sole function is to ventilate and air condition these rooms properly so that they will be physically comfortable without adding any acoustical hazards. UNIT OF NOISE MEASUREMENT According to an international standard, two terms are used for noise measurement. The decibel (db) is the physical unit for expressing in tensity or pressure levels. The phon is the unit of loudness level. The loudness level, in phons, of any sound is by definition equal to the in tensity level in decibels of a thousand cycle tone which sounds equally loud. The decibel is defined by the relation N = 10 logTM -y-, where N i'the number of decibels by which the intensity flux Ii exceeds the intensity flux I0. The intensity flux is the measure of the intensity of a sound wave and is defined in terms of watts- per square centimeter passing through a, unit area of wave front in a freely traveling plane wave. It is usually more convenient to select an arbitrary reference intensity for I0 and express all other intensities in terms of decibels above that level. For this purpose a reference intensity of 10"w 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 for sound in air at usual room temperatures. 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 based on a ratio, it can only be employed when related to a reference threshold level as given. Noise levels, which vary with frequency 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 Standards1 published by the American Standards Association. `American Standards for Noise Measurement, Z24.2-1942, American Standards Association. Sound Control 585 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 response 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 le\^l and usually contains ,a weighting 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 mation* 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: :. 1. To determine the noise level existing without the equipment. 2. To ascertain the noise level which would exist if the equipment were installed . without sodkd control. 3. To provide as a part of the installation sufficient sound control appliances to reduce the noise level substantially to that found in Item 1. 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. 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. Information concerning the noise levels created by ventilating and air conditioning equipment such as fails, motors, air washers, and similar items is not yet oh 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. A sound test code for fans ' has been developed by the National Association of Fan Manufacturers ' which uses the flat response network of the sound level meter. However, when determining the noise generated by an air distribution system, it is- customary to use the noise level of the fan as determined by the weighting network most-nearly approaching the noise level of the space. In most cases this will be the noise level of the fan as determined on the 40 decibel weighting network. Refer to Table 1 for typical noise levels. 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 the necessity of knowing the noise level at the source, and L'Vd MeterS for Murtrhent of Noise and Other Sounds, Z24.3-1944,