Document Ne8Dy1ZVkbGnDOXdpB3rewMmg

American Society of Heating arid Ventilating Engineers Guide, 1937^ ' 10 In Fig. 3, a static pressure of 85 per cent of blocked tight pressure co, responds to three different volumes, namely 11 per cent, 30 per cent and 48 1^ cent of wide open volume. What will determine which volume the fan delivers* The fan ran operate only at the intersection of its pressure-volume curve and the systf characteristic. The type of system, together with the specification of the volume at"1 certain static pressure, completely defines the system characteristic. a As illustrated in Fig. 5, a given system characteristic will intersect the fan curve in onlv one point. *. " If the 85 per cent value for static pressure is specified for the 48 per cent value of volume, it is at once obvious that the same system will not have the same resistance at any otW volume. i< f Si 4I S 332 Chapter 18 SOUND CONTROL Decibel Defined, Apparatus for Measuring Noise, The Sound Control Problem, Acceptable Noise Levels, Controlling Vibration from Machine Mountings, Controlling Noise through Room Wall Surfaces, Controlling Noise Transmitted Through Ducts 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 rs not assumed that the ventilating and air conditioning engineer will attempt to improve the acoustics of the space that is being con-, ditioned, but the designer should have at least enough fundamental knowledge of the acoustical effects of the system which is being designed to be sure that no damaging effects occur to the existing acoustical properties. 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 In the United States and England the unit of noise measurement is the decibel (db). In Germany this unit is called the phon. The decibel is defined by the relation N = 10 log ~< where N is the number of decibels In by which the intensity flux If exceeds the intensity flux IQ. The in tensity flux is the measure of the energy contained in a sound wave and is defined in terms of microwatts per square centimeter of wave front in a freely traveling plane wave. It is usually more convenient to select an arbitrary reference intensity for Ia and express all other intensities in terms of decibels above that level. For this purpose the threshold' of audibility for the average human ear at a frequency of 1000 cycles per second lias been selected. This reference threshold is 10-16 watts per square centimeter or 10~10 microwatts per square centimeter. 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 333