Criteria for Acceptable Air-Handling Units and HVAC System Noise Levels
The disturbance caused by a noise depends on its intensity (equivalent pressure level L in dB(A)), its frequency spectrum (that is its energy distribution), and the acoustic characteristics of the medium in which the listener is located.
Concerning the sound pressure level, when a noise generated bv an HVAC system or an air-handling unit increases the ambient background noise by } dB, the noise increase is just perceptible. On the contrary, an incrc. iv.- ui 5 dB or more is clearly perceptible.
As regards the noise spectrum, the different situations can be analyzed ap proximately with NC (noise criterion) and NR (noise rating) curves (Fig. 9.63). NC and NR curves define the octave band limits of an acceptable back ground noise: each of them is characterized by a number representing the sound pressure level at 1000 Hz.
The procedure must be carried out in this manner: the noise spectrum is superimposed on NC or NR diagrams, and the highest intercepted NC or NR curve represents the noise. For example, when a noise is represented by an NC 50 curve, it means that its spectrum does not exceed the NC curve, in correspondence of which at 1000 Hz the pressure level is equal to 50 dB(A). If the intercepted point is placed between 250 and 1000 Hz, the noise is classified as neutral; under 250 Hz is called rumbly, while over 1000 Hz is classed as hissy.
A more recent method of analysis uses the RC (room criterion) curves. In this case it must be calculated the arithmetical mean of sound pressure level at 500, 1000, and 2000 Hz. The obtained value identifies the specific RC curve. The noise is classified as rumbly (with excess of energy at low frequencies) if it is under 500 Hz and its sound pressure level exceeds the RC value by 5 or more dB. The noise is classified as hissy (with excess of energy at high frequencies) if it is over 500 Hz and its sound pressure level exceeds the RC value by 3 or more dB.
The acceptability of a noise is fixed from the designer on the basis of specific data for the area depending on state and country laws. In some countries there are regulations requiring that the workers cannot be exposed to a noise exceeding set limits.
Unsteady noise can be evaluated by a phonometer, which measures the sound pressure level for a time period of noise fluctuation and gives the time- weighted average value.
The actual noise levels produced by HVAC systems can vary considerably, and it is not possible to generalize the problems that may be encountered. From a safety point of view, it is advisable to start hearing conservation programs for workers. Permanent hearing damage will result when the noise levels exceed 80 dB(A) for a given time period. Whenever possible, it is desirable to control noise pressure levels to meet the requirements of speech communication; in this case noise should not exceed 65-70 dB(A).
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