Criteria for Acceptable Air-Handling Unit and HVAC System Noise Levels

3 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 kicated. 

Concerning the sound pressure level, when a noise generated by an HVAC system or an air-handling unit increases the ambient background noise by a dB, the noise increase is just perceptible. On the contrary, an increase oi 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. 9.6.3). 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. 

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