Noise rating (NR) curves

It is apparent that the combination of a single figure index such as dBA, with more information on the shape of the frequency content would be useful. Noise rating curves (N R) were evolved by ISO to meet this need, largely replacing the very similar NC curves which did not follow mathematical laws and were there­fore more difficult to handle on a computer. Nevertheless, such curves continue to proliferate and we now have PNC curves and who knows what else.

NR curves consist of a family of octave band spectra, with each curve marked with its own NR rating number. The octave band spectrum of the noise being analysed is plotted on the same grid and the NR rating of that noise corresponds to the highest NR curve touched by the noise spectrum.

Figure 14.33 shows a set of NR curves and Table 14.10 gives recommended levels for various environments. The spectrum of a noise with an NR rating of 35 is also shown on the grid.

NR ratings are particularly suitable for selecting and assessing suitable background noise levels for ventilating and air condi­tioning systems.

Warning: NR curves assume SPLs in the environment and are not directly applicable to fans without knowing the room charac-









O 40



= 95 dBW = 92 dBW = 3dB = 1.7 dBW = 95+ 1.7 = 96.7 dB


1. Fan Motor

2. Difference

3. Correction (from chart)

4. Total noise

Equ 14.30


10 log

Where both inlet and outlet are ducted from the room, (Figure 14.31), it should be noted that SWL values may not specifically cover sound radiated from the fan housing. This is not a serious shortcoming since the housing radia­tion will not be the primary source of sound.

In most cases there are two other sources of sound that will pre­dominate. One is the flexible connection used in most fan in­stallations to isolate the fan vibration from the ductwork. Usually this is relatively light flexible material and becomes a source of sound far more important than that radiated from the fan. Sec­ondly, the ductwork is, in most cases, of lighter construction than the fan housing and more sound will be transmitted through the duct walls than through the fan casing.

Depending on the fan size and casing thickness, and based on experience with installations of this kind, it is recommended that the total sound power level be reduced by up to 20 dBW to esti­mate the sound level in the fan house. In installations where special isolation points (special flexible connections) and heavy ductwork are used, there can be a reduction of up to 35 dBW in the occupied space.

Posted in Fans Ventilation A Practical Guide