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Strutt|Criteria|Curves|Sound Levels|NR inserts the Noise Rating (NR) curve into the active row of the worksheet. The user may enter the curve number into Column D of the active row to return a specific NR curve.

The NR curve is defined for integer curve numbers, and is calculated from the following formula:

`NR = A + B(NR#)`, where `NR#` is the curve number

and `A` and `B` are frequency-dependent coefficients defined as:

Frequency	31.5 Hz	63 Hz	125 Hz	250 Hz	500 Hz	1 kHz	2 kHz	4 kHz	8 kHz
A	55.4	35.4	22.0	12.0	4.8	0.0	-3.5	-6.1	-8.0
B	0.681	0.790	0.870	0.930	0.974	1.000	1.015	1.025	1.030

Note:

• BS8233 provides the coefficients to calculate the NR curves. However, there is an error in the coefficients, particularly at 500 Hz. The A and B coefficients from ISO R 1996 are used in Strutt, as per the table above.
• Strutt rounds each curve value to 1 decimal place, using the `A` and `B` values above as defined in the standard.

Reference:

• BS8233 (1999)
• ISO R 1996