Acoustics - Fundamentals

SWEGON AB Indoor Climate Systems 2004 - Air distribution products - Rev. 5 June, 2007
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To estimate how disturbing a sound is, the frequency analysis of the sound can be compared to normalised noise curves, (so-called NR curves. The NR value is given as the highest number on the highest NR-curve, which is a tangent of the curve for frequency analysis.A direct conversion of a dB(A) value to an NR value cannot be done. As a guideline we can say that a dB(A) value is roughly five units higher than the NR value. The difference depends completely on the sound frequency distribution.

Figure 67. NR-curves.
Lp (dB) = Sound pressure level
A = Mid-frequency (Hz)


Octave band	Mid-frequency	Band limits	Wavelengths
No.	Hz	Hz	m
2	125	88-177	2.720
3	250	177-354	1.360
4	500	354-707	0.680
5	1000	707-1410	0.340
6	2000	1410-2830	0.170
7	4000	2830-5660	0.085
8	8000	5660-11300	0.043

Table 11. Recommended octave bands according to ISO.


Mid-frequency	Filter A	Filter B	Filter C
Octave band	(dB)	(dB)	(dB)
Hz