AC-03Acoustic Fundamentals
A, B, C & Z Frequency Weighting
IEC 61672 weighting curves, and the overall dB(A) / dB(C) of an octave-band spectrum.
Single frequency
Hz
Octave-band spectrum (unweighted)
dB
dB
dB
dB
dB
dB
dB
dB
A @ 100 Hz
-19.1dB
C weighting
-0.3dB
A − C
-18.8dB
Weighting curves - IEC 61672-1
AB (obsolete)CZ
Overall Z
79.9dB
Overall A
72.1dB(A)
Overall C
79.6dB(C)
C − A
7.4dB
balanced spectrum
Band levels
Unweighted (Z)A-weighted
§ Theory of operation
Why weight a spectrum?
Human hearing is far less sensitive at low and very high frequencies, especially at moderate levels. Frequency weightings are standardised filters that approximate this sensitivity so a single number can rank how “loud” or harmful a noise is.
- A-weighting follows roughly the 40-phon equal-loudness contour. It is used for almost all occupational and environmental limits (dB(A), L_\mathrm{Aeq}).
- C-weighting is nearly flat between 31.5 Hz and 8 kHz and is used for peak sound pressure (L_\mathrm{Cpeak}) and for judging low-frequency content.
- Z-weighting (zero) is flat, it replaced the manufacturer-dependent “linear” setting.
- B-weighting sits between A and C; it is obsolete and no longer part of IEC 61672.
Analytical definition (IEC 61672-1)
\begin{aligned} R_A(f) &= \dfrac{12194^2\,f^4}{(f^2 + 20.6^2)\,\sqrt{(f^2 + 107.7^2)\,(f^2 + 737.9^2)}\,(f^2 + 12194^2)} \\ A(f) &= 20\,\log_{10} R_A(f) + 2.00\ \mathrm{dB} \\ R_C(f) &= \dfrac{12194^2\,f^2}{(f^2 + 20.6^2)\,(f^2 + 12194^2)} \\ C(f) &= 20\,\log_{10} R_C(f) + 0.06\ \mathrm{dB} \end{aligned}
The constants normalise both curves to 0 dB at 1 kHz.
Overall weighted level from bands
Add the weighting correction to each band level, then sum energetically:
L_A = 10\,\log_{10} \sum 10^{(L_i + A_i)/10}
The difference L_C - L_A is a quick indicator of low-frequency content: above roughly 15–20 dB, the noise is dominated by low frequencies and A-weighting may underestimate annoyance. It is also used to select hearing protectors (HML method).