NOTE 1 – The kind of quantity must be specified, such as by (squared) sound pressure spectrum level.

NOTE 2 – In view of the fact that filters have finite bandwidths, practically the sound pressure spectrum level L_ps is obtained for the centre frequency of the band by the formula:

${\displaystyle L_{\text{ps}}=10{\text{log}}_{10}\frac{\left(p^2/B\right)}{\left(p_0^2/B_0\right)}\text{dB}}$

where p and p₀ are respectively the given field quantity and the reference quantity; B et B₀ are respectively the effective bandwidth of the filter and the reference bandwidth of 1 Hz.

When L_p is the band pressure level observed through the filter, the above relation reduces to

$L_{\text{ps}}=L_{\text{p}}-{\text{log}}_{10}\left(B/B_0\right)\text{dB}$