| level of the limit, as the width of the band approaches zero, of the ratio of a specified quantity distributed within a frequency band to the width of the band
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 Lps is obtained for the centre frequency of the band by the formula:
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 Lp is the band pressure level observed through the filter, the above relation reduces to