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SV 977D User Manual - Appendixes

F.2.3. Determination of prominent tones

Bandwidth and centre frequency of critical bands

The widths Δf

of the critical band about the tone frequency f

is given in equation (C.1):

The corner frequencies can be delivered as given in equations (C.2) and (C.3), since the central frequency of

the critical band is a geometric mean of the boundaries:

For the definition of the centre frequency of a critical band, only tones with level higher than the adjoining ones

should be regarded as significant.

NOTE. A tonal audibility is performed for a tone only if its distinctness is at least 70%, that means it should

satisfy the set of requirements that is given by the following standard together with edge steepness and maximal

bandwidth.

Mean narrow-band level of the masking noise, L

The mean narrow-band level is derived in an iterative procedure from the lines of the critical band about the

line under investigation. The investigation range is determined by boundaries of the critical band of a pending

tone.

The procedure commences with the energy averaging of all lines of the critical band with the exception of the

central frequency line. The next step is repeating the procedure excluding the lines above the mean level

calculated in the previous step by 6 dB.

The mean narrow-band level is given by equation (C.4).

The procedure ends, if the subsequent mean narrow-band sound level of the masking noise is equal to the

calculated in the current calculations loop within a tolerance of ±0.005 dB. The procedure also ends, if the

number of lines contributing to the subsequent mean narrow-band level to the right or left of the line under

investigation falls below a value of 5.

NOTE 1. The second addend of the equation (C.4) could be simplified and used as a constant summand equal

to -1.76 dB. It’s possible since we apply the Hanning window, that provides the effective bandwidth Δf

be 1.5

times the frequency resolution Δf.

NOTE 2. If the iteration is discontinued, because the remaining number of spectral lines to be averaged on one

or both sides fall below 5, then the audibility may be somewhat greater than the audibility calculated with this

mean narrow-band level.

NOTE 3. In principle, a tone may only be present if the level of the spectral line considered is at least 6 dB

greater than the corresponding mean narrow-band level L

Svantek SV 977D User Manual

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