Coustic XM6 Manuel d'utilisateur télécharger pdf (Page 45)

To be ANSI/TIA-470.130-C

Annex B (Normative) – Calculation of Loudness Ratings 779

This Annex details the loudness rating calculations and weighting factors relevant to this document. 780

Loudness ratings are used to insure that the loudness of a connection from the Mouth Reference 781

Point (MRP) of the talker to the Ear Reference Point (ERP) of the far end listener is at a satisfactory 782

level. The loudness of the complete path is designated as the Overall Loudness Rating (OLR). The 783

MRP to electrical component is referred to as the Send Loudness Rating (SLR). The electrical to 784

ERP component is the Receive Loudness Rating (RLR). The loudness of the sidetone path of 785

handsets and headsets is referred to as the Sidetone Masking Rating (STMR). 786

Loudness ratings are used rather than simple level measurements because of better subjective 787

correlation. Loudness ratings more closely account for changes in the ear’s sensitivity at different 788

frequencies and its nonlinear response to varying sound levels. The following calculations are based 789

on the 1999 revision of ITU-T Recommendation P.79. Older versions of P.79 should not be used. 790

ITU-T P.79 provides information on the derivation of the loudness rating algorithm. 791

For use in these loudness ratings formulae, frequency response measurements shall be converted to 792

the R10 format, by using band averaging according to IEEE Std 269. 793

At each ISO R10 preferred frequency 794

′













∑

() log10

795

where 796

H’(f) = response at the new preferred ISO R10 frequency 797

f = preferred ISO R10 frequency 798

N = number of response values within the 1/3 octave band centered at f 799

i = index for each response value within the 1/3 octave band 800

= measured response value (in dB) 801

802

For the lowest frequency within the band, i = 1. For the highest included frequency, i = N. The 1/3 803

octave pass band limit frequencies can be calculated according as: 804

10 0 05(/ ) .

805

Where n is the band number. 806

B.1 Send Loudness Rating (Handset): 807

Band 17

808

SLR

- 57.1 log

∑

(0.1 * 0.175 * (S

– W

–

))

809

i = Band 4

810

where: 811

i Frequency bands from Table 1 of ITU-T P.79 (1999), bands 4-17. 812

Send frequency response data (Sensitivity, Mouth-to-Junction) in dB(V/Pa). 813

Send weighting factor from Table 1 of ITU-T P.79 (1999), see Table B.1 814

Impedance sensitivity correction = 1.76 dB when 900 ohm termination is used in the 815

measurement. 816

817

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Coustic XM6 Manuel d'utilisateur Page 45

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