scope:

The sending, receiving or sidetone sensitivity/frequency characteristic of a local telephone system (LTS) is usually measured directly.

NOTE 1 - The sending, receiving or sidetone sensitivity/frequency characteristic can also be calculated provided that the relevant information of the telephone line and feeding bridge is known. Some of the information required for sidetone is outside the scope of existing Recommendations.

NOTE 2 - The same principles also apply to the measurement of microphones and earphones.

Since electro-acoustical measurements of the type being considered may be required for different purposes, it is important to distinguish the following:

a) supplying the designer of a transducer with information concerning the success he has achieved in aiming at a given sensitivity/frequency response;

b) checking that the manufactured product meets the specified requirements;

c) supplying sensitivity/frequency characteristics suitable for use in calculating loudness ratings or estimating other subjectivity-determined quantities.

This Recommendation is mainly concerned with c), but the principle is also applicable to a) and b). For these purposes, especially for c), measurements under real conditions must form the basis. Artificial mouths and artificial ears must be used with due regard to obtaining good agreement between these measurements and those from real mouth and ear determinations. Measurements under real conditions are complicated, time-consuming and not reproducible with great precision. This Recommendation describes measurement methods using recommended forms of artificial mouths and artificial ears (see [ITU-T P.51] and [ITU-T P.57]).

This Recommendation applies mainly to LTSs with handset telephones. However, the principles also apply to other types of telephones. Specific considerations for headsets are described in [ITU-T P.380] and, for loudspeaker telephones, [ITU-T P.340].

See [ITU-T P.76] for general principles concerning the determination of loudness ratings.