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IEEE 661 79th Edition, January 1, 1979
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Determining Objective Loudness Ratings of Telephone Connections
Includes all amendments and changes through Reaffirmation Notice , September 25, 2008
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This IEEE Standard Method for Determining Objective Loudness Ratings of Telephone Connections has been prepared in response to a need by the telecommunications industry for a standard method of testing transmission performance of partial and overall telephone connections in a manner that reflects the attribute of subjective loudness. This document complements and is dependent on IEEE Std 269-1971 , Standard Method for Measuring Transmission Performance of Telephone Sets. Individuals from private and government design, manufacturing, and research organizations have given freely of their time in preparing, testing, and reviewing this standard.


The purpose of this standard is to describe a practical and reproducible method of determining the loudness ratings of telephone connections. Loudness rating may be measured directly using appropriate test instruments and facilities, as specified in this standard, or may be computed using the amplitude-frequency characteristic of the connection measured in accordance with IEEE Std 269-1971 [5].

Loudness ratings determined using this standard may not exactly duplicate subjective loudness evaluations. Moreover, measured ratings and computed ratings determined using this standard may not agree, particularly if the connection incorporates nonlinear elements. However, differences are expected to be acceptably small for engineering purposes. For example, differences for connections utilizing linear microphones are expected to be small, resulting only from instrument and measurement error, while for connections employing nonlinear microphones, for example, carbon microphones, the differences may be larger.

NOTE – For the purposes of this standard, a linear microphone is one that exhibits a one-to-one relation between input pressure and output voltage over its normal operating range.

The procedures given in this standard may be used for determining the loudness rating of partial and complete telephone connections. For complete telephone connections, comprising overall and sidetone transmission paths, the procedures involve measurement of acoustic input and output pressures.

For partial telephone connections comprising transmitting, receiving, or connection paths, the procedures involve measurement of acoustic pressures and electric voltages.

Transmission planning of switched telephone networks is generally accomplished by working with parts of connections, for example, customer loops (telephone sets, line facilities, and feeding bridges) or trunks, to ensure that when these parts are switched together, the resulting overall connections perform satisfactorily. When planning is based on loudness, the sum of the loudness losses determined for parts of a connection should closely approximate the loudness loss of the overall connection. For such purposes the recommended analysis bandwidth is 300-3300 Hz, which approximates the bandwidth of the more band-limiting components typically found in modern telephone networks. There may be cases in which a wider bandwidth would be desirable (see Appendix A), and the methods of this standard will accommodate such wider bandwidths.

The method of this standard is based on specification of (1) characteristics of a suitable artificial mouth, (2) a standard artificial ear, and (3) appropriate auxiliary measuring instruments and facilities. The method is intended for rating connections involving telephone sets equipped with handsets. The applicability of the method for the measurement of telephone sets incorporating special devices such as noise-exclusion transmitters, distant-talking transmitters, ear insert receivers, or noise-exclusion receivers (receivers equipped with circumaural earpads) has not been reviewed. However, the loudness theory on which this standard is based is sufficiently general that advances in the art may permit measurement of many such special devices using the methods specified in this standard.