Loudness ratings of telephone speech paths

Telephone engineers rely a great deal on loudness ratings of speech paths for the framing of national transmission plans and for international recommendations to secure proper co-ordination between national networks. Loudness ratings for such use are, by convention, based on subjective loudness comparisons against a specified high-quality reference system (NOSFER), and the measurements are difficult to reproduce accurately. A great deal of work has already been done to enable such ratings (called `reference equivalents¿) to be determined, by calculation from instrumentally measured sensitivities of the telephone sets and local lines. However, little practical use can be made of these results because of a lack of properly specified instrumental measuring methods. The situation may improve when international agreement has been achieved on the characteristics and method of use of artificial mouths and artificial ears for telephony. The paper presents a review of available fundamental information marshalled into a form convenient for use as soon as suitable electroacoustical measuring methods have been defined and accepted. The precision and reliability of loudness ratings can also be improved by replacing the wideband reference system by an `intermediate reference system¿ more like the commercial telephone equipment to be tested. Particulars suitable for such a system are suggested.