Apparatus standards of telephonic transmission, and the technique of testing microphones and receivers

The first part of this paper deals with the selection, calibration, maintenance and operation of apparatus standards selected as representative samples of the types of telephones in general use. Particulars of the acoustical and electrical characteristics of microphones and receivers are given. The second part of the paper deals with telephone standards giving high-quality transmission and specially designed for standardization purposes, and of which the acoustical and electrical characteristics are capable of precise specification and measurement. The Appendix deals with the range of frequencies required for various classes of telephonic transmission.