The value is demonstrated of the frequency/amplitude characteristics of telephone systems or apparatus in determining the transmission efficiency, including volume and articulation efficiency. The audio-frequency ranges of importance for speech and music are then considered.Methods are described for the direct and indirect production of constant acoustic output over the audio range, and the calibration of acoustic measuring instruments is dealt with.The next section of the paper deals with the production of electrical energy of constant value over the required frequency range, and a description is given of a special form of oscillator, together with associated apparatus for the direct recording of frequency characteristics.The concluding part of the paper gives frequency characteristics of each part of the apparatus used in a telephone connection, and also of audio-frequency amplifiers and loud-speakers. For the purely electrical portions of the circuit, current or voltage characteristics are given, whilst for apparatus such as transmitters and receivers the characteristics are given in terms of acoustic pressures on the diaphragm. The characteristics of some commercial types of granular transmitters are first given, then under the heading of receivers will be found the characteristics of both the Bell pattern and some typical loud-speakers of the horn and hornless types. Particulars are given of apparatus designed to measure diaphragm motion, some results obtained by an aural balancing method being included.In the section dealing with lines will be found characteristics for unloaded open wires and cables and for loaded cables. An impedance/frequency characteristic is also given and its application to fault location illustrated.Some notes on telephone line repeater characteristics are included, and the next section deals with exchange cord circuits and subscriber's instrument characteristics.Under audio-frequency amplifiers some typical intervalve transformers of the type used for broadcast reception are characterized, and an extension is given of the theory of this type of transformer covering the effects of resistance and capacity loads.