Transmitted Frequency Range for Telephone Message Circuits
William H. Martin · Bell System Technical Journal · 1930
In the Bell System the general objective which has been set up for the transmitted frequency range for new designs of telephone message circuits is a range having a width of 2,500 cycles. extending from about 250 cycles to a bout 2,750 cycles. In determining the frequency range of such a circuit, the cutoff points are taken as those at which the attenuation reaches a value 10 db greater than that at 1,000 cycles.