A Revised Telephone Transmission Rating Plan
P. W. Blye, O. H. Coolidge, H. R. Huntley · Bell System Technical Journal · 1955
Although the telephone appears to be a relatively simple device, it is in fact one of the more complex elements of the telephone business, primarily because its operation depends on transforming sound waves into electrical waves and vice-versa. This fact makes its performance difficult to evaluate and measure. It is not surprising, therefore, that the problem of evaluation and measurement, that is, of “rating”, has been the subject of much thought and work over many years and that even today widely different methods are used in different parts of the world. The Bell System has done its full share of work in this connection and as the art and state of knowledge have changed, several different rating methods have been used. In the past twenty-five years or so the methods have come almost full circle from “loudness” through “effective transmission” to “loudness” again; but with a significant difference. The earlier ratings (loudness originally, effective later) were always in terms of so many db poorer or better than a standard rating. While this standard was specifically defined, only a few people were sufficiently familiar with it to appreciate thoroughly whether something which was rated, say, 6 db better than the standard was good, bad or indifferent. The new rating system avoids this problem by not having any fixed standard at all. It is based primarily on the ratio of the loudness of the sounds into the listener's ear to the loudness of the sounds out of the talker's mouth but with suitable modifications to take care of factors other than loudness which affect ease of conversation.