In a paper in Metrologia 2001,*38*, pp 353-361 the authors argue that the neper is the SI unit of logarithmic decay rather than the bel. They argue this by analogy with the radian.
I contend that this is a false analogy. The radian is a measure of angle such that a torque of T newton metres = T joules per radian. If the torque produces one revolution the work done is 2 pi T joules. If a sound has an amplitude e (= 2,718..) times the amplitude of a base souund, it has a sound level difference of 1 neper. Auditory power of a simple harmonic signal varies as the square of the amplitude of the sound. If a sound has an auditory power of 1 bel, its power is 10 times that of the sound taken as base. Note that the neper and the bel differ not only in their logarithmic base, but also in what they measure. The metric system was originallly known as the decimal metric system to distinguish it from the many metric systems that preceeded it. For that reason, and because it measures the quantity of human interest rather the square root of that quantity. I favor the bel. Judging from the relative frequency of references to the neper and the bel, I think the technical consensus agrees with me. Joseph B.Reid 17 Glebe Road West Toronto M5P 1C8 Tel. 416 486-6071
