SI unit for the ratio of two numbers expressed using a natural logarithm formula:

For two scalar values a and b, the value of Np is ln(a/b)/2

1 Np is equal to a ratio of e2 (I am not making this up!) or about 7.4 and corresponds to approximately 8.7dB. -1 Np is about 0.135 (1/e2). A decibel is 10 log(a/b)