Preparatory Notes for ASNT NDT Level III Examination - Ultrasonic Testing, UT

Like the decibel, the Neper is a unit in a logarithmic scale. While the Bel uses
the decadic (base-10) logarithm to compute ratios, the Neper uses the natural
logarithm, based on Euler's number (e ≈ 2.71828).
The value of a ratio in Neper is given by
where x1 and x2 are the values of interest, and ln is the natural logarithm.
In the ISQ, the Neper is defined as 1 Np = 1.
1 Np = ln (2.718) when the ration of = 2.718
The neper is defined in terms of ratios of field quantities (for example, voltage
or current amplitudes in electrical circuits, or pressure in acoustics), whereas
the decibel was originally defined in terms of power ratios. A power ratio 10
log r dB is equivalent to a field-quantity ratio 20 log r dB, since power is
proportional to the square (Joule's laws) of the amplitude.