The neper (symbol: Np) is a logarithmic unit for ratios of measurements of physical field and power quantities, such as gain and loss of electronic signals. The unit's name is derived from the name of John Napier, the inventor of logarithms. As is the case for the decibel and bel, the neper is a unit defined in the international standard ISO 80000. It is not part of the International System of Units (SI), but is accepted for use alongside the SI.
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 nepers is given by
where and are the values of interest (amplitudes), and ln is the natural logarithm. When the values are quadratic in the amplitude (e.g. power), they are first linearised by taking the square root before the logarithm is taken, or equivalently the result is halved.
In the ISQ, the neper is defined as 1 Np = 1.
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. Hence the neper and decibel are related via:
The decibel and the neper have a fixed ratio to each other. The (voltage) level ratio is
The neper is a natural linear unit of relative difference, meaning in nepers (logarithmic units) relative differences add rather than multiply. This property is shared with logarithmic units in other bases, such as the bel.
Particular to the neper, however, is that the derived unit of centineper is asymptotically equal to percentage difference for very small differences – since the derivative of the natural log (at 1) is 1; this is not shared with other logarithmic units, which introduce a scaling factor due to the derivative not being unity. The centineper can thus be used as a linear replacement for percentage differences. The linear approximation for small percentage differences,
is widely used, particularly in finance—see for example the Fisher equation. However, it is only approximate, with error increasing for large percentage changes. Measured instead in centinepers, these linear approximations can be replaced with exact equalities, and applicable to any magnitude change, by defining the following centineper quantity for any change
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