The human ear and an eye have logarithmic characteristic of sensitivity. Therefore for expression of relative change of intensity of a stream of the radiation perceived by the person it is convenient to use logarithmic units: in decibels and neper. The first of them are most widespread.
Instruction
1. Calculate the relation of the measured value of any given size to standard reference. For power it is one milliwatt, for tension of a low-frequency signal in the household audio system - one volt, for stress of a high-frequency signal removed from the reception antenna - one microvolt, for tension of a low-frequency signal in professional audio equipment - 0.775 volts, for sound pressure - one pascal. Before calculation of the relation transfer the measured size to the same units, as reference. If two sizes are given (before change and after that), divide the second into the first - any standard in this case is not required. The result of division will turn out dimensionless - it will be expressed in times.
2. Find a decimal logarithm of result of division. Do not mix it with natural (it is used at calculation not of decibels, and Napierian). On domestic scientific calculators the key of LG, on import - log is for this purpose intended. The decimal logarithm is in the majority of programming languages by means of the log or LOG function after which the argument is specified in brackets (sometimes without brackets and through a gap).
3. In case the measured size is capable to depend on other size quadratically (for example, power at an invariable tension quadratically depends on resistance), increase the result of logarithming received during performance of the previous step by ten. The calculated size will also be expressed in decibels.
4. If the measured size is not capable to depend quadratically on another (as, for example, tension), increase result of logarithming not by ten, and by twenty.
5. The so-called VU meter represents the magnetoelectric arrow indicator which characteristic of sensitivity is mechanically brought artificially closer to logarithmic. Having read indications of this device, do not carry out with them any mathematical operations. If entrance chains of the indicator are adjusted correctly, the measured size will be expressed in decibels with an accuracy sufficient for practical purposes at once. Consider that the lag effect that it did not react to signal peaks is often specially entered into VU meter. Other indicator - peak is intended for their registration.