The decibel (dB) is defined by the following expression:
where: LdB = The signal level in dB
L1 = Vibration level in Acceleration, Velocity, or Displacement
Lref = Reference level, equivalent to 0 dB
The Bell Telephone Labs introduced the concept of the decibel before 1930. It was first used to measure relative power loss and signal to noise ratio in telephone lines. It was soon pressed into service as a measure of acoustic sound pressure level.
The vibration velocity level in dB is abbreviated VdB, and is defined as:
The reference, or "0 dB" level of 10-9 meter per sec is sufficiently small that all our measurements on machines will result in positive dB numbers. this standardized reference level uses the SI, or "metric," system units, but it is not recognized as a standard in the US and other English-speaking countries. (The US. Navy and many American industries use a zero dB reference of 10-8 m/sec, making their readings higher than SI readings by 20 dB.)
The VdB is a logarithmic scaling of vibration magnitude, and it allows relative measurements to be easily made. Any increase in level of 6 dB represents a doubling of amplitude, regardless of the initial level. In like manner, any change of 20 dB represents a change in level by a factor of ten. Thus any constant ratio of levels is seen as a certain distance on the scale, regardless of the absolute levels of the measurements. This makes it very easy to evaluate trended vibration spectral data; 6 dB increases always indicate doubling of the magnitudes.
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