The dB (decibel) is defined as 10 times of the logarithm to the ratio of the reference value of selected physical amount. Generally, the dB value is a relative value, but the unit for the power spectrum dB value before the physical calibration is V^{2}, so the dBV which sets 1 V^{2} (the square of rms) as a reference value is displayed as the absolute unit. According to this, if the power spectrum of the selected frequency is x^{2}, the dB value can be calculated in the following expression (1);

(1) 
Conversely, if the power spectrum value is y (dBV), the linear spectrum x can be calculated in the following expression (2);

(2) 
Please refer to the example values of the linear spectrum x (V) and power spectrum y (dBV) in the following diagram 1.
Diagram 1: The specific example value of the power spectrum and linear spectrum
Power spectrum(V^{2})  Power spectrum(dBV)  Linear spectrum(V) 

0.0001  40  0.01 
0.01  20  0.1 
0.5  3.01  0.707 
10  10  3.16 
100  20  10 
The power spectrum is a square average value (effective value), so sometimes the unit "dBVr" is displayed to make it more understandable.
Revised:2009/11/16