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 V², so the dBV which sets 1 V² (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 x², the dB value can be calculated in the following expression (1);

 

 

Conversely, if the power spectrum value is y (dBV), the linear spectrum x can be calculated in the following expression (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(V2)	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.