Basic FAQ for FFT Analyzer

dB (decibel) is defined as ten times the logarithm of the ratio to the reference value of a physical quantity, and is usually a relative value. However, the dB value of the power spectrum is measured in V when the physical quantity does not calibration ² Therefore, 1 V ² (RMS squared) as the reference value dBV is displayed as the absolute unit. In other words, the power spectrum of a certain frequency is x² If so, the dB value is:

 

（1）

is required. Conversely, when the power spectrum value is set to y (dBV), the linear spectrum x is;

（2）

 

Table 1 shows examples of numerical values for the linear spectrum x (V) and power spectrum y (dBV).

Table 1: Specific numerical examples of 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

 

(Note) The power spectrum is the square mean value (RMS), so it is sometimes displayed in a more understandable unit, dBVr.

 

4-1 dB logarithmic scale (what is a sound level meter) Decibels (dB) (FFT Basic Terminology) 4.4 Y-axis Values (About the FFT Analyzer) Linear/Log Conversion of Power Spectrum (FFT Basic Terminology)