FFT Analyzer Basic FAQ

dB (decibels) A dB value is defined as 10 times the logarithm of the ratio of a certain physical quantity to a reference value, and is usually a relative value, but the dB value of a power spectrum is in units of V when the physical quantity is not calibration. ² Therefore, 1 V ² (RMS squared) was used as the reference value. dBV It is displayed as an absolute unit. That is, the power spectrum of a certain frequency x² Therefore, the dB value is:

 

（1）

This can be calculated as follows. Conversely, if the power spectral value is y (dBV), the linear spectral value x is:

（2）

 

Numerical examples of the linear spectrum x (V) and power spectrum y (dBV) are shown in Table 1.

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) Since the power spectrum is a mean square (RMS) value, it is sometimes expressed in dBVr for easier understanding.

 

A logarithmic scale of 4-1 dB (what is a sound level meter?) Decibels (dB) (FFT Basic Glossary) 4.4 Y-axis values (Regarding FFT analyzers) Linear/logarithmic conversion of power spectrum (FFT basic terminology glossary)