Relative Units (dB)

Use relative units, such as dB, to display scalar and spectrum results when you want to show large and small components on the same scale.

Use the following equation to convert linear units to relative units in dB for amplitude values.

dB = 20*log(EU/EU_ref)

Use the following equation to convert linear units to relative units in dB for power values.

dB = 10*log(EU/EU_ref)

where log denotes the base 10 logarithm, EU represents the value in engineering units, EU_ref is the reference in engineering units, and EU^2 represents a value in squared engineering units, such as V^2 rms.

You typically use relative units of dB reference to 20 μPa to report acoustic measurements such as sound pressure level and fractional-octave spectra. For sound power measurements the reference is 1 pW. Harmonic distortion measurements commonly use the amplitude of the fundamental component as the reference and represent in decibels relative to the carrier (dBc). For frequency response measurements, you often use a gain of one as the dB reference. In this case, negative dB values for the magnitude indicate attenuation, positive dB values indicate gain, and 0 dB is equivalent to a gain of one.

Each measurement domain might use a specific reference. Thus you need to specify the dB reference when reporting results in dB.