Week 14: Lecture - Signals & the dBs

The dB is a logarithmic unit of measurement. There are two ways of expressing the signal levels:

dBu: Professional level – referenced to 0.775Vrms

dBV: Consumer level – referenced to 1Vrms

The ‘u’ and ‘V’ are known as the reference level and are used with dB to give a voltage value. The value in front of the dBu/V tells us how much higher or lower the voltage is to the reference.

e.g. -7dBV = 7 decibles lower than 1V
+10dBu = 10 decibles higher than 0.755 Vrms

To calculate the voltage level you would use this formula:

dB = 20log(A/B) [A=output B=input]

dB can be used to express a voltage level with the addition of a reference unit and also the gain of a system without the addition of the reference level.

The dBSPL

Sometimes written as ‘dB’, the dBSPL is used to express sound pressure levels and is related to the range of pressures which the human ear can deal with. Only positive values are used because 0dBSPL is the smallest amount of sound pressure which the human ear can detect. 140dBSPL is the threshold of pain.

The dBFS

The dBFS is used for metering in digital systems. The ‘FS’ stands for ‘full scale’. dBFS does not have fixed reference values like dBu and dBV. When the 0dBFS level is surpassed clipping occurs, therefore only negative values of dBFS can be used.