The origin of the Decibel Unloaded (dBu) lies with the American telephone company Bell. This company had determined a reference power of 1 mW at a resistance of 600 ohms for the level determination. The impedance of 600 ohms comes from telephone technology. This level corresponds to 0 dBm ( decibel milliwatt).

If we relate the reference power to a voltage value (U), then the power (P) is calculated from 'U^2/R': the square of the voltage divided by the resistance of 600 ohms. This results in a voltage of 0.7746 V. This voltage level was the reference value for 0 dBv. However, since in Europe the dBV (capital "V") is used as the reference value for 1.0 V, the dBv was renamed dBu to avoid confusion.

Absolute voltage value as a function of the dBu value

The reference value can be used to calculate the absolute voltage value. Here, all levels that are smaller than 0 dBu have a negative sign, those that are larger than 0 dBu have a positive sign.

You can convert any dBu value to a dBV value and vice versa: 1 dBV corresponds to +2.2 dBu, 1 dBu corresponds to 0.869 V, 4 dBu, the nominal level used in professional audio, corresponds to 1.228 V, -10 dBV, the level used in semi-professional audio, corresponds to -7.8 dBu.

Informations:
 Englisch: decibel unloaded - dBu Updated at: 22.01.2022 #Words: 206 Links: telephone company (TELCO), power (P), level, impedance (Z), decibel milliwatt (dBm) Translations: DE Sharing: