dBu(decibel unloaded)
“dBu” is a unit used to represent signal levels in audio systems.The reference point for dBu is 0.775V (RMS). This logarithmic scale unit indicates signal strength, with positive values representing levels higher than 0.775V and negative values pointing to levels below 0.775V.
It’s important to differentiate dBu from dBm. While dBm is a unit that signifies power with 1mW at 600 ohms as its reference, dBu is a unit typically used with the assumption of audio connections. In most instances, the transfer occurs from low impedance outputs to high impedance inputs. Therefore, dBu designates 0.775V as its standard irrespective of impedance, under the presumption of an audio connection rather than a telephone line (historically, telephone equipment matched all devices at 600 ohms).
The term “unloaded” essentially means “without a load” or “without resistance”.
In essence, while dBm focuses on power and references the 600-ohm standard, dBu centers on voltage, independent of resistance.
To sum it up, dBu is a unit used in professional audio systems to denote signal strength, with 0.775V as its benchmark.