This tool converts from dBu to dBm.
Formula
dBm = dBu + 27.78 – 10*Log10(Z)
where Z is the impedance in ohm (Ω)
Example Calculations
A value of 0 dBu is equivalent to 10.79 dBm or 0.012 Watt when the impedance is 50 ohm
dBm and dBu are numerically equal when the impedance is 600 ohm.
Background
What is dBu?
dBu is a unit of measurement used to express the voltage level of an audio or electrical signal. It is widely employed in audio engineering and telecommunications to describe signal levels in decibels (dB) relative to a reference voltage of 0.775 volts RMS (Root Mean Square).
- dB: A logarithmic unit representing a ratio of two quantities, commonly used to measure power, voltage, or sound levels.
- u: Refers to an “unloaded” circuit, meaning the measurement is not tied to a specific impedance.
The reference of 0.775 volts was historically chosen because it corresponds to 1 milliwatt of power in a 600-ohm system, a standard impedance in early telecommunications and audio systems.
Formula
To calculate dBu, use the formula:
dBu = 20⋅Log10(VRMS/0.775 V)
Where:
- VRMS is the RMS value of the signal in volts.
- 0.775 V is the reference voltage.
Example Conversions:
- A signal with 0.775 V RMS is 0 dBu.
- A signal with 1.55 V RMS (double the reference) is +6 dBu.
- A signal with 0.387 V RMS (half the reference) is -6 dBu.
Applications of dBu
- Audio Engineering:
- Describes signal levels in professional audio equipment to maintain proper gain staging and prevent distortion.
- Helps standardize levels across devices.
- Broadcasting:
- Ensures consistent levels in audio signal transmission and processing.
- Telecommunications:
- Measures voltage levels in circuits without assuming a specific load impedance.
What is dBm?
dBm stands for deciBel referenced to 1 milliwatt. It is an absolute unit of power and there’s an easy conversion from Watt to dBm.
PdBm = 10*log10(1000*PW)
where
- PW is the power expressed in watt.
- PdBm is the power expressed in dBm