Deutsche Version 
The index u at decibel (dB) means unloaded source, V (volt). Some say: The "u" in dBu implies also that the load impedance is unspecified, or unterminated and is likely to be high. The index m at dB (decibel) means 1 milliwatt (power) as reference value for 0 dB. If the reverse conversion dBu, or dBV to volts is needed, go to the converter at the bottom. ● What is dBu? That has nothing to do with power. A logarithmic voltage ratio with a reference voltage of V_{0}= 0.7746 volt ≡ 0 dBu ● What is dBV? A logarithmic voltage ratio with a reference voltage of V_{0} = 1.0000 volt ≡ 0 dBV The max. domestic recording level of −10 dBV means 0.3162 volts, that is −7.78 dBu
● What is dBm? That has nothing to do with voltage. A logarithmic ratio with a reference power of P_{0} = 1.000 milliwatt ≡ 0 dBm. The pseudo unit "dBm" is not used in audio and in sound recording. With the known impedance value you can convert voltage V to level dBm (power) and vice versa. Often an impedance of 600 ohms or 50 ohms is assumed. For radio frequency systems (RF), impedance matching or power matching is absolutely essential.

In sound engineering there is no Power matching or Impedance matching. In audio we use only Voltage bridging or high Impedance bridging. 
Enter two values, the third value will be calculated.
Voltage V, level L, and impedance Z.
dBm indicates that the reference power is P_{0} = 1 milliwatt = 0.001 watt ≡ 0 dB_{m}
To use the calculator, simply enter a value. The calculator works in both directions of the ↔ sign. 
Telephone lines need an input impedance and
an output impedance of 600 ohms for
impedance matching (power matching).
Reference voltage at 600 ohms – 1 mW according to 0 dBm Reference voltage at 50 Ohm – 1 mW according to 0 dBm 
dBu is a voltage standard where "zero" (as in zero dB on your meters) is considered to be 0.775 volts and +4 dBu is therefore 1.23 volts. dBm is a power measurement (the "m" stands for milliwatts), where +4 also happens to be 1.23 volts if the load impedance is 600 ohms. In many cases (and this is "old school" audio) a 600 ohm load can be assumed, however, in modern day interconnects input impedances are usually much higher, which kind of renders the dBm standard useless. This is one reason why most gear is rated in dBu, or in dBV (another voltage standard). 
Impedance matching Z_{2} = Z_{1 }for telephone (phone) lines
Transformer matched 600 ohm phone transmission line
● Definition of dBm dBm is defined as power ratio in decibel (dB) referenced to one milliwatt (mW). It is an abbreviation for dB with respect to 1 mW and the "m" in dBm stands for milliwatt. dBm is different from dB. dBm represents absolute power, whereas in audio engineering the decibel is usually a voltage ratio of two values and is used then to represent gain or attenuation of an audio amplifier, or an audio damping pad. If you use modern audio equipment in sound engineering, calculations used at typically 600 ohms impedance for telephone lines or 50 ohmsfor antennas, are not correct. Matching impedance for power is not used in the field of audio. You will only use voltage bridging or impedance bridging Z_{out} << Z_{in} (Z_{source} << Z_{load}) Here are adequate calculators for audio voltage and voltage levels: 
Converter: Decibel to Voltage and vice versa
Measurement of input impedance and output impedance
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