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The reference voltage for 0 dBu is 0.775 volt (0.77459667 V) and for 0 dBV it is exactly 1.0 volt.
Scroll down to find the formulas for voltage and power and the calculation of the absolute level.
The origin of the index of dBu comes from "u = unloaded" and dBV comes from "V = 1 volt". Some say: The "u" in dBu implies that the load impedance is unspecified, unterminated, and is likely to be high. What is dBu? 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 home recording level (consumer audio) of −10 dBV means 0.3162 volts, that is −7.78 dBu. The studio recording level (pro audio) of +4 dBu means a voltage of 1.228 volts. The maximum undistorted level of audio amplifiers is +18 dBu. In USA it is +24 dBu. Domestic gear with a −10 dBV level is usually unbalanced. Studio gear with a +4 dBu level is always balanced. 
Scale: Level in dBu and dBV in comparison to the voltage in V
Level dBu 
Voltage volt 
Level dBV 

Studio level international  +4 ●  1.228  +1.78 
Standard 1 Volt  +2.22  1  0 ref. 
Standard 0.775 Volt  0 ref.  0.775  −2.22 
Domestic level  −7.78  0.316  −10 ● 
The level difference between +4 dBu studio level and −10 dBV consumer level is Δ L = 11.78 dB (12 dB). The level difference between dBu level and dBV level is 2.2 dB. 0 dBV equals 2.2 dBu or 0 dBu equals −2.2 dBV. The conversion from level (dBu) to voltage (volt) is V = 0.775 × 10^{(L/20)}. The conversion from voltage (volt) to level (dBu) is L = 20 × log (V/0.775). 
All field sizes, like the voltage or the sound pressure are
always true RMS values, if not otherwise stated. In mathematics, the root mean square (abbreviated RMS or rms), also known as the quadratic mean, is a statistical measure of the magnitude of a varying quantity. 
For sinusodial voltages or currents with ohm's loads calculations can made easier with RMS = amplitude / √2 
Level  Voltage 
Note  Comparing dBSPL and dBA: There is no conversion formula for measured dBA values to sound pressure level dBSPL or vice versa. Also you cannot convert "dBA to volts" and vice versa. Conversion is only possible for measuring one single frequency. 
Pro audio equipment often lists an Aweighted noise spec – not because it correlates well with our hearing – but because it can "hide" nasty hum components that make for bad noise specs. Words to bright minds: Always wonder what a manufacturer is hiding when they use Aweighting. *) 
*) http://www.google.com/search?q=Always+wonder+what+a+manufacturer+Rane&filter=0
We don't use the dBm in audio engineering. That belongs to power, we don't need here. If you really need dBm with "m = 1 milliwatt or one thousandth (10^{−3}) of a watt", go to: The dBm calculator and power matching or impedance matching 
Explanation: What is "dBFS"? (Digital Audio)
dBFS  Digital recording level
Analog levels and digital levels are different realms. 
♦ An often posted question: "Please, can you help me convert from dBFS to dBu".
Never express analog signal levels in terms of dBFS.
Follow this and you will not confuse anyone.
Notice  Comparing dBu and dBFS: There is really no fixed world standard like e.g. −20 dBFS = +4 dBu = 0dBVU. The digital peak scale is not equivalent to the analog RMS scale. 
dBu is volts  you measure it with a volt meter. dBFS is in contrast a binary number. 
There is no such thing as peak volts dBu *) It is incorrect to state peak voltage levels in dBu. 
dBFS must have a minus sign at the beginning. There is not something like +6 dBFS.
There is no such standardized reference. x dBFS is a digital voltage level
(peak) and y dBVU or dBu is an analog voltage level (RMS). Digital and analogue are two totally different realms. That's why there is no relation between dBFS and dBVU or dBu, whatsoever. Analog meter (ppm): attack time 10 to 300 ms − reading rms values. Digital meter: attack time < 1 ms − reading peak values. That is really some difference. Advice: Watch only your digital meters and go up to 0 dBFS, but never go over this mark. We use "headroom" in the analog domain. That is OK, but we don't need digital "headroom" as an always "unused" forbidden zone. You are free to choose your private headroom, if you like that, but there is no standard that you have to do that. The demand for a high modulation level, stand in the contrary to the claim, to avoid overloading. Never take the following funny guessing game for granted. Use it only as a rough guide: European & UK calibration for Post & Film is −18 dBFS = 0 VU BBC spec: −18 dBFS = PPM "4" = 0 dBu American Post: −20 dBFS = 0 VU = +4 dBu Orchestral −18 dBFS = 0 VU = +4 dBu Rock and / or Radio −16, or −14, or −12 dBFS = 0 VU = +4 dBu Digi 002 is only capable of −14 dBFS. German ARD & studio PPM +6 dBu = −10 (−9) dBFS. +16 (+15) dBu = 0 dBFS. No VU. • EBU R682000  The European Broadcasting Union recommends: digital level −9 dBFs (maximum). You have to keep the upper 9 dBs empty without any use. The reference level is −18 dBFs. 0 dBFs is equal to +15 dBu. Notice: 0 dBFS is the permitted maximum digital level. The EBU broadcasters have a problem, because they want to use the old slow meters with the dBu scale (attack 10 ms, quasipeak) of the analog times for digital recordings. The rest of the world looks always at the fast digital meters (attack < 1 ms, peak) with the dBFS scale. Forget looking at the dBu scale of the meters. It seems to come a change from QPPMmodulation to loudness (ITU/EBU) and truepeak. Look at: EBU R 128. 
Note: The guidelines of the EBU to set the maximum gain
to −9 dB dBFS should not apply if not working for the
European Broadcasting Union. Whose maximum levels of
digital CD masters are −9dBFS, should not be surprised if
the CDs are not loud enough. 9 dB up to the top are left free with really no use. 
More readable papers of the topic "Loudness and Level": 10 things you need to know about ... EBU R 128  the EBU loudness recommendation Florian Camerer: Loudness On the way to nirvana  audio levelling with EBU R 128 
LUFS = Loudness Units relative to Full Scale 
The formulas for voltage and power
and the calculation of the absolute level
To use the calculator, simply enter a value. The calculator works in both directions of the ↔ sign. 
dBm indicates that the reference power is P_{0} = 1 milliwatt = 0.001 watt ≡ 0 dB
Conversion of voltage or power ratios to decibels dB  table and chart
Table of Sound Pressure Levels and Corresponding Sound Pressure and Sound Intensity
RMS voltage, peak voltage and peaktopeak voltage
The parameters of the mains or "power" sine wave form are summarized at the table below:
Average voltage  RMS voltage (V_{RMS})  Peak voltage (V_{p}) = (Û)  Peaktopeak voltage (V_{pp}) 
0 volts  117 volts = V_{RMS} = ~V  165 volts = √2×V_{RMS} = 0,5 × V_{pp}  330 volts = 2×√2×V_{RMS} = 2 × V_{p} 
0 volts  230 volts = V_{RMS} = ~V  325 volts = √2×V_{RMS} = 0,5 × V_{pp}  650 volts = 2×√2×V_{RMS} = 2 × V_{p} 
The value V_{RMS} of an alternating voltage V (t) = V_{0} × f(t)is defined so that the effective DC power corresponds V_{RMS}^{2} / R = V_{RMS} × I_{RMS} to an ohmic resistance of the middle resistive power of this AC voltage to the same resistance. 
The crest factor means the ratio of the peak voltage to the RMS voltage.
If you need to calculate an attenuator (attenuation calculation) you calculate a voltage divider.
Voltage conversions
Voltage  V_{RMS} = ~V  V_{p}  V_{pp} 
Average voltage RMS V_{RMS} =  −  0.7071 × V_{p}  0.3535 × V_{pp} 
Peak voltage V_{p} =  1.414 × V_{RMS}  −  0.5000 × V_{pp} 
Peaktopeak voltage V_{pp} =  2.828 × V_{RMS}  2.000 × V_{p}  − 
Different voltage levels
Level  Level L in dB  Voltage RMS  Voltage peaktopeak 
European studio level  ARD broadcast level  +6 dBu  1.55 V  4.38 V 
International studio level  USA  +4 dBu  1.228 V  3.47 V 
Domestic recording (Consumer units)  −10 dBV  0.3162 V ≡ −7.78 dBu  0.894 V 
Sound pressure level (auditory threshold)  0 dB  2×10^{−5} Pa ≡ 0 dBSPL  5.66×10^{−5} Pa 
Reference studio level re 0.775 volt  0 dBu  0.7746 V  2.19 V 
Reference studio level re 1 volt  0 dBV  1.0000 V  2.828 V 
International reference values
Physical unit  Level unit  Reference value  Note 
Voltage  V_{0} = 0.775 V  ≡ 0 dBu  Audio engineering, no impedance reference! 
Voltage  V_{0} = 1 V  ≡ 0 dBV  Audio engineering, USA 
Voltage  V_{0} = 1×10^{−6} V  HF receiver and amplifier technology  
Voltage  V_{0} = 0.224 V  HF technology  Reference 1 mW at Z = 50 Ω  
Voltage  V = 1.228 V  Studio level +4 dBu, USA  Reference 0.7746 V  
Voltage  V = 1.55 V  Studio level +6 dBu, ARD  Reference 0.7746 V  
Voltage  V = 0.3162 V  Home recording level −10 dBV  Reference 1.0 V ≡ −7.78 dBu  
Sound pressure  p_{0} = 2×10^{−5} Pa  ≡ 0 dB  Sound Pressure Level SPL, auditory threshold (Sound field size) 
Sound particle velocity  v_{0} = 5×10^{−8} m/s  ≡ 0 dB  
Sound intensity  I_{0} = 1×10^{−12} W/m^{2}  ≡ 0 dB  Threshold of pain at 1 W/m^{2} (Sound energy size) 
Power  P_{0} = 1 W  ≡ 0 dBW  The reference impedance must always be told 
Power  P_{0} = 1 mW  ≡ 0 dBm  Z = 600 Ω (telephones) or Z = 50 Ω (antennas) 
Electric field strength  E_{0} = 1×10^{−6} V/m 
Decibels (dB) Calculator Decibels are defined as ten times the log of a power ratio. Decibels convert multiplication and division calculations into simple addition and subtraction operations. This calculator converts between decibels, voltage gain (or current), and power gain. Just fill in one field and the calculator will convert the other two fields. Equations: Level in dB: L = 20 × log (V_{1}/V_{2}) = 10 × log (P_{1}/P_{2}) 
The dBm is a logarithmic measure of power compared to 1 mW,
that means it is power dependent. It can be converted to a voltage, if the load impedance is known. Typically the impedance (load) is 600 ohms. Equation: Level in dBm: L_{P} = 10 × log (P / 0.001) 
Simple rule of thumb: When working with power: 3 dB is twice, 10 dB is 10 times. When working with voltage or current: 6 dB is twice, 20 dB is 10 times. 
Why is the bandwidth and the cutoff frequency found at the level of "−3 dB"? Full width at half maximum (FWHM). That is the point where the energy (power) is fallen to the 1/2 value or 0.5 = 50 percent of the initial energy size, that is −6 dB. There the voltage is fallen to the value of √(1/2) = 1/√2 or 0.71 = 70.1 percent of the initial voltage as field size; that is −3 dB. 
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