Math & Units

dBV

Q: What is the difference between dBV and dBu?

Both reference voltage in decibels but use different references. dBV uses 1 volt RMS, while dBu (and the older dBv) uses 0.7746 volts RMS, the voltage that develops 1 milliwatt in a 600 ohm load. The fixed offset between them is 20 log10(0.7746) = -2.218 dB, so a level in dBV is always 2.218 dB lower numerically than the same voltage expressed in dBu. For example, +4 dBu, the professional audio nominal level, equals +1.78 dBV, and the consumer -10 dBV level equals -7.78 dBu. Because dBu carries the 600 ohm power heritage, it remains common in pro-audio gear, whereas dBV is the more general voltage reference used in measurement and EMC work.

Q: How do you convert dBV to dBm?

Converting between dBV (a voltage unit) and dBm (a power unit) requires knowing the reference impedance, because power equals V squared divided by R. The general relation is P(dBm) = level(dBV) + 10 log10(1000/R) for an RMS voltage across resistance R. In a 50 ohm RF system the offset is 10 log10(1000/50) = +13.01 dB, so 0 dBV across 50 ohm equals +13.01 dBm. In a 75 ohm system the offset is +11.25 dB. This impedance dependence is why dBV and dBm are not directly interchangeable: a voltage figure alone does not define power until the load resistance is specified.

Q: Why is dBV defined with a factor of 20 instead of 10?

The decibel is fundamentally a power ratio defined as 10 log10(P1/P2). Because power in a resistor is proportional to voltage squared (P = V squared / R), substituting voltage gives 10 log10(V1 squared / V2 squared), and the squared term moves outside the logarithm as 20 log10(V1/V2). The factor of 20 therefore appears whenever a decibel is computed directly from a voltage ratio rather than a power ratio. This holds for dBV, dBu, and dBuV. A consequence is that a 2:1 voltage ratio is about 6.02 dB, while a 2:1 power ratio is about 3.01 dB.

/dee-bee-VEE/

A logarithmic unit that expresses a voltage in decibels relative to a fixed reference of 1 volt RMS, calculated as 20 log₁₀(V / 1 V). Because the reference is a voltage rather than a power, the scale uses the factor 20: 0 dBV corresponds to exactly 1 V RMS, +20 dBV to 10 V, and −60 dBV to 1 mV. Unlike dBm, dBV carries no implied load impedance, so a dBV figure describes a voltage at a node regardless of the source or termination resistance. It is the natural unit for high-impedance probe measurements, consumer-audio line levels (−10 dBV nominal), and conducted-emission limits in EMC standards.

Reference: 1 V RMS

Formula: 20 log₁₀(V/1V)

0 dBV: 1.000 V RMS

How the dBV Voltage Reference Works

The decibel was originally a power ratio, but engineers frequently measure voltage rather than power, so a family of voltage-referenced decibel units evolved. dBV pins the reference at exactly 1 volt RMS, a clean, impedance-free anchor that makes it easy to read levels straight off an oscilloscope, a spectrum analyzer in voltage mode, or a high-impedance EMC receiver. A level of 0 dBV is 1 V RMS, every additional 20 dB multiplies the voltage by ten, and every 6.02 dB roughly doubles it. The key distinction from power units is that dBV says nothing about how much current flows; two nodes at the same dBV deliver very different power if their impedances differ.

That impedance independence is both a strength and a trap. In consumer audio the line-level reference is −10 dBV (about 316 mV RMS), while professional gear specifies +4 dBu, and the 2.218 dB offset between the dBV and dBu references is a common source of gain-staging errors when interconnecting equipment. In RF and EMC work, conducted-emission and immunity limits are routinely written in dBµV (decibels relative to 1 microvolt), which is simply dBV shifted by 120 dB, so a CISPR class-B conducted limit of 60 dBµV equals −60 dBV at the measurement port.

Relating dBV to Power and to Field-Strength Units

To turn a dBV reading into power you must supply the load resistance, because P = V²/R. Across a 50 ohm RF load, 0 dBV (1 V RMS) dissipates 20 mW, which is +13.01 dBm; across 600 ohm the same voltage is only 1.67 mW, or +2.22 dBm. This is why test specifications that mix voltage and power units always state the reference impedance. For receiver sensitivity and antenna work, voltage-referenced figures such as dBµV tie cleanly back to dBV, so a single 20 log relationship spans audio, conducted EMC, and RF front-end measurements.

Common dBV Reference Points

Definition (RMS voltage):
L_dBV = 20 × log₁₀(V_RMS / 1 V)

Inverse (voltage from level):
V_RMS = 1 V × 10^{(L_dBV / 20)}

dBV to dBu offset:
L_dBu = L_dBV + 2.218 dB (0 dBu ≈ 0.7746 V RMS)

dBV to dBm (across R):
P_dBm = L_dBV + 10 log₁₀(1000 / R) → +13.01 dB for R = 50 Ω

Example: −10 dBV (consumer line level) = 0.316 V RMS = −7.78 dBu = +3.01 dBm into 50 Ω.

dBV Compared with Related Voltage and Power Units

Unit	Reference	0-unit value	Factor	Offset from dBV	Typical use
dBV	1 V RMS	1.000 V	20 log	0 dB (baseline)	Consumer audio, scope/probe levels
dBu (dBv)	0.7746 V RMS	0.7746 V	20 log	+2.218 dB	Professional audio (+4 dBu)
dBµV	1 µV RMS	0.000001 V	20 log	+120 dB	EMC conducted emissions, receivers
dBmV	1 mV RMS	0.001 V	20 log	+60 dB	Cable TV / 75 Ω distribution
dBm	1 mW	1 mW	10 log	+13.01 dB at 50 Ω	RF and microwave power levels

Common Questions

Frequently Asked Questions

What is the difference between dBV and dBu?

Both express voltage in decibels but use different references: dBV uses 1 V RMS while dBu uses 0.7746 V RMS (the voltage that develops 1 mW in 600 Ω). The fixed offset is 20 log₁₀(0.7746) = −2.218 dB, so any level in dBV is 2.218 dB lower numerically than the same voltage in dBu. Pro-audio +4 dBu equals +1.78 dBV; consumer −10 dBV equals −7.78 dBu.

How do you convert dBV to dBm?

You must know the load impedance, since P = V²/R. The relation is P_dBm = L_dBV + 10 log₁₀(1000/R). In a 50 Ω system the offset is +13.01 dB, so 0 dBV = +13.01 dBm; in 75 Ω it is +11.25 dB. A voltage figure alone cannot define power until R is specified.

Why is dBV defined with a factor of 20 instead of 10?

The decibel is fundamentally 10 log₁₀(P₁/P₂). Because power scales as voltage squared (P = V²/R), substituting voltage gives 10 log₁₀(V₁²/V₂²), and the square moves out as 20 log₁₀(V₁/V₂). So a 2:1 voltage ratio is about 6.02 dB while a 2:1 power ratio is about 3.01 dB.

How does dBµV relate to dBV in EMC measurements?

dBµV is simply dBV shifted by 120 dB, since 1 V is 10⁶ µV and 20 log₁₀(10⁶) = 120 dB. A CISPR conducted-emission limit of 60 dBµV therefore equals −60 dBV (1 mV RMS) at the measurement port. EMC receivers report in dBµV because the signals of interest are typically microvolts to millivolts.

Related Terms

← dBuV dBW →

Measurement & Test

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