dBu, dBV, dBm, dBFS, dB SPL: What Do Decibel Suffixes Mean?
The short answer
A decibel is a ratio on a logarithmic scale, so a dB figure means nothing until you state what it is compared against — a rule set out in the NIST Guide to the SI. The suffix is how a spec sheet names that reference: dBV against 1 volt, dBu against about 0.775 volt, dBm against 1 milliwatt, dBFS against digital full scale, dB SPL against a reference sound pressure. Figures with different suffixes are different quantities and do not convert without additional information about the equipment.
Why the suffix is not optional
The bel and the decibel express the logarithm of a ratio between two quantities of the same kind. NIST treats a level as inherently relative: the unit alone carries no information about magnitude, only about the ratio, so the reference must accompany it. A bare "-18 dB" on a meter is readable only once you know what the meter's zero represents.
A second detail follows from the same rule. For field quantities such as voltage and sound pressure, the level is twenty times the log of the ratio; for power quantities it is ten times. That is why doubling a voltage is about 6 dB while doubling a power is about 3 dB — the same decibel, two kinds of quantity. The basics of amplitude and decibels are laid out plainly in the National Park Service's Understanding Sound explainer.
Voltage and power references
- dBV — referenced to 1 volt RMS. Common on consumer and instrument specifications, where -10 dBV is a typical nominal line level.
- dBu — referenced to approximately 0.7746 volt RMS, the voltage that dissipates 1 milliwatt in 600 ohms. The reference is a voltage, so dBu applies regardless of the load impedance actually connected. Professional nominal line level is often quoted as +4 dBu.
- dBm — referenced to 1 milliwatt of power. Because power depends on impedance, a dBm figure is only complete when the impedance is stated. It survives mainly in radio frequency and legacy telephone contexts.
dBu and dBV differ by a fixed amount a little under 2.2 dB, since both reference voltages are known constants. That makes them the one pair in this list with a clean conversion.
The digital reference: dBFS
dBFS is referenced to digital full scale — the largest sample value the word length can represent. All normal readings are negative, and 0 dBFS is a hard numeric boundary rather than a performance figure. Because full scale is defined by the number format, dBFS says nothing about how loud the signal will be; that depends on the converter, the amplifier, and the transducer downstream.
dBFS is also distinct from the loudness scales that share the same zero point. A LUFS reading applies frequency weighting and time averaging per ITU-R BS.1770, and dBTP is an estimate of the reconstructed waveform's peak rather than of individual sample values. See LUFS vs. dBFS and true peak vs. sample peak.
The acoustic reference: dB SPL
dB SPL is referenced to a standard sound pressure of 20 micropascals, a figure chosen to sit near the threshold of hearing at mid frequencies. It describes pressure in air, which is why it is the scale used on the charts that list environments and sound sources. Readings are usually qualified further by a frequency weighting, such as A-weighting, and by a time response — details that change the number for the same sound. How to read a decibel chart covers those qualifiers.
Why there is no universal conversion
A signal chain crosses all three domains: acoustic pressure to voltage at the microphone, voltage to digital numbers at the converter, digital numbers back to voltage and then to pressure at the output. Each crossing has a gain that depends on the specific equipment and its settings. So "-20 dBFS equals X dB SPL" is true only for a system that has been calibrated, and only at that calibration.
The practical reading habit: find the reference first, then the weighting, then the number. For headphone and speaker figures, where sensitivity is quoted in dB per stated input, see headphone impedance and sensitivity specs and what a decibel actually measures.