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Decibels
The decibel (dB) is a logarithmic unit that expresses the ratio between two quantities, most often two powers or two amplitudes. It is one tenth of a bel, and because it is based on a logarithm, multiplying ratios becomes adding decibels, which makes the unit convenient for describing systems that span a very wide range of values. The decibel is dimensionless on its own: it always describes a ratio rather than an absolute amount, unless a reference level is specified.
For a power ratio, the value in decibels is ten times the base-ten logarithm of the ratio. A consequence of this definition is that a tenfold increase in power equals ten decibels, and a doubling of power is close to three decibels. When the quantities being compared are amplitudes, such as voltage or sound pressure, a factor of twenty is used instead of ten, because power is proportional to the square of amplitude. This relationship lets gains and losses through a chain of stages be added directly when expressed in decibels.
To express an absolute level, a reference is attached to the decibel, producing units such as dBm, referenced to one milliwatt of power, or sound levels referenced to a standard pressure. Decibels are used throughout acoustics to state sound pressure and loudness, in audio and communication systems to describe signal levels and signal-to-noise ratios, and in radio engineering to express amplifier gain and antenna gain. Their logarithmic nature also matches the way human hearing responds to sound, which is roughly proportional to the logarithm of intensity.
Frequently asked questions
- Why use a factor of 10 for power but 20 for voltage?
- Power is proportional to the square of amplitude. Taking the logarithm of a squared quantity doubles the multiplier, so amplitude ratios use 20 while power ratios use 10.
- How much is a 3 dB change in power?
- An increase of about 3 decibels corresponds to roughly a doubling of power, and a decrease of about 3 decibels to roughly halving it.
- What does dBm mean?
- dBm is a decibel value referenced to one milliwatt. It expresses an absolute power level rather than a relative ratio, with 0 dBm equal to one milliwatt.
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Decibels: audio and RF related topics 
related topics: Acoustics, Antennas, Logarithms and exponentials part of mathematics |
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Bels,
decibels en dB pdf file,
in Dutch |
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Characteristics of Sound and the Decibel Scale |
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Characteristics of Sound and the Decibel Scale |
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Common-mode
rejection ratio Common-mode rejection ratio (CMRR) is a measure of the
capability of an instrument, such as an operational amplifier, to reject a
signal that is common to both input leads |
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Decibel: définition et usage en Français |
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Decibel pdf file |
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Decibel
calculator excel file |
| Decibel
in communications
The dB in Communications systems, The db (decibel) is a relative unit of
measurement commonly used in communications for providing a reference for input
and output levels |
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Decibels decibels, dB,
what are Decibels, Decibels explained, pdf file |
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Decibels
what is a Decibel |
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Decibels The
decibel (dB) is a measure of the ratio between two quantities, and is used in a
wide variety of measurements in acoustics, physics and electronics, ... |
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Decibels
what are the different types of decibel measurement: dB, dBA, dBC, dBV, dBm and
dBi? |
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Decibels
dBi, dBd, dBm, dBmV, Decibels related to dipole
antenna, ..., pdf file |
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Decibels decibels, dB,
dBF, decibels referred to 1 femtowatt, dBk the signal level in DECIBELS referred to 1,000 watts, dBm
decibels referred to 1 milliwatt, dBm signal
level in DECIBELS referred to 1 microwatt, dBV decibels referred to 1 volt RMS,
dBW decibels referred to 1 WATT, Signal-to-Noise ratio, dynamic range of a
sound, sound pressure, sound pressure level |
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Decibels and power The sensitivity tells you how loud a sound you'll get out
of the speaker when you put a given amount of electrical power in, Hi-Fi power
amplifiers, loudspeakers |
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Decibels and their use in radio systems understanding Decibels and their use in radio systems, dBi,
dBd, EIRP, This article discusses the basic unit of measurement used in radio
signals: the decibel or dB. It is named after Alexander Graham Bell - that is
why the “B” is capitalized, pdf file |
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Decibels in radio systems
dB (Decibel), dBm (dB milliWatt), dBd (dB dipole), dBi (dB isotropic), EIRP
(Effective Isotopic Radiated Power), FSL (Free Space Loss) |
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Decibel Scales: Sound Pressure
Level and Sound Intensity Level The most physically understandable quantity
used in determining the size of a sound signal is the Pressure Amplitude.
Pressure amplitude is a measure of the size of the variation in air pressure
caused by a sound wave, ... |
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Decibels
demystified |
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Decibel levels chart
Decibel (Loudness) Comparison Chart |
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Decibels |
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Decibels: what is the difference between dB, dBi, dBu, dBm, dBuV, and other units?
dBi = dBd + 2.15, i: isotropic antenna, d: half-wavelength dipole |
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Equal loudness tester
This site allows you to measure equal loudness curves--the frequency response of
your own ears, loudness |
| Intensity and the decibel scale
Sound Properties and Their Perception
Intensity and the Decibel Scale |
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Power Conversions Power
Conversions chart, pdf file |
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RF Calculator
Radio and RF units conversion/JavaScript application,
Conversion calculator from Watt to dBm and from dBm to Watt, ...,
a tip |
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Understanding dB
dB is an abbreviation for "decibel". One decibel is one tenth of a Bel, named
for Alexander Graham Bell, ..., dBFS - dB Full Scale, dB SPL, sound pressure
level, A-Weighting dB(A), Relationship between Frequency and Level, |
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What is a Decibel
And what are the different types of decibel measurement: dB, dBA, dBC, dBV, dBm
and dBi? How are they related to loudness? |
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What is a decibel the dB is a logarithmic unit used to describe a ratio. The
ratio may be power, sound pressure, voltage or intensity |
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Working with decibels - a tutorial |
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Unit |
General electronics |
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CMRR (dB) |
CMRR = 20 log Ad/As |
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dBu or dBv |
voltage referenced to 0.775 volt, RMS value, not related to any impedance, u
stands for unterminated, audio equipment: input level is +4 dBu, which
corresponds to 1.23 volts RMS value |
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dBm or dBmW |
power measurement relative to 1 milliwatt,
dBm = 10 log P/PO;
when impedances are equal |
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dBm(75) |
power measurement relative to 1 milliwatt, measured with respect to a 75Ω load,
RF systems, cable television |
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dBm(600) |
power measurement relative to 1 milliwatt, measured with respect to a 600Ω load,
telephony line impedance, 0 dBm corresponds to 0.775 volt, RMS value |
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dBm(50) |
power measurement relative to 1 milliwatt, measured with respect to a 50 ohm
load |
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dBV |
RMS voltage of a signal in a wire, relative to 1 volt, not related to any
impedance |
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dBmV |
relative to 1 mV across a given impedance, RMS voltage |
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dBW |
power amplification relative to 1 watt, 50Ω load, RF power amplifiers |
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Acoustics |
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dB(SPL) |
sound pressure level, relative to 20 micropascals (PO) or 20 mN/m2, 1kHz, the quietest sound
a human can hear,
dB(SPL) = 20 log P/PO |
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IdB |
sound intensity relative to a standard threshold of hearing intensity Io:
Io = 10-12 watts/m2; IdB =
10 log I/IO |
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RF Communications |
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dBc |
power of one signal referenced to a carrier signal |
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dBd |
gain of an antenna compared to a dipole antenna |
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dBf |
referred to 1 femtowatt, signal levels at the RF inputs of FM tuners |
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dBk (dB) |
referred to 1000 watts, used in radio and television transmitting stations |
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dBi |
gain of an antenna compared to an isotropic antenna, mostly used for antennas
above 1 GHz, dBi = dBd + 2.15 |
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dBu (dBµV/m) |
electric field intensity: microvolt/meter |
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dBmV |
RF level relative to 1 mV |
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Last updated on:
2026-06-24
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