
For other uses, see Decibel (disambiguation).
The decibel (dB) is a measure of the ratio between decible two quantities, and is used in a wide variety of measurements in acoustics, physics and electronics. decibell While originally only used for power and intensity ratios, it decibal has come to be used more generally in engineering. The decibel is widely used decibel in measurements of the loudness of sound. It is a "dimensionless unit" decibels like percent. Decibels are useful because they decibel meter allow even very large or small ratios to be represented with a conveniently small number (similar to decibel chart scientific notation). This is achieved by using a logarithm.
Contents
 1 History decibel levels of bels and decibels
 2 Definition
 3 Examples
 4 Electrical decibel products decibel scale circuits
 5 Standards
 5.1 Merits
 5.2 Difficulties
 6 Uses
 6.1 Acoustics
 6.1.1 Relation decibel meters decibel sones to Loudspeakers
 6.1.1.1 A practical example
 6.1.2 Frequency decibel measure weighting
 6.1.3 In water
 6.2 Electronics
 6.3 Optics
 6.4 Telecommunications
 6.5 Seismology
 7 Typical decibel level decibel antennas abbreviations
 7.1 Absolute measurements
 7.1.1 Note decibels chart regarding absolute measurements
 7.2 Relative measurements
 8 Reckoning
 8.1 Round numbers
 8.2 The sound decibel chart 4 → 6 osha noise decibel levels safety energy rule
 8.3 The "789" rule
 8.4 −3 sound decibel dB ≈ ½ power
 8.5 6 dB per bit
 8.6 dB chart
 8.6.1 Commonly what is a decibel used dB values
 8.6.2 Other 200 decibel car audio dB values
 9 See also
 10 References
 11 External ge spacemaker dishwasher decibel rating links

History of bels and decibels
The bel (symbol B) is mostly used in telecommunication, electronics, and decibel ear protection acoustics. Invented by engineers of the Bell Telephone Laboratory to quantify decibel garden the reduction in audio level decibel measurement over a 1 mile (1.6 km) length of standard telephone cable, it define decibel was originally called the transmission unit or TU, but was renamed in 1923 or 1924 in honor ireland decibel meter of the laboratory's founder and telecommunications what do we perceive for one decibel change pioneer Alexander Graham Bell.
The bel was too large for everyday use, calculators decibel amps so the decibel (dB), equal to 0.1 bel (B), became more commonly used. The bel is still used to represent noise power decibel antenna levels in hard drive specifications. The Richter scale uses numbers decibel comparison expressed in bels as well, though they are not decibel credit union labeled with a unit. In spectrometry and optics, the absorbance unit decibel diagram used to measure optical density is equivalent to −1 B. decibel levels rodents In astronomy, the apparent magnitude measures the brightness of stars logarithmically, since just as the ear decibel of generators responds logarithmically to acoustic power, the eye responds logarithmically to brightness.
Definition
A decibel is defined in decibel to watts chart two common ways.
When referring to measurements of power it is:
But when sones decibels referring to measurements of voltage it is:
where X_{0} is advantage of using a decibel amplitude scale a specified reference. In many converting decibel to decimal cases, the reference is 1 and so is ignored. Which one people use depends on decibel club convention and context. When the impedance is held constant, the power is proportional to the square of the decibel competitions voltage, and so the above two decibel festival definitions become consistent.
An intensity I or power P can be expressed in decibels decibel levels and noise ordinances with the standard equation
where I_{0} and P_{0} are decibel machine a specified reference intensity and power.
Examples
As examples, if P_{dB} is 10 dB greater decibel magazine than P_{dB0}, then P is ten times P_{0}. If P_{dB} is 3 decibel mp3 dB greater, the power ratio is very close to a factor of two .
For sound fosgate decibels intensity, I_{0} is typically chosen to be 10^{−12} W/m^{2}, which is roughly the threshold hair 165 decibels of hearing. When this choice is made, the units are said to be "dB SIL". For how do you measure decibels sound power, P_{0} is typically chosen to be 10^{−12} W, koi no decibel and the units are then "dB SWL".
Electrical circuits
In electrical circuits, the dissipated power is typically air nozzle decibels proportional to the square of the voltage V, and for austin zoning decibel limits sound waves, the transmitted power is similarly proportional to the square of the average decibels from a portable power saw pressure amplitude p. Effective sound pressure is related to sound intensity I, density ρ and speed of sound c by the following backupalarm 72 volt 112 decibel equation:
Substituting a measured voltage convert mv to decibels or pressure and a reference voltage or pressure and rearranging terms leads to decibel band room the following equations and accounts for the difference between the multiplier of 10 for decibel comparisons intensity or power and 20 for voltage or pressure:
where V_{0} and decibel detector p_{0} are a specified reference voltage and pressure. This means a 20 decibel hz dB increase for every factor 10 increase in the voltage or decibel level dirt bike pressure ratio, or approximately 6 dB increase for decibel meter audio every factor 2. Note that in physics, decibels refer to power ratios only; it is incorrect to use them if the electrical decibel monitor or acoustic impedances are not the same at the two points decibel o fortuna mp3 where the voltage or pressure are decibel products antennas measured, though this usage is very common in engineering. For example, the power decibel products home carried by a sound wave at atmospheric pressure is only proportional to the squared pressure decibel ratings amplitude as long as the decibel sensor latter is much smaller than 1 atmosphere.
Standards
The decibel is not an SI unit, although the International Committee for Weights and decibel sound measurement Measures (BIPM) has recommended its inclusion in the SI decibel speakers system. Following the SI convention, the d is lowercase, decibels and hearing loss as it is the SI prefix deci, and the B is capitalized, as it is federal high decibel shotgun shell an abbreviation of a namederived unit, the bel, named for Alexander Graham Bell. Written out it free coke decibels becomes decibel. This is standard English capitalization.
Merits
The use headset decibel of decibels has a number of merits:
 It is more convenient how do decibel meters work to add the decibel values of, for interior decibel level in buick lacrosse quiet tuning instance, two consecutive amplifiers rather than to multiply their motorcycle exhaust system decibel rating comparison amplification factors.
 A very large range of ratios can be expressed with decibel values sound level decibels in a range of moderate size, allowing one to clearly visualize huge changes of some quantity. (See Bode plot and half logarithm graph.)
 In acoustics, air conditioning decibel level the decibel scale was adopted for measuring sound intensity, which, according air hose decibel level to Fechner's law is a good fit to loudness perception. However buy a decibel meter “not long after they had adopted clemson 126 decibels the decibel scale for measuring sound intensities, the engineers coke music decibel code noted that equal steps on the logarithmic (decibel) scale do not behave like equal steps [in sensation], for a level 50 db comparison of sones to decibels above threshold does not sound at all convert decibel to psi like half of 100 db, as Fechner's dbc decibel law implies it should.” (Stevens, 1957: 163). This led to the development of Stevens' decibel antenna 08844h65exy power law which is generally found to be decibel at different distances a better fit to data. Stevens (1957) suggested replacing the decibel scale with the sone scale, but it did not seem decibel conversion to take root.
Difficulties
The use of decibels frequently causes confusion:
 It is unclear decibel conversion chart to many users whether any unit requires the decibel data logger 20log10 or 10log10 formulation.
 The 'deci' formulation causes confusion decibel effects sound  understanding that this is merely bels times ten, and that a one bel increase means an increase of 10 to decibel examples the power 1, i.e. a factor decibel level tables of ten increase, may add clarity.
Uses
Acoustics
The decibel unit is commonly used in acoustics to quantify decibel levels for classrooms sound levels relative decibel levels in varying tools comparison to some 0 dB reference. Commonly, sound intensities are specified as a sound pressure level (SPL) relative to 20 decibel products mirage omni antenna micropascals (20 µPa) in gases and 1 µPa in other decibel ratings of generators media (standardized in ANSI S1.11994).^{[1]} 20 decibel ratings of rv generators µPa corresponds to the threshold of human hearing (roughly the sound of a mosquito flying 3 decibel ratings vacuum cleaners m away). Often, the unit dB(SPL) is decibel track reading used, implying the standard reference, though this is discouraged by the Acoustical Society of America, which recommends explicitly decibel values stating the reference level for each measurement; decibel versus spl "100 dB re 20 µPa". ^{[2]}^{[3]}. In decibels db the remainder of this section, the reference level of 20 µPa is implied.
A reason for decibels examples using the decibel is that the ear is capable of detecting a very large range of sound pressures. dishwasher noise decibel comparison The ratio of the sound pressure that eb 3500x honda decibel causes permanent damage from short exposure to the limit that (undamaged) high decibel sound ears can hear is above a how loud is a decibel million. Because the power in a sound wave is proportional to the square of the pressure, the ratio instrument that measures decibels of the maximum power to the minimum power is above one jet engine decibel levels (short scale) trillion. To deal with such a range, logarithmic units are useful: the log of a trillion is map decibel hotel 12, so this ratio represents a difference of 120 dB.
Psychologists have mortar detection by sound acoustic decibel signature debated whether loudness perception is better described as roughly noise decibels logarithmic (see the WeberFechner law) or as a power law (see Stevens' power law), where the noise pollution decibel scale latter is now generally more accepted. online shop specialized decibel helmet A consequence of either model is that a volume control dial on a typical audio amplifier that osha decibel levels is labeled linearly in program for mesuring decibels in a car voltage amplification will affect the loudness much more for lower numbers than higher ones. This is que es 1 decibel why some are labeled in relation to decibels, i.e. the numbers are related rosier exhaust decibels to the logarithm of school cafeteria decibel meters stop light intensity.
Various frequency weightings are used to allow the result of an acoustical sone decibel comparison chart measurement to be expressed as a single sound level. The weightings approximate the changes in sensitivity of the ear to different frequencies sones decibel at different levels. The two most commonly used weightings are the A and C the decibels weightings; other examples are the B traffic decibel levels and Z weightings.
In air, sound pressure levels above 85 dB are considered harmful, while 95 tutorial decibel conversion dB is considered unsafe for prolonged what are decibels periods and 120 dB causes an immediate perforation of the ear drum white noise decibels (tympanic membrane). Windows break at about 163 dB. Jet aircraft cause Aweighted levels 125 decibel horns of about 133 dB at 33 m, or 100 dB at 170 m. In air 150 decibel horns at atmospheric pressure, the simple relationship between pressure and power of a sound wave breaks down for 165 decibels hair pressures on the order of or greater than 20000 decibels 1 atmosphere, which corresponds to an SPL of 40 decibel noise reduction 194 dB re 20 µPa (i.e. ). Waves with higher pressures 5 watt amp vs 30 watt amp decibel level are more properly called shock waves rather than sound waves; their properties are very different from those of normal sound waves. 600 ohm impedence decibel conversion One could extend the meaning of sound pressure level in 85 decibels order to describe the pressure acura 3.2 tl car horn decibels waves emitted by processes such as earthquakes and explosions, and get numbers exceeding 194 dB, adding decibels but these numbers should only be used if it air pressure decibel level is clear how the measurable quantities are converted into SPL. An extensive list air pressure decibels can be found at makeitlouder.com.
dB (SPL) 
Source (with amount of decibels noise ordinence in georgia distance) 
194 
Theoretical limit for a sound wave at 1 atmosphere environmental pressure; pressure waves with a greater intensity behave as shock waves. 
180 
Krakatoa volcano automotive decibel ratings explosion at 1 mile in bar exam july 2006 noise decibel level of 80 db air [1] 
160 
M1 Garand being fired at 1 meter (3 ft) 
150 
Jet engine at 30 m (100 ft) 
140 
Low Caliber Rifle being fired at below 20 decibels 1 m (3 ft) 
130 
Threshold of buy decibel reader pain; civil defense siren at 100 ft (30 m) 
120 
Train horn calculate decibel at 1 m (3 ft) 
110 
Football stadium during kickoff at 50 yard line at 5 m chart of decibel levels (16 ft) ; chainsaw at 1 m (3 ft) 
100 
Jackhammer at clemson football loud 126 decibels record 2 m (7 ft); inside disco 
90 
Loud factory, heavy truck at 1 m (3 ft) 
80 
Vacuum cleaner clemson loud 126 decibels record at 1 m (3 ft), curbside compare decibel levels of common noises of busy street, PLVI of City 
70 
Busy traffic at 5 m (16 ft) 
60 
Office or restaurant inside 
50 
Quiet restaurant inside 
40 
Residential area at night 
30 
Theatre, no talking 
20 
Whispering 
10 
Human breathing consumer opinion craftsman air compressor decibels at 3 m (10 ft) 
0 
Threshold of human hearing continuous yelling decibel harmful (with healthy ears); sound of a convert decibel to hertz mosquito flying 3 m (10 ft) away 
Note that the SPL emitted by an object changes with distance from the object. converting decibels to watts Commonlyquoted measurements of objects like jet engines or jackhammers are dangerous decibels meaningless without distance information. The measurement is not of the object's noise, dba decibel but of the noise at a point in the air near decibel 844g65vtzasx that object. For instance, it is intuitively obvious that the noise level of a volcanic eruption decibel average in excel will be much higher standing inside the crater than it would be decibel db493 measured from 5 kilometers away.
Sound pressure levels are applicable only to the specific decibel db854dg85esx position at which they are measured. decibel death The levels change with the distance from the source of the sound; in general, the level decreases decibel definition as the distance from the source increases. If decibel example the distance from the source is unknown, it is difficult to estimate the sound pressure level decibel hi fi at the decibel hotel source. Measurements that refer to the "threshold of pain" or the threshold at which ear damage occurs are decibel hotel map measuring the SPL at a point near the ear itself. Measurements of ambient noise decibel incorporated do not need a distance, since the decibel level calculation noise level will be relatively constant at any point in the area (and are usually only rough approximations anyway).
Under controlled conditions, decibel level chart in an acoustical laboratory, the trained healthy human ear is able decibel level for a dodge caliber to discern changes in sound levels of 1 dB, decibel level of 300 ultra mag when exposed to steady, decibel level of drivein speakers single frequency ("pure tone") signals in the midfrequency range. It is widely accepted that the average healthy ear, however, can decibel levels and classrooms barely perceive noise level changes of 3 dB.
On this scale, the normal range of human decibel levels measuring hearing extends from about 0 dB(SPL) to about 140 dB(SPL). 0 dB(SPL) is decibel levels resulting in death the decibel limits;ohio threshold of hearing in healthy, undamaged human ears at 1 kHz; 0 dB(SPL) decibel measurer is not an absence of sound, and it is possible for people with exceptionally good decibel measuring test equipment hearing to hear sounds at −10 dB(SPL). A 3 dB increase in decibel meter diy the level of continuous noise doubles the sound power, however experimentation has determined that the response decibel meter sound of the human ear results in a perceived decibel millivolts doubling of loudness for approximately every 10 dB increase (part of decibel noise meters Stevens' power law).
Relation to Loudspeakers
Speaker sensitivity is usually given decibel products db854dg85esx in dBSPL @ 1 Watt @ 1 meter.
The equation for dBSPL is :.
This means that a decibel products/andrew corp doubling in sound pressure output from decibel rating chart a speaker relates to a 6 dBSPL increase.
A practical example
A fictional 2 way speaker (A box with separate driver for high("Treble") decibel ratings of vacuum cleaners and low("Bass") decibel reader ) has the following specs:
High driver: 92 dBSPL @ 1W @ 1m. A Low driver: 86 dBSPL decibel reading of pots and pans clanging together @ 1W @ 1m. B
Now if we want to match the output of the decibel readings two speakers so the sound is "equally loud" we need to do the following:
Get the difference decibel readings for construction equipment between the two by decibel readings on human voice subtracting the sensitivity:
Difference in sensitivity = AB
= decibel thunderstorm texas 92 dBSPL  86 dBSPL
= 6 dBSPL
As we concluded earlier this 6dB difference requires that we double the power decibel to watts converter delivered to the low driver. Since a doubling in [power] relates to 3 dB, decibel tutorials we need to adjust the crossover unit in this system so that decibels corvette the [gain] of the Low signal is 3dB more than the decibels explained Highs. If there is no crossover decibels in air hose you can always adjust the Amplifier's output to be 3dB more.
Frequency weighting

Main article: decibels of a car alarm Frequency weighting
Since the human ear digital decibel meter is not equally sensitive to all the frequencies of sound within the entire spectrum, noise earplug decibel ratings levels at maximum human sensitivity — middle A and its higher harmonics (between explain 110 decibels 2 and 4 kHz) — are factored more heavily into sound descriptions using a process called funny decibels readings frequency weighting.
The most widely used frequency weighting hearing decibel chart is the "Aweighting", which roughly corresponds to the inverse hearing decibels of the 40 dB (at 1 kHz) equalloudness curve. Using this filter, the sound level meter is less sensitive high decibel fire alarm speaker to very high and very low frequencies. high decibel noise output The A weighting parallels the sensitivity of the human ear when highest decibel in school hallway it is exposed to normal levels, and frequency weighting C is suitable for use when the ear is exposed to how loud is 110 decibels higher sound levels. Other defined frequency weightings, such as B and how loud is 126 decibels Z, are rarely used.
Frequency weighted sound levels are still expressed in how many decibels is 49m v decibels (with unit symbol dB), although it is common to how sound is measured in decibels using a logarithmic scale see the incorrect unit symbols dBA or human whistle decibels dB(A) used for Aweighted sound levels. Performance characteristics for professional and consumer audio products are commonly measured with Aweighted filtering.
In water
For the international fire service decibels to hurt ears same source pressure at 1 m, the underwater sound ireland decibel testing equipment pressure level will be higher by 62 dB, due to loud car horn decibels the difference in reference levels (20 µPa vs 1 µPa = 26.0 dB difference), and the noise from air hose pressure decibel difference in acoustic impedance between air and water noise levels decibels (3600 times = 35.6 dB difference).^{[4]}
Electronics
The decibel is used noise levels in decibels, how to add decibels rather than arithmetic ratios or percentages because when certain online decibel meter types of circuits, such as amplifiers and attenuators, are connected osha jet engine decibel levels in series, expressions of power level in decibels may be arithmetically added and subtracted. It is quiet dishwashers decibels comparison also common in disciplines such as audio, in which the properties of the signal retail decibel meter are best expressed in logarithms due to rifle detection by sound acoustic decibel signature the response of the ear.
In radio electronics, the decibel is used saskatchewan and hearing loss and decibel to describe the ratio between two measurements of electrical power. It can also be combined with a school cafeteria decibel meters suffix to create an absolute unit smart alarm 72 volt 112 decibel of electrical power. For example, it can be combined with snoring decibels "m" for "milliwatt" to produce the "dBm". Zero dBm is one milliwatt, sound decibel levels and 1 dBm is one decibel greater than 0 dBm, or about 1.259 mW.
Although decibels were originally used for power sound decibels ratios, they are also used spl decibels normal speech osha in electronics to describe voltage or current ratios. In a constant resistive load, power is static in the ears at certain decibels proportional to the square of the voltage or current the decibel in vibration monitoring in the circuit. Therefore, the decibel ratio of two voltages V_{1} and V_{2} is defined as 20 log_{10}(V_{1}/V_{2}), and similarly for current top 10 subwoofers with high decibel readings ratios. Thus, for example, a factor of 2.0 in voltage town noise ordinence decibel level of calhoun,ga is equivalent to 6.02 dB what decibel can your hearing be damaged (not 3.01 dB!). Similarly, a ratio of 10 times gives 20 dB, and one tenth gives what do we pereive for one decibel change −20 dB.
This practice is fully consistent what instrument measures decibels with powerbased decibels, provided the circuit resistance remains constant. However, voltagebased decibels are frequently used to what is the decibel of a train horn express such quantities as the voltage gain of an amplifier, where what is the difference between 57 decibels and 67 decibels the two voltages are measured in different circuits which may have very different resistances. For example, a unitygain where to buy a decibel meters buffer amplifier wisconsin motorcycle noise decibel regulations with a high input resistance and a low output resistance may be said to have a "voltage gain of 0 dB", even though it is actually providing a considerable power gain when driving a lowresistance load.
In professional audio, a popular unit is the dBu (see below for all the units). The "u" stands for "unloaded", and was probably chosen to be similar to lowercase "v", as dBv was the older name for the same thing. It was changed to avoid confusion with dBV. This unit (dBu) is an RMS measurement of voltage which uses as its reference 0.775 V_{RMS}. Chosen for historical reasons, it is the voltage level at which you get 1 mW of power in a 600 ohm resistor, which used to be the standard impedance in almost all professional audio circuits.
Since there may be many different bases for a measurement expressed in decibels, a dB value is considered an absolute measurement only if the reference value (equivalent to 0 dB) is clearly stated. For example, the gain of an antenna system can only be given with respect to a reference antenna (generally a perfect isotropic antenna); if the reference is not stated, the dB value is a relative measurement, such as the gain of an amplifier.
Optics
In an optical link, if a known amount of optical power, in dBm (referenced to 1 mW), is launched into a fibre, and the losses, in dB (decibels), of each electronic component (e.g., connectors, splices, and lengths of fibre) are known, the overall link loss may be quickly calculated by simple addition and subtraction of decibel quantities.
Telecommunications
In telecommunications, decibels are commonly used to measure signaltonoise ratios and other ratio measurements.
Decibels are used to account for the gains and losses of a signal from a transmitter to a receiver through some medium (free space, wave guides, coax, fiber optics, etc.) using a Link Budget.
Seismology
Earthquakes were formerly measured on the Richter scale, which is expressed in bels. (The units in this case are always assumed, rather than explicit.) The more modern moment magnitude scale is designed to produce values comparable to those of the Richter scale.
Typical abbreviations
Absolute measurements
Electric power
 dBm or dBmW
 dB(1 mW) — power measurement relative to 1 milliwatt.
 dBW
 dB(1 W) — same as dBm, with reference level of 1 watt.
Electric voltage
 dBu or dBv
 dB(0.775 V) — (usually RMS) voltage amplitude referenced to 0.775 volt. Although dBu can be used with any impedance, dBu = dBm when the load is 600 Ω. dBu is preferable, since dBv is easily confused with dBV. The "u" comes from "unloaded".
 dBV
 dB(1 V) — (usually RMS) voltage amplitude of a signal in a wire, relative to 1 volt, not related to any impedance.
Acoustics
 dB(SPL)
 dB(Sound Pressure Level) — relative to 20 micropascals (μPa) = 2×10^{−5} Pa, the quietest sound a human can hear.^{[1]} This is roughly the sound of a mosquito flying 3 metres away. This is often abbreviated to just "dB", which gives some the erroneous notion that "dB" is an absolute unit by itself.
Radio power
 dBm
 dB(mW) — power relative to 1 milliwatt.
 dBμ or dBu
 dB(μV/m) — electric field strength relative to 1 microvolt per metre.
 dBf
 dB(fW) — power relative to 1 femtowatt.
 dBW
 dB(W) — power relative to 1 watt.
 dBk
 dB(kW) — power relative to 1 kilowatt.
Note regarding absolute measurements
The term "measurement relative to" means so many dB greater than or less than the quantity specified.
Some examples:
 3 dBm means 3 dB greater than 1 mW.
 −6 dBm means 6 dB less than 1 mW.
 0 dBm means no change from 1 mW, in other words 0 dBm is 1 mW.
Relative measurements
 dB(A), dB(B), and dB(C) weighting
 These symbols are often used to denote the use of different frequency weightings, used to approximate the human ear's response to sound, although the measurement is still in dB (SPL). Other variations that may be seen are dB_{A} or dBA. According to ANSI standards, the preferred usage is to write L_{A} = x dB, as dBA implies a reference to an "A" unit, not an Aweighting. They are still used commonly as a shorthand for Aweighted measurements, however.
 dBd
 dB(dipole) — the forward gain of an antenna compared to a halfwave dipole antenna.
 dBi
 dB(isotropic) — the forward gain of an antenna compared to an idealized isotropic antenna.
 dBFS or dBfs
 dB(full scale) — the amplitude of a signal (usually audio) compared to the maximum which a device can handle before clipping occurs. In digital systems, 0 dBFS would equal the highest level (number) the processor is capable of representing. This is an instantaneous (sample) value as compared to the dBm/dBu/dBv which are typically RMS.(Measured values are usually negative, since they should be less than the maximum.)
 dBr
 dB(relative) — simply a relative difference to something else, which is made apparent in context. The difference of a filter's response to nominal levels, for instance.
 dBrn
 dB above reference noise See also dBrnC.
 dBc
 dB relative to carrier — in telecommunications, this indicates the relative levels of noise or sideband peak power, compared to the carrier power.
Reckoning
Decibels are handy for mental calculation, because adding them is easier than multiplying ratios. First, however, one has to be able to convert easily between ratios and decibels. The most obvious way is to memorize the logs of small primes, but there are a few other tricks that can help.
Round numbers
The values of coins and banknotes are round numbers. The rules are:
 One is a round number
 Twice a round number is a round number: 2, 4, 8, 16, 32, 64
 Ten times a round number is a round number: 10, 100
 Half a round number is a round number: 50, 25, 12.5, 6.25
 The tenth of a round number is a round number: 5, 2.5, 1.25, 1.6, 3.2, 6.4
Now 6.25 and 6.4 are approximately equal to 6.3, so we don't care. Thus the round numbers between 1 and 10 are these:
Ratio 1 1.25 1.6 2 2.5 3.2 4 5 6.3 8 10
dB 0 1 2 3 4 5 6 7 8 9 10
This useful approximate table of logarithms is easily reconstructed or memorized.
The 4 → 6 energy rule
To one decimal place of precision, 4.x is 6.x in dB (energy).
Examples:
 4.0 → 6.0 dB
 4.3 → 6.3 dB
 4.7 → 6.7 dB
The "789" rule
To one decimal place of precision, x → (½ x + 5.0 dB) for 7.0 ≤ x ≤ 10.
Examples:
 7.0 → ½ 7.0 + 5.0 dB = 3.5 + 5.0 dB = 8.5 dB
 7.5 → ½ 7.5 + 5.0 dB = 3.75 + 5.0 dB = 8.75 dB
 8.2 → ½ 8.2 + 5.0 dB = 4.1 + 5.0 dB = 9.1 dB
 9.9 → ½ 9.9 + 5.0 dB = 4.95 + 5.0 dB = 9.95 dB
 10.0 → ½ 10.0 + 5.0 dB = 5.0 + 5.0 dB = 10 dB
−3 dB ≈ ½ power
A level difference of ±3 dB is roughly double/half power (equal to a ratio of 1.995). That is why it is commonly used as a marking on sound equipment and the like.
Another common sequence is 1, 2, 5, 10, 20, 50 ... . These preferred numbers are very close to being equally spaced in terms of their logarithms. The actual values would be 1, 2.15, 4.64, 10 ... .
The conversion for decibels is often simplified to: "+3 dB means two times the power and 1.414 times the voltage", and "+6 dB means four times the power and two times the voltage ".
While this is accurate for many situations, it is not exact. As stated above, decibels are defined so that +10 dB means "ten times the power". From this, we calculate that +3 dB actually multiplies the power by 10^{3/10}. This is a power ratio of 1.9953 or about 0.25% different from the "times 2" power ratio that is sometimes assumed. A level difference of +6 dB is 3.9811, about 0.5% different from 4.
To contrive a more serious example, consider converting a large decibel figure into its linear ratio, for example 120 dB. The power ratio is correctly calculated as a ratio of 10^{12} or one trillion. But if we use the assumption that 3 dB means "times 2", we would calculate a power ratio of 2^{120/3} = 2^{40} = 1.0995 × 10^{12}, giving a 10% error.
6 dB per bit
In digital audio linear pulsecode modulation, the first bit (least significant bit, or LSB) produces residual quantization noise (bearing little resemblance to the source signal) and each subsequent bit offered by the system doubles the (voltage) resolution, corresponding to a 6 dB ratio. So for instance, a 16bit (linear) audio format offers 15 bits beyond the first, for a dynamic range (between quantization noise and clipping) of (15 × 6) = 90 dB, meaning that the maximum signal (see 0 dBFS, above) is 90 dB above the theoretical peak(s) of quantization noise. The negative impacts of quantization noise can be reduced by implementing dither.
dB chart
As is clear from the above description, the dB level is a logarithmic way of expressing not only power ratios, but also voltage ratios The following tables are cheatsheets that provide values for various dB power ratios and also "voltage" ratios.
Commonly used dB values
dB level 
power
ratio 

dB level 
voltage
ratio 
−30 dB 
1/1000 

−30 dB 
= 0.03162 
−20 dB 
1/100 

−20 dB 
= 0.1 
−10 dB 
1/10 

−10 dB 
= 0.3162 
−3 dB 
1/2 (approx.) 

−3 dB 
= 0.7071 
3 dB 
2 (approx.) 

3 dB 
= 1.414 
10 dB 
10 

10 dB 
= 3.162 
20 dB 
100 

20 dB 
= 10 
30 dB 
1000 

30 dB 
= 31.62 
Other dB values
dB level 
power
ratio 

dB level 
voltage
ratio 
−12 dB 
1/16 (approx.) 

−12 dB 
1/4 (approx.) 
−9 dB 
1/8 (approx.) 

−9 dB 
0.36 (approx.) 
−6 dB 
1/4 (approx.) 

−6 dB 
1/2 (approx.) 
−1 dB 
0.79 (approx.) 

−1 dB 
0.89 (approx.) 
1 dB 
1.26 (approx.) 

1 dB 
1.12 (approx.) 
6 dB 
4 (approx.) 

6 dB 
2 (approx.) 
9 dB 
8 (approx.) 

9 dB 
2.82 (approx.) 
12 dB 
16 (approx.) 

12 dB 
4 (approx.) 
See also
 Equalloudness contour
 ITUR 468 noise weighting
 Noise (environmental)
 Signal noise
 Sound pressure level
 Weighting filter — discussion of dBA
References
 ^ ^{a} ^{b} Glossary of Noise Terms — Sound pressure level definition
 ^ Rane Pro Audio Reference definition of "dBSPL"
 ^ ASACOS Rules for Preparation of American National Standards in Acoustics, Mechanical Vibration and Shock, Bioacoustics, and Noise
 ^ Air to Water conversion
 Martin, W. H., "DeciBel — The New Name for the Transmission Unit", Bell System Technical Journal, January 1929.
 Stevens, S. S. (1957). On the psychophysical law. Psychological Review 64(3):153—181. PMID 13441853.
External links
 What is a decibel?
 Description of some abbreviations
 Noise Measurement OSHA 1
 Noise Measurement OSHA 2
 Understanding dB
 Rane Professional Audio Reference entry for "decibel"
 Hyperphysics description of decibels
 Decibel chart
Converters
 V_{peak}, V_{RMS}, Power, dBm, dBu, dBV converter
 Conversion: dBu to volts, dBV to volts, and volts to dBu, and dBV
 Conversion of sound level units: dBSPL or dBA to sound pressure p and sound intensity J
 Conversion: Voltage V to dB, dBu, dBV, and dBm
 Only Power: dBm to mW conversion
 Decibel  Description and calculations
Categories: Units of measure  Sound  Acoustics