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What is a decibel?
A decibel is a logarithmic ratio between a sound and a fixed reference, not an amount of sound. For sound pressure that reference is 20 micropascals, so 0 dB is the quietest audible pressure rather than silence. Adding 3 dB doubles the energy, and adding 10 dB sounds roughly twice as loud.
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It is a ratio, so it always needs a reference
On its own, "decibel" means a ratio expressed on a logarithmic scale. It says how many times one quantity is bigger than another, and until you name the other one the number is incomplete. Sound level meters use sound pressure level, written dB SPL, whose reference is 20 micropascals of pressure — roughly the quietest sound a healthy young ear detects at 1 kHz. That is why a reading of 0 dB does not mean silence. It means the sound is sitting exactly at the reference, and quieter sounds go below it as negative numbers.
Other fields keep the word and change the reference. In digital audio, dBFS compares a signal with the loudest value a file can store, so its numbers are almost always negative. In radio and electronics, dBm compares a power with one milliwatt. The arithmetic is identical; only the thing being compared against changes. Our decibel calculator converts between several of them.
Why a logarithm, and not ordinary numbers
Hearing spans an extraordinary range. The threshold of hearing is about 0.00002 pascals of pressure and the threshold of pain about 20 pascals: a factor of one million. Carrying numbers like that around would be hopeless, so the decibel takes the logarithm of the ratio and multiplies it, turning a million-to-one span into a scale that runs from 0 to about 120. The compression is a convenience, but it also matches perception, because the ear judges ratios rather than differences: the step from 40 to 50 dB feels much like the step from 90 to 100.
The consequence catches people out constantly. Decibels cannot be added like ordinary numbers. Two machines of 60 dB each do not make 120 dB; they make 63 dB, because the energies add and doubling the energy is a 3 dB step.
What each step actually means
| Change | In physics | To a listener | Where you meet it |
|---|---|---|---|
| +3 dB | Twice the sound energy | Just noticeably louder | A second identical machine starts up |
| +6 dB | Twice the sound pressure | Clearly louder | You halve your distance from a point source |
| +10 dB | Ten times the energy | About twice as loud | Ten identical machines instead of one |
| +20 dB | A hundred times the energy | About four times as loud | A quiet conversation becomes a raised voice at the same distance |
The perception column is a rule of thumb from psychoacoustics, not a measurement: how loud a change seems depends on the frequency, the starting level and the listener. The physics column is exact.
Some anchors on the scale
| Level | Roughly |
|---|---|
| 0 dB | The reference: 20 micropascals, the quietest pressure a healthy young ear detects at 1 kHz |
| 30 dB | A quiet library |
| 60 dB | Normal conversation about a metre away |
| 85 dB | Where NIOSH begins counting daily exposure time |
| 120 dB | A siren at close range; discomfort for most people |
| 140 dB | A jet engine nearby; pain and immediate risk |
The full list, with a source for every row, is on the decibel levels chart, and each level has its own page if you want the exposure time that goes with it.
dB, dBA and dBC
A plain dB SPL reading treats every frequency equally. Your ear does not: at ordinary levels it is much less sensitive to deep bass than to the middle of the range. Weighting filters correct for that. A-weighting, written dBA, discounts the low frequencies and is what nearly every workplace limit, noise ordinance and hearing-damage threshold is expressed in. C-weighting, dBC, is much flatter at the bottom and is used for peaks and for low-frequency complaints. The same sound can read 85 dBC and 70 dBA, which is exactly why the letter matters as much as the number. There is more detail in the glossary entry on dBA against dBC.
What a decibel does not tell you
- Not how harmful it is, by itself. Damage depends on level and time together: 85 dBA is safe for a few hours and 100 dBA for a few minutes. The safe levels page has the exposure table.
- Not what the sound is made of. Two sounds at 70 dB can be a bass rumble and a whistle. The spectrum analyser shows the frequencies behind the number.
- Not how loud it will be somewhere else. Level falls with distance, about 6 dB per doubling in the open; the distance calculator does that arithmetic.
- Not a legal fact. Ordinances specify a weighting, a measurement height, a duration and often a time of day. Our noise ordinance pages quote the local wording.
What is a decibel in simple terms?
A decibel is a way of writing a ratio between two quantities on a compressed scale. For sound it compares the pressure of what you are hearing with a fixed reference pressure of 20 micropascals. The number tells you how many times bigger, not how much bigger in absolute terms, which is why 0 dB is not silence: it is the reference itself.
How loud is one decibel?
One decibel is about the smallest change in level a person can notice under good conditions, and in ordinary rooms most people miss it entirely. It is not a quantity of sound on its own. A single decibel added to a quiet room and a single decibel added to a factory are the same ratio but very different amounts of energy.
Why is the decibel scale logarithmic?
Because hearing covers a range of about a million to one in pressure, from the threshold of hearing to the threshold of pain. Written linearly those numbers would run from 0.00002 to 20 pascals, which is unusable in conversation or on a dial. The logarithm compresses that span into roughly 0 to 140, and it happens to track perception better as well, since the ear responds to ratios rather than differences.
Is 0 dB silence?
No. Zero decibels is the reference pressure, not the absence of sound, and levels below it are perfectly real and written as negative numbers. A recording studio or an anechoic chamber can be quieter than 0 dB SPL. True silence has no decibel value at all, because the logarithm of zero is undefined.
What is the difference between dB and dBA?
dBA is a decibel reading after A-weighting, a filter that reduces the contribution of low frequencies to match how the ear responds at everyday levels. A deep rumble and a mid-range whine that measure the same in plain dB will differ by a lot in dBA. Almost every noise regulation and hearing-protection limit is written in dBA for that reason.
Can decibels be negative?
Yes, and it simply means the quantity is below the reference. Minus 10 dB SPL is a real, extremely quiet sound level. In digital audio, where the reference is full scale, nearly every value is negative, which is why levels in recording software normally read somewhere between about minus 60 and 0.
See it for yourself
The quickest way to make the scale concrete is to watch it move. The free online decibel meter reads your room in real time, so you can talk, tap the desk and hear where 40, 60 and 80 dB actually sit. Readings from a phone or laptop microphone are uncalibrated estimates, within roughly 10 dB, which is fine for understanding the scale and not enough for a legal or workplace measurement.