What Is a Decibel? The Decibel Scale Explained
A decibel compares a sound to the quietest one we can hear. Why +10 dB sounds twice as loud, +3 dB doubles the energy, and two 60 dB sources make 63 dB.
A decibel (dB) is a unit that compares one quantity with a reference on a logarithmic scale. For sound, the reference is the quietest sound a healthy young ear can detect, and the decibel tells you how many times stronger a sound is than that, in a compressed form that is easy to work with.
That compression is the whole point. The loudest sound you can stand is about a million times stronger in pressure than the quietest one you can hear. The decibel scale squeezes that enormous range into numbers from 0 to about 130.
The reference point: 0 dB
Sound is a small, rapid change in air pressure. Sound level in decibels, more precisely called sound pressure level (SPL), is calculated against a reference pressure of 20 micropascals. That is roughly the faintest 1 kHz tone a person with excellent hearing can detect in a quiet lab.
- 0 dB is that reference. It is not silence, just the threshold of hearing.
- Negative decibels exist. They describe sounds quieter than the reference, which only lab equipment can pick up.
- 130 dB is around the threshold of pain. The loudest sounds go well beyond it.
The formula is:
Sound level (dB) = 20 × log₁₀(measured pressure ÷ 20 µPa)
You never need to calculate it by hand, but it explains every rule below.
Why the scale is logarithmic
A logarithmic scale counts multiplications instead of additions. Each step of 10 dB means the sound carries 10 times more energy (sound intensity) than the step below it.
| Level | Sound pressure | Energy compared with 0 dB | Example |
|---|---|---|---|
| 0 dB | 20 µPa | 1× | Threshold of hearing |
| 20 dB | 200 µPa | 100× | Rustling leaves |
| 40 dB | 0.002 Pa | 10,000× | Library |
| 60 dB | 0.02 Pa | 1,000,000× | Conversation |
| 80 dB | 0.2 Pa | 100,000,000× | Alarm clock |
| 100 dB | 2 Pa | 10,000,000,000× | Earbuds at full volume |
| 120 dB | 20 Pa | 1,000,000,000,000× | Siren up close |
| 140 dB | 200 Pa | 100,000,000,000,000× | Gunshot |
Even a gunshot changes air pressure by only a fraction of a percent of normal atmospheric pressure. Our ears are extremely sensitive pressure sensors, and the log scale is the only practical way to describe what they handle.
There is a second reason. Human hearing itself works roughly logarithmically. We notice ratios, not absolute differences. Going from 40 to 50 dB feels about the same as going from 80 to 90 dB, even though the second step adds vastly more energy.
The rules of thumb
+10 dB sounds about twice as loud
A sound 10 dB higher carries 10 times the energy, but most listeners judge it to be about twice as loud. So 70 dB seems roughly twice as loud as 60 dB, and 80 dB about four times as loud as 60 dB.
This is a perceptual rule, measured across many listeners, not an exact law. It works well in the middle of the range and less well at very low or very high levels.
+3 dB is double the energy
Every 3 dB adds a doubling of sound energy. That sounds like a lot, yet to the ear a 3 dB change is only just noticeable in everyday listening.
This is why hearing safety rules use a 3 dB exchange rate. NIOSH allows 8 hours at 85 dB and halves the time for every 3 dB above it: 4 hours at 88 dB, 2 hours at 91 dB. The ear receives the same total energy in each case. The how loud is too loud guide has the full table.
+6 dB is double the pressure
Because the formula uses 20 × log for pressure, doubling the pressure adds 6 dB. You will see this in the distance rule below.
How big a change can you hear?
| Change | What you notice |
|---|---|
| 1 dB | Detectable in a lab, side by side |
| 3 dB | Just noticeable in daily life |
| 5 dB | Clearly noticeable |
| 10 dB | About twice (or half) as loud |
| 20 dB | About four times (or a quarter) as loud |
Adding two sounds together
Decibels do not add like ordinary numbers. Two sources at 60 dB do not make 120 dB. They make 63 dB, because two equal sources deliver twice the energy, and twice the energy is +3 dB.
When the two sources are different, the louder one dominates:
| Difference between the two levels | Add to the louder one |
|---|---|
| 0 dB | +3 dB |
| 1 dB | +2.5 dB |
| 2 dB | +2 dB |
| 3 dB | +1.8 dB |
| 4 dB | +1.5 dB |
| 6 dB | +1 dB |
| 8 dB | +0.6 dB |
| 10 dB or more | less than +0.5 dB |
Some examples:
- A 70 dB vacuum cleaner and a 70 dB TV together: about 73 dB.
- A 80 dB blender and a 70 dB radio: about 80.4 dB. The radio barely registers.
- Ten identical 60 dB sources: 70 dB (ten times the energy is +10 dB).
The same logic works in reverse. To make a room noticeably quieter, you have to deal with the loudest source first. Silencing the quieter ones changes almost nothing.
Distance and decibels
Outdoors, away from walls, sound from a small source drops by about 6 dB every time you double the distance. If a mower measures 90 dB at 1 m, expect around 84 dB at 2 m and 78 dB at 4 m.
Indoors the drop is smaller, because reflections from walls, floors and ceilings keep feeding sound back into the room. This is also why any measurement should state where it was taken.
dB, dBA and other variations
A plain “dB” figure for sound usually means sound pressure level with no frequency weighting. Most real-world numbers, including noise limits, are given in dBA, which adjusts the reading to match how the ear hears different pitches. The difference is explained in dB vs dBA.
The decibel is also used outside acoustics. Audio engineers use dBFS for digital levels, radio engineers use dBm for signal power, and so on. They share the logarithmic idea but use different references, so the numbers are not interchangeable with sound levels.
Where the name comes from
The unit is named after Alexander Graham Bell. The original unit, the bel, was introduced by engineers at Bell Telephone Laboratories in the 1920s to measure signal loss in telephone lines. It proved too coarse, so the tenth of a bel, the decibel, became the standard. One bel equals 10 decibels.
See the scale for yourself
The easiest way to get a feel for decibels is to measure familiar sounds. Try the free meter on our home page in your browser, then compare the numbers with the decibel chart.
Decibel Meter: dB Sound Level for iPhone and Apple Watch shows the live level in dBA, dBC or dBZ, with min, average (Leq), max and peak for each session and a spectrum view that shows which frequencies make up a sound. A built-in reference table puts each reading in context. The app is free to download, and live measuring is part of the subscription. See what the app does.
