Decibel scale
The Decibel Scale Explained
The decibel scale measures sound level logarithmically, not linearly. Each 10 dB step means ten times more sound intensity and sounds about twice as loud, while +3 dB doubles the sound energy. That is why 90 dB is not slightly louder than 80 dB, but roughly twice as loud and ten times as intense.
Why decibels are logarithmic
Human hearing covers an enormous range: the loudest sound you can stand has about a million million (1012) times the intensity of the quietest sound you can hear. Writing that as plain numbers is unwieldy, so the decibel scale compresses it with a logarithm:
L = 10 · log10(I / I0) — where I is the sound intensity and I0 is the reference intensity at the threshold of hearing (0 dB). Ten times the intensity adds 10 dB; a hundred times adds 20 dB.
The +3, +6 and +10 dB rules
| Change | Sound intensity (energy) | Sound pressure | How it sounds |
|---|---|---|---|
| +3 dB | ×2 | ×1.4 | Slightly louder (just noticeable) |
| +6 dB | ×4 | ×2 | Clearly louder |
| +10 dB | ×10 | ×3.2 | About twice as loud |
| +20 dB | ×100 | ×10 | About four times as loud |
Two rules of thumb come out of the table: +3 dB doubles the intensity (two identical machines are 3 dB louder than one), +6 dB doubles the sound pressure, and +10 dB is ten times the intensity, which most people hear as roughly twice as loud.
dB SPL vs dBA vs dBC vs dBFS
- dB SPL — sound pressure level in the air, relative to 20 micropascals (0 dB).
- dBA — SPL with A-weighting, which turns down low and very high frequencies to roughly match how we hear at moderate levels. Hearing-safety limits such as NIOSH’s 85 dBA use it.
- dBC — C-weighting, much flatter, keeps more of the bass. Used for peak and loud low-frequency noise.
- dBFS — decibels relative to digital full scale: 0 dBFS is the loudest level a device can record, so readings are negative. Browsers and phones measure this first; an offset turns it into an estimated dB SPL.
dBA vs this meter: Safety limits are in dBA (A-weighted). This meter shows an unweighted estimate, so use it as a rough guide only, not to check workplace compliance.
Distance and decibels
For a small (point) source in open space, every doubling of distance lowers the level by about 6 dB. A sound of 80 dB at 1 m is about 74 dB at 2 m and 68 dB at 4 m. Indoors, reflections from walls mean the level drops less than that.
Mini decibel chart
| dB | Example | Zone |
|---|---|---|
| 0 | Threshold of hearing | Comfortable |
| 30 | Whisper, quiet library | Comfortable |
| 60 | Normal conversation | Comfortable |
| 80 | Garbage disposal, alarm clock 60 cm away | Loud |
| 100 | Listening with earphones, tractor | Harmful quickly |
| 120 | Jet take-off, thunderclap (painful) | Painful / danger |
| 140 | Firearms (peak) | Painful / danger |
See the full decibel chart of about 30 everyday sounds and the safe decibel levels for each zone.
Try it: the meter
Open the free online decibel meter, tap Start, then clap or speak and watch how a few dB change the reading. It shows an approximate, unweighted dB value from your device’s microphone.
Frequently asked questions
Is 0 dB silence?
No. 0 dB is the reference level near the threshold of hearing for a healthy young ear, not the absence of sound. Sounds quieter than that reference have negative decibel values, and specialised microphones can measure them.
Is 100 dB twice as loud as 50 dB?
No, far more. Each 10 dB step sounds roughly twice as loud, so 50 extra decibels is five doublings: 100 dB sounds about 32 times as loud as 50 dB and carries 100,000 times the sound intensity.
What does dBA mean?
dBA is a sound level measured with A-weighting, a filter that reduces low and very high frequencies to match how human hearing responds at moderate levels. Hearing-safety limits, such as NIOSH’s 85 dBA for 8 hours, are written in dBA.
Why does my meter show dB, not dBA?
dB Meter Online shows an unweighted estimate: it measures the full signal from your microphone without an A-weighting filter, and phone mics roll off deep bass anyway. Treat the number as a rough guide and don’t compare it directly with dBA safety limits.