HowLoudWhat Is a Decibel? The dB Scale Explained Simply
A decibel (dB) is a unit that compares two quantities on a logarithmic scale. For sound, it compares the pressure of a sound wave with the quietest pressure a young healthy ear can detect, defined as 0 dB. Every 10 dB step is ten times the sound energy and sounds roughly twice as loud; every 20 dB step is ten times the sound pressure.
Why a logarithmic scale?
Human hearing spans an enormous range. The pressure of a sound at the threshold of pain is about a million times greater than at the threshold of hearing, and the ear responds to ratios rather than differences: a doubling of pressure feels like the same step whether you start quiet or loud. A linear scale would need seven-digit numbers; a logarithmic scale compresses the same range into 0 to 120. That is the decibel.
Formally, sound pressure level is L = 20 × log10(p / p0), where p is the measured pressure and p0 = 20 micropascals, the reference. The "deci" prefix just means the unit is one tenth of a bel, a unit named after Alexander Graham Bell that turned out to be too coarse for practical use.
The three rules of thumb
- +3 dB doubles the sound energy. Two identical sources are 3 dB louder than one. This is why noise limits halve the allowed exposure time for every 3 dB.
- +10 dB is ten times the energy and about twice as loud to the ear. 70 dB sounds twice as loud as 60 dB.
- +20 dB is ten times the sound pressure. Microphones and meters measure pressure, which is why the formula uses 20 rather than 10.
What different dB levels sound like
| Level | Example |
|---|---|
| 0 dB | Threshold of hearing; not silence, just the quietest audible sound |
| 30 dB | A whisper |
| 60 dB | Normal conversation |
| 85 dB | Heavy traffic; hearing damage begins with 8-hour exposure |
| 120 dB | Thunder overhead; pain threshold |
The full decibel chart runs from 0 to 160 dB.
dB SPL, dBA, dBC and dBFS: which decibel?
"Decibel" on its own only says "a logarithmic ratio". The letters after it say what is being compared:
- dB SPL — sound pressure level against 20 µPa. What a sound level meter shows with no weighting (also written dB(Z) or dBZ).
- dB(A) or dBA — dB SPL with A-weighting, a filter that mimics the ear's lower sensitivity to bass and very high treble. Used by all noise regulations. The HowLoud meter reads dB(A) by default.
- dB(C) or dBC — a flatter weighting that keeps more bass, used for peak measurements and loud music. See dBA vs dBC.
- dBFS — decibels relative to full scale in digital audio. 0 dBFS is the loudest sample a file can hold; everything else is negative. A phone microphone delivers dBFS, and a meter app adds a calibration offset to turn it into an estimate of dB SPL.
Can decibels be negative?
Yes. 0 dB SPL is a reference, not the absence of sound, so a sound quieter than the reference has a negative level. Anechoic chambers measure around −10 to −20 dB. In digital audio all normal levels are negative because 0 dBFS is the maximum.
Frequently asked questions
What does 0 decibels mean?
0 dB SPL is the reference pressure of 20 micropascals, roughly the quietest 1 kHz tone a young healthy ear can hear. It is not silence.
Is 10 dB twice as loud as 5 dB?
No. Loudness roughly doubles every 10 dB, so 15 dB is about twice as loud as 5 dB, and 10 dB is about 1.4 times as loud as 5 dB.
Why do meters use A-weighting?
Because the ear is far less sensitive to low frequencies at moderate levels. A-weighted readings match how loud people say a sound is, and decades of hearing-loss research are expressed in dB(A).