HowLoudNegative Decibels: Can Sound Be Below 0 dB?
Yes. 0 dB is not silence: it is a reference point, roughly the quietest sound a young, healthy ear can detect. A sound quieter than that reference has a negative level. The quietest room ever measured, a specially built anechoic chamber, reached about −20 dB(A). In digital audio every normal level is negative because 0 dB is the maximum.
Why 0 dB is not silence
A decibel value is a ratio written on a logarithmic scale. Sound pressure level compares a pressure with a reference of 20 micropascals, chosen in the 1930s because it is about the faintest 1 kHz tone people can hear. The formula is 20 × log10(p / 20 µPa). When p equals the reference the logarithm is 0, so the level is 0 dB. When p is smaller than the reference the logarithm is negative, and so is the level: half the reference pressure is −6 dB, a tenth is −20 dB. True silence, zero pressure variation, would be minus infinity. See what is a decibel for the full explanation.
Where negative decibels occur
| Level | What it is |
|---|---|
| 0 dB | Threshold of hearing at 1 kHz (the reference) |
| −5 to −10 dB | Best hearing thresholds of young people around 2–4 kHz, where the ear canal resonates |
| ≈ −9 dB | Background in commercial anechoic test chambers |
| ≈ −20 dB(A) | The quietest room on record, an anechoic chamber at a technology lab in the US |
| ≈ −23 dB | Theoretical floor from the random motion of air molecules against the eardrum |
People who sit in such rooms describe hearing their own heartbeat, blood flow and the creak of their joints. Most find it uncomfortable within minutes because the brain, starved of sound, turns up its own sensitivity.
Negative decibels in digital audio (dBFS)
Audio software, recorders and phone microphones use dBFS, decibels relative to full scale. 0 dBFS is the largest value a digital sample can hold; anything louder clips. So every real signal is negative: a loud recording peaks around −1 to −6 dBFS, quiet room noise from a phone microphone sits around −70 to −60 dBFS. A sound level meter converts dBFS into dB SPL by adding a calibration offset, which is exactly what the HowLoud meter does.
Why your meter never shows 0 dB
Phone and laptop microphones have their own electronic noise, typically equivalent to 25–35 dB. A truly silent room therefore still reads about 30 dB. Even professional Class 1 meters need special low-noise microphones to measure below about 15–20 dB.
Frequently asked questions
Can decibels be negative?
Yes. Decibels compare a value with a reference; anything smaller than the reference is negative.
What does −10 dB mean?
For sound pressure level, a sound with about a third of the reference pressure, below the hearing threshold of most people. In audio gain, −10 dB means the signal is reduced to about a third of its amplitude.
What is the quietest place on Earth?
Purpose-built anechoic chambers, which absorb nearly all reflections and block outside noise. The record is around −20 dB(A).