A decibel (dB) is the unit used to measure the intensity of sound. It sits on a logarithmic scale — each step represents a proportional increase in energy rather than an equal addition — and that’s by design, because your hearing works the same way.
The scale runs from 0 dB at the threshold of human hearing to around 130–140 dB at the point of immediate pain and physical damage. Normal conversation sits at about 60 dB. A lawnmower runs at 85–90 dB, the range where prolonged exposure starts causing permanent hearing damage.
Where the Word Comes From
“Decibel” is one-tenth of a Bel — named after Alexander Graham Bell, who developed the measurement in the 1920s during telephone signal work. The full Bel was too large for precise use, so engineers divided it by ten. The prefix “deci-” means one-tenth.
One Bel equals 10 decibels. In practice, you’ll almost always see decibels — the Bel alone is rarely used.
How the Scale Works
The decibel scale is logarithmic, not linear. Equal steps represent equal multiples of energy, not equal additions. The full reasoning is explained in the guide to why the decibel scale is logarithmic.
In practice:
- A 10 dB increase makes a sound about twice as loud to your ears
- A 10 dB increase represents 10× more acoustic intensity
- A 3 dB increase represents double the acoustic energy
The jump from 60 dB to 70 dB feels roughly the same as the jump from 90 dB to 100 dB — even though the second gap involves far more raw energy. This is your auditory system’s natural compression at work.
The Decibel Scale: Real-World Examples
| Sound | Typical Level | Notes |
|---|---|---|
| Threshold of hearing | 0 dB | Quietest sound a healthy young adult can detect |
| Breathing | 10 dB | Near silence |
| Whisper at 1 meter | 30 dB | Extremely quiet |
| Quiet library | 40 dB | Barely noticeable background sound |
| Light rainfall | 50 dB | Quiet environment |
| Normal conversation | 60 dB | Standard reference point |
| Vacuum cleaner | 70 dB | Noticeable but safe indefinitely |
| City traffic | 80–85 dB | Approaching the safe exposure threshold |
| Lawnmower | 85–90 dB | At 85 dB, NIOSH limit is 8 hours |
| Motorcycle | 95–100 dB | Safe for under 15–30 minutes |
| Power saw | 100–110 dB | Under 15 minutes at 100 dB |
| Rock concert | 110–120 dB | Potentially harmful in under 2 minutes |
| Ambulance siren (close range) | 120 dB | Fewer than 10 seconds before risk |
| Gunshot | 140–165 dB | Above the immediate damage threshold |
To check where your environment sits right now, the free online decibel meter gives a live reading directly in your browser — no app needed.
What dB Actually Measures
The decibel is a ratio — it expresses one value relative to a reference. For sound in air, the standard reference is 20 micropascals (μPa): the softest sound a young adult with healthy hearing can detect in ideal conditions.
When you see “dB” in sound contexts, it almost always means dBSPL — decibels relative to sound pressure level, using that 20 μPa reference. Most people drop the SPL and just say “dB.”
Other dB Variants You’ll See
The base dB unit gets a modifier depending on context.
dBA (A-weighted) filters the measurement to match how the ear weights different frequencies. Most noise regulations use dBA because the ear is less sensitive to very low and very high frequencies.
dBFS (Full Scale) is used in digital audio — the reference is the maximum signal the system can handle, not a physical pressure. The dB vs dBFS page covers this distinction in full.
dBm is used in electronics and radio engineering, referenced to 1 milliwatt of power. The dB vs dBm vs dBW guide breaks down all three.
What dB Doesn’t Measure
Frequency — the pitch of a sound — is measured in hertz (Hz), not decibels. A 100 dB sound can be a deep bass rumble or a piercing shriek; the decibel tells you how intense it is, not what it sounds like. The dB vs Hz guide covers that distinction directly.
When Does dB Become Dangerous?
NIOSH recommends no more than 85 dBA over an 8-hour day. Above that, every 3 dB increase halves the safe exposure window. At 100 dBA, you’re down to 15 minutes. At 115 dBA, no unprotected exposure at all.
The damage targets hair cells in the cochlea — the inner ear’s sound-sensing structure. These cells don’t regenerate once damaged, making noise-induced hearing loss permanent and cumulative. The hearing damage decibel chart maps everyday sounds to their risk level, and the noise exposure time limits guide covers exact safe times at every level.
How Decibels Are Measured
A sound level meter converts acoustic pressure at a calibrated microphone into a dB reading. Professional instruments are Type 1 (±0.7 dB) or Type 2 (±1.5 dB). You can also use a smartphone app or measure directly in a browser.
For reliable readings, microphone calibration matters. The microphone calibration guide walks through improving your phone’s accuracy step by step — a properly calibrated phone mic typically gets within ±2–3 dB of a Type 2 instrument.
FAQ
Is a higher dB always louder?
Yes in terms of acoustic intensity. Perceived loudness can be influenced by frequency — ears are more sensitive around 2–5 kHz — which is why dBA weighting exists. But for any given frequency, higher dB means louder.
Can decibels be negative?
Yes. Negative dB means the measured sound is below the reference level (20 μPa). Most adults can’t hear sounds below 0 dBSPL, though some young people with sensitive hearing can detect around −5 dBSPL.
What is a safe decibel level?
For continuous 8-hour exposure, 85 dBA is the NIOSH upper limit. Below 80 dBA, current evidence shows no daily limit is needed. The concern is total accumulated noise dose over time, not any single brief exposure.
How do you measure decibels?
With a calibrated microphone connected to a meter that converts the signal to dB. This can be a professional instrument, a smartphone app, or an online meter in your browser.
What does “sound pressure level” mean?
Sound pressure level (SPL) is the technical name for dB measured in air relative to 20 μPa. It’s the full term for what most people mean when they say “decibel level.”