Short answer: A decibel (dB) is the unit used to measure how loud a sound is. It compares a sound’s pressure with a reference on a logarithmic scale: 0 dB is the quietest sound a young, healthy ear can hear, normal conversation is about 60 dB, and 120 dB is painfully loud. Every 10 dB increase sounds about twice as loud.
Decibels are used for sound, but also for electronics, radio and audio signals, which is why you meet so many versions (dBA, dBFS, dBm…). This guide explains the definition, where the word comes from, how the scale works, what sound is actually measured in, and when decibels become dangerous.
Decibel Definition
A decibel is one tenth of a bel, a unit for the ratio between two values on a base-10 logarithmic scale. For sound in air, the ratio is between the measured sound pressure and 20 micropascals: L = 20 × log10(p ÷ 20 µPa). Because it is a ratio, a decibel value always needs a reference; for everyday sound it is the level called dB SPL (sound pressure level). The bel was named after Alexander Graham Bell by Bell Telephone Laboratories in the 1920s; engineers soon found the bel too large and used tenths instead.
How the Decibel Scale Works
- +10 dB = 10 times the sound energy and about twice as loud.
- +3 dB = double the energy, just noticeable.
- +6 dB = double the sound pressure, clearly louder.
- Two equal sounds together add 3 dB, not double the decibels: 60 dB + 60 dB = 63 dB.
This logarithmic behaviour lets one scale cover everything from a whisper to a jet engine. The logarithmic decibel scale guide shows the maths, and the decibel calculator adds and averages decibels for you.
The Decibel Scale: Everyday Examples
| Level | Example | Effect |
|---|---|---|
| 0 dB | Threshold of hearing | Barely audible |
| 20 dB | Rustling leaves, a quiet recording studio | Very quiet |
| 40 dB | Library, quiet bedroom | Quiet |
| 60 dB | Normal conversation | Moderate |
| 80 dB | Heavy traffic, alarm clock | Loud |
| 85 dB | Blender, busy kitchen | Risk with 8 hours a day |
| 100 dB | Car horn at 5 m, concert | Harmful within about 15 minutes |
| 120 dB | Siren up close, thunder | Painful; damage in seconds |
| 140 dB | Jet engine at 30 m, gunshot | Immediate damage possible |
For 20+ more sounds with safe listening times, open the decibel chart.
What Is Sound Measured In?
| Property | Unit | Measured with |
|---|---|---|
| Loudness level (how strong) | Decibels (dB, dB(A)) | Sound level meter or decibel meter |
| Pitch (how high or low) | Hertz (Hz) | Frequency analyzer |
| Pressure | Pascals (Pa) | Calibrated microphone |
| Perceived loudness | Phons and sones | Loudness models (ISO 532) |
| Loudness of recordings | LUFS | Loudness meter |
So the unit of sound level is the decibel, and the unit of pitch is the hertz; they can’t be converted into each other (see dB vs Hz). Recordings use LUFS, which you can check with the audio loudness meter.
Common Types of Decibels
- dB SPL: sound pressure level in air, unweighted.
- dB(A) and dB(C): SPL weighted to human hearing or keeping the bass; see dB vs dBA.
- dBFS: digital audio level relative to the maximum.
- dB HL: hearing level on an audiogram.
- dBm, dBW, dBV, dBu: electrical power and voltage levels. The decibel units guide covers them all.
What Decibels Don’t Tell You
A single dB number doesn’t say how long a sound lasts, what its pitch is, or how annoying it is. A dripping tap at 40 dB can keep you awake, while steady rain at 50 dB can help you sleep. For hearing risk, both the level and the time matter, which is why limits are written as “85 dB(A) for 8 hours”.
When Do Decibels Become Dangerous?
Long daily exposure above 85 dB(A) can cause permanent hearing loss; NIOSH allows 8 hours at 85 dB and halves the time for every 3 dB more, so 100 dB is safe for only about 15 minutes. Levels above about 120 dB are painful and can cause damage within seconds. See the safe noise levels chart for every level.
Decibels Outside Acoustics
The decibel isn’t only a sound unit. Because it simply expresses a ratio, engineers use it wherever values span a huge range: amplifier gain (+20 dB means ten times the voltage), cable and signal loss, radio signal strength in dBm (decibels relative to one milliwatt), Wi-Fi reception, noise figures, and digital audio levels in dBFS, where 0 is the loudest possible value and everything else is negative. In every case the rule is the same: 10 × log10 of a power ratio, or 20 × log10 of an amplitude ratio such as voltage or pressure.
0 dB, Negative Decibels and the Top of the Scale
Because the scale starts at a reference rather than at zero sound, 0 dB is not silence: it is the reference level itself. A sound can be quieter than the reference and have a negative level, and some young people hear tones slightly below 0 dB SPL around 3–4 kHz. At the other end, the threshold of pain is around 130 dB, and in air at sea level about 194 dB is the largest possible undistorted sound, where the pressure swing equals atmospheric pressure.
How Decibels Are Measured
A sound level meter, or a phone or laptop running the free sound decibel meter, converts the microphone signal into dB SPL, applies a weighting and averages it over time. Our step-by-step guide on how to measure decibels explains where to hold the device and which setting to use.
Frequently Asked Questions
What does dB stand for?
Decibel: “deci” (one tenth) + “bel”, after Alexander Graham Bell. The B is capital because the unit is named after a person.
Is 0 dB silence?
No. 0 dB is the reference level, roughly the threshold of hearing. Quieter sounds have negative decibels.
How many decibels is too loud?
Long exposure above 85 dB(A) is too loud for hearing; at night, bedroom noise above about 30–40 dB can disturb sleep.
Why are decibels logarithmic?
Because the range of sound pressures we can hear is enormous and our ears judge loudness by ratios, not differences.