Decibels Explained
Understanding Sound Measurement
Learn About DecibelsThe decibel (dB) is the standard unit for measuring sound intensity. Its logarithmic scale matches how our ears perceive loudness, making it ideal for describing everything from whispers to jet engines.
What Is a Decibel?
A decibel is one tenth of a bel, a unit named after Alexander Graham Bell. It expresses the ratio between two power or intensity levels rather than an absolute amount, so it is always measured against a fixed reference. Because the scale is logarithmic, each equal step means a multiplication rather than an addition, which lets one scale span a faint rustle and a jet engine's roar.
Common Sound Levels
| Source | Level (dB) |
|---|---|
| Breathing | 10 dB |
| Whisper | 30 dB |
| Normal conversation | 60 dB |
| Vacuum cleaner | 70 dB |
| Lawn mower | 90 dB |
| Rock concert | 110 dB |
| Jet takeoff (100m) | 130 dB |
How Sound Is Measured
In acoustics, the decibel usually describes sound pressure level (SPL). The reference pressure for airborne sound is 20 micropascals (20 µPa), close to the quietest tone a healthy young ear can detect near 1 kHz. A sound level meter samples the pressure changes in the air and reports them on this scale.
A-Weighting
The ear is not equally sensitive to every frequency. Meters apply a filter called A-weighting to match that response, and readings taken this way are marked dBA. Most workplace and environmental noise limits are written in dBA because the value tracks perceived loudness.
Working With the Decibel Formula
Sound pressure level follows L = 20 × log₁₀(p / p₀), where p is the measured pressure and p₀ is the 20 µPa reference. For power or intensity, the multiplier is 10 rather than 20. Several familiar results drop straight out of the logarithm:
- Doubling the sound pressure adds about 6 dB.
- Doubling the power or intensity adds about 3 dB.
- A shift of roughly 10 dB is heard as twice or half as loud.
Levels also do not add directly. Two machines at 80 dB each do not reach 160 dB. Their combined power doubles, so the total rises by 3 dB to 83 dB.
Where Decibel Measurements Are Used
Decibel readings guide decisions across many fields. In workplaces, safety officers compare noise against exposure limits and provide hearing protection where needed. A common guideline flags sustained exposure above 85 dBA across an eight-hour day. Audio engineers rely on the scale to set recording levels, while planners measure road and aircraft noise near homes. Distance helps as well: for a small source outdoors, the level falls about 6 dB each time you double the distance.
Conclusion
Decibels provide a practical way to measure sound that aligns with human perception. Understanding the logarithmic nature helps interpret what dB values actually mean. A 10 dB increase sounds about twice as loud, and sustained exposure above 85 dB can damage hearing over time.