Knowledge Centre · Understanding Noise
Understanding Decibels
A decibel (dB) is simply a way of measuring sound — but because it uses a logarithmic scale, decibels don’t add or compare the way ordinary numbers do.
Read time
2 minutes
Relevant to
Anyone new to reading a noise report
A logarithmic scale
A decibel (dB) is simply a way of measuring sound.
Unlike distance or weight, sound covers an enormous range of levels, from the quietest sounds we can hear through to jet aircraft and rock concerts.
Because of this huge range, sound is measured using a logarithmic scale rather than a normal linear scale.
This means that:
- 60 dB is not twice as loud as 30 dB.
- 80 dB is not twice as loud as 40 dB.
In fact:
- A 3 dB increase represents approximately double the sound energy.
- A 10 dB increase is generally perceived as approximately twice as loud.
- A 20 dB increase sounds roughly four times as loud.
Typical sound levels
| Sound source | Typical level |
|---|---|
| Leaves rustling | 20 dB |
| Quiet office | 40 dB |
| Normal conversation | 60 dB |
| Busy road | 70 dB |
| Lawn mower | 85 dB |
| Rock concert | 100 dB |
Related reading
Try the calculator
See how two sound levels combine — the logarithmic way, not by ordinary addition.
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