Most of you reading this post will know that a decibel is a logarithmic measurement of noise. But you might not understand exactly what it measures, or what all those weightings mean. Let alone the numerous confusing acronyms that often accompany this.
Defining the decibel
A decibel is one tenth of a bel, which kind of makes sense: deci = ten. But what’s a bel? Good question. It’s a bit of a circular definition, actually: a bel is ten decibels. The IEC (IEC 801-22-02) gives a slightly clearer definition:
unit of level of a quantity proportional to power when the base of the logarithm is ten. Also, the unit of level of a field quantity when the base of the logarithm is the square root of ten.
If that makes your head hurt, you’re in good company. I find the best way to visualise what a decibel means is to use relatable sounds, like in the picture below. You can see why certain noises are so deafening when you think that this is not a linear scale.
A Soundmask system will be set around 37-43dBA, which is within that average room noise level.
Sometimes you might see the term “SPL” or sound pressure level used in relation to decibels.
What’s an SPL?
Decibels are the unit of measurement used to calculate the sound pressure level (SPL). Sound pressure level (SPL) is defined as follows by IEC 801-22-07:
logarithm of the ratio of a given sound pressure to the reference sound pressure in decibels is 20 times the logarithm to the base ten of the ratio.
In other words, SPL is a sound field quantity that uses the 20 log factor, for example:
6 dB = a factor of 2 in SPLs (double or half)
10 dB = a factor of 3 in SPLs
20 dB = a factor of 10 in SPLs.
The following table demonstrates a range of pascals we hear and the more practicable SPLs in decibels.
| Sources at 1 m | Sound Pressure | SPL re 20 μPa |
| Rifle | 200 Pa | 140 dB |
| Threshold of pain | 20 Pa | 120 dB |
| 2 power mowers | 2 Pa | 100 dB |
| 1 power mower | 1 Pa | 94 dB |
| Street traffic | 0.2 Pa | 80 dB |
| Talking | 0.02 Pa | 60 dB |
| Library | 0.002 Pa | 40 dB |
| TV studio | 0.0002 Pa | 20 dB |
| Reference sound pressure | 0.00002 Pa | 0 dB |
What about all those weightings?
If you’re working in a built environment, like we do at Soundmask, you may have seen “dB(A)” or “dBA” or even “LAeq” next to sound levels. But what does that mean, and what are all the other weightings?
Acoustic consultants will use an A-weighting within the built environment. The weighting mimics how the human ear perceives loudness across frequencies. They’ll use a C-weighting for low frequency sounds, including measuring peak or impulsive noise events (shown as dB(C) or C-Peak). Z-weighting (zero weighting) is useful in environmental and octave band analyses (shown as dB(Z)). It is important to use the correct weighting otherwise this can invalidate noise measurements, especially for regulatory or occupational purposes. You can find a bit more detail here.
Hopefully now you have a better understanding of the decibel. If you have any questions, please contact us.