What a Decibel Is
Almost every confusion about noise comes from treating decibels as ordinary units. They are not, and four consequences follow.
When employers review noise dose alongside working patterns, online timesheets can provide operational context, while exposure limits still need to be assessed from sound level and duration.
What it is
A ratio, expressed logarithmically, against a reference.
For sound pressure the reference is 20 micropascals — approximately the quietest sound a healthy young ear detects. 0 dB is not silence; it is the threshold of hearing.
For standards, background, or a second perspective, consult ILO safety and health at work resources.
Because it is a ratio, a decibel figure with no reference is incomplete, which is why the same number can describe sound pressure, sound power or an electrical level and mean three different things.
Consequence one: 3 dB is a doubling
Every 3 dB is twice the sound energy.
Which is why the exchange rate is 3 dB — double the energy, half the permitted time.
And why 88 dBA is not "slightly more than 85". It is twice as much, and it is the single most consequential thing to know about the scale.
Consequence two: you cannot add them
60 dB plus 60 dB is 63 dB, not 120.
Two identical machines produce twice the energy, which is 3 dB more. Ten identical machines produce 10 dB more, not ten times the number.
The practical version: silencing one of two identical noise sources buys you 3 dB, which is real and is much less than intuition suggests.
Consequence three: the weighting matters
dBA is the A-weighted scale, adjusted to approximate how human hearing responds across frequencies — less sensitive at the low end.
dB with no letter is unweighted and will read higher in a room with heavy low-frequency content.
Exposure limits are written in dBA. A figure quoted in plain dB is not comparable to them, and equipment specifications frequently use one while regulations use the other.
Consequence four: perceived loudness is a different scale again
A 10 dB increase is commonly described as roughly a doubling of perceived loudness, while being ten times the energy.
So perception, energy and the scale all move at different rates, and any sentence about something being "twice as loud" needs to say which it means.
This is also why quiet-seeming reductions matter. A 3 dB reduction halves the dose and is barely noticeable, which makes it easy to dismiss and worth doing.
The reference points worth memorising
Around 40–55 dBA: a typical office. (Industry sources.)
85 dBA: the level at which occupational protection begins. (NIOSH.)
Raise your voice to be heard at arm's length: likely at or above 85 dBA. (NIOSH.)
Three anchors, and everything else can be placed relative to them.
The short version
- A decibel is a logarithmic ratio against a reference, and 0 dB is the threshold of hearing rather than silence
- Every 3 dB is twice the sound energy, which is why 88 dBA is not slightly more than 85
- Decibels do not add arithmetically: two identical sources give 3 dB more, ten give 10 dB more
- dBA is weighted for human hearing and is what exposure limits use; plain dB is a different quantity
- A 10 dB rise is roughly a doubling of perceived loudness while being ten times the energy — three scales moving at different rates
- A 3 dB reduction halves the dose and is barely audible, which makes it easy to dismiss and worth doing