What Is a Decibel? The dB Scale, Adding Levels and a Table

What a decibel is

A decibel is a ratio on a logarithmic scale, not an absolute quantity. For airborne sound, the sound pressure level Lp is defined as 20 log10 (p / p0), where p0 is the reference pressure of 20 micropascals — roughly the quietest sound a healthy ear detects at 1 kHz. So 0 dB does not mean silence; it means equal to the reference.

The logarithm is there because the ear copes with an enormous range: from the threshold of hearing to the threshold of pain the pressure changes by about a million times. Compressing that into 0–120 dB keeps the numbers manageable, and it tracks how loudness is perceived far better than a linear scale would.

The rules worth memorising: 3 dB, 6 dB and 10 dB

• +3 dB = double the sound energy (two identical machines instead of one), barely audible as a change
• +6 dB = double the sound pressure, or half the distance from a point source
• +10 dB = ten times the energy, perceived by most people as "about twice as loud"
• +20 dB = ten times the pressure, a hundred times the energy
• 1 dB is the smallest change most people detect under controlled conditions; in real life it takes about 3 dB to be noticed

These rules are why a 10 dB reduction is a big deal — it removes 90 % of the sound energy — and why a measure that "saves 2 dB" is rarely worth its cost.

Adding decibels

Decibels cannot be added directly; they have to be converted back to energy first: Ltotal = 10 log10 (10^(L1/10) + 10^(L2/10) + …). In practice this table is enough.

Examples: 80 dB plus 80 dB is 83 dB; 80 dB plus 70 dB is 80.4 dB. The practical lesson is that the loudest source dominates the total. Quietening a source that is already 10 dB below the loudest changes almost nothing at the receiver — the loudest machine always comes first.

dB versus dBA

The ear is not equally sensitive at every frequency; very low and very high frequencies sound quieter than mid frequencies. A-weighting adjusts each frequency band for that sensitivity and gives dBA, the unit almost every Thai limit is written in — environmental, nuisance and occupational. dBC is used for peak levels and low-frequency sources; dBZ is the unweighted value.

The difference between dB and dBA is not academic. In a noise-control contract, "reduced by 10 dB" and "reduced by 10 dB(A)" give very different results, and the gap is where a contractor claims the contract was met while the problem is still there. See 10 dB or 10 dB(A)?, and for the metrics themselves noise measurement and decibels.

Typical sound levels

The table is indicative. The legal limits, with the measurement conditions attached to each, are on the Thailand noise limits reference.

Decibels and distance

For a point source — one machine, one loudspeaker — in the open, the level falls by 6 dB each time the distance doubles. For a line source — a road with continuous traffic, a railway — it falls by only 3 dB per doubling. That is why traffic noise carries further than people expect, and it is one of the inputs a calculation model uses to predict the level at a receiver before a project is built: see noise modelling.

Measuring decibels properly

A phone app can be wrong by 10 dB and carries no legal weight. A measurement that can be relied on uses a Class 1 sound level meter to IEC 61672-1, calibrated by an ISO/IEC 17025 accredited laboratory, checked with an acoustic calibrator before and after, and follows the Pollution Control Department method. Geonoise provides field measurement; anyone who needs to buy an instrument or have one calibrated can contact PLACID, the distributor, which is run separately from Geonoise's consulting work.

Frequently asked questions