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The decibel & SPL

Why loudness is measured on a logarithmic scale, and what SPL means.

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Why it matters

Loudness, power, and sensitivity are all quoted in decibels. Read them as linear numbers and the maths comes out wrong by a wide margin — you over-buy amplifiers and misjudge how loud a build will go.

How it works

The decibel is logarithmic because the ear is. We hear a huge range of pressures, so a log scale keeps the numbers manageable.

Three relationships do most of the work:

  • +3 dB is double the power. Going from 100 W to 200 W adds 3 dB.
  • +6 dB is double the voltage or double the SPL measured at a point.
  • +10 dB sounds roughly twice as loud to a listener.

Two more facts that catch people out:

  • Doubling distance from a point source drops level about 6 dB (the inverse-square law).
  • SPL adds logarithmically. Two identical sources are +3 dB over one, not double the number.

SPL is referenced to 20 micropascals (0 dB SPL, the quiet threshold). Conversation sits near 60 dB; a loud build can hit 110 dB or more, which is hearing-damage territory in minutes.

Common mistakes

Simple example

Sound & frequency Sound is a pressure wave in air. Frequency is how many times that wave cycles per second, measured in hertz, and it maps directly to pitch. Sensitivity Sensitivity tells you how loud a speaker gets from a known input. It helps estimate output, amplifier needs, and how well drivers will match. Wattage Wattage is electrical power. In audio, it helps describe amplifier output and speaker power handling, but it does not directly tell you loudness or sound quality.

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