What Does a Decibel Actually Measure?
The short answer
A decibel (dB) is not a fixed quantity of sound. It is a ratio, expressed on a logarithmic scale, comparing one level against a reference level. Ten decibels corresponds to a tenfold difference in sound energy; twenty decibels, a hundredfold. The decibel says nothing by itself — it only means something once you know what reference it is measured against.
The National Park Service's acoustics explainer covers the physics cleanly: sound is a pressure wave, its amplitude determines how loud it seems, and the range of pressures the ear handles is so enormous that a linear scale would be unusable. The logarithm compresses that range into workable numbers.
Why logarithmic
Human hearing spans pressure variations from a faint rustle to levels millions of times stronger. Rather than write out those multipliers, the decibel scale counts factors of ten. Three consequences follow:
- Decibels add where energy multiplies. Two equal sources together measure about 3 dB higher than one, not twice the number.
- Small dB differences are large physical differences. A 10 dB increase is ten times the sound energy — and, as a rule of thumb, is perceived as roughly twice as loud.
- Zero dB is not silence. It is the reference level. Values below the reference come out negative, which is why digital meters read in negative numbers.
The suffix is the meaning
The letters after "dB" tell you which reference is in play, and they are not interchangeable:
- dB SPL — sound pressure level in air, referenced near the quietest sound a typical young ear can detect. This is the scale used for real-world environments.
- dBA — dB SPL filtered by "A-weighting," which discounts very low and very high frequencies to better track how the ear responds. Noise measurements are usually stated in dBA.
- dBFS — decibels relative to full scale in a digital system, where 0 dBFS is the maximum a file can represent and everything real sits below it, in negative territory. See LUFS and dBFS explained for how this differs from perceived loudness.
A number like "85 dB" quoted without a suffix or a reference is incomplete. Context usually implies dBA for environmental noise, but a careful spec says so.
Where hearing enters the picture
Sound level matters to people mostly because of what sustained loud sound does to hearing over time. The mechanics of that — how the inner ear's hair cells respond to intense sound — are laid out in the NIDCD's reference on noise-induced hearing loss. That page, not this one, is the place for questions about exposure; personal questions belong with an audiologist. For decoding the charts that pair dB values with everyday sounds, see how to read a decibel chart.