A decibel is not a regular linear unit like a volt or a metre. It expresses a ratio on a logarithmic scale, which lets a very wide range of acoustic power fit into useful numbers. A change of 10 dB represents a tenfold change in power ratio, while a change of 20 dB represents a tenfold change in pressure-amplitude ratio when the same reference and conditions apply. That is why a 60 dB sound is not simply twice as loud as a 30 dB sound.
Which Decibel Reference?
The reference must be named before a decibel value is meaningful. Sound pressure level (dB SPL) uses 20 micropascals in air as its reference pressure; electrical and audio specifications may instead use dBm, dBW, or dBV, each with a different reference. The Sound Converter is useful for transparent reference conversions, but it cannot decide which reference a measurement instrument or datasheet intended.
Pressure, Power, and Intensity Ratios
For power or intensity ratios, the decibel change is 10 log₁₀(P₂/P₁). For pressure or voltage ratios under equivalent impedance and acoustic conditions, it is 20 log₁₀(A₂/A₁). Doubling acoustic power therefore adds about 3.01 dB, while doubling pressure amplitude adds about 6.02 dB. Do not mix those equations simply because both results carry the dB label.
Practical Comparisons
A quiet library may be around 30–40 dB SPL, normal conversation near 60 dB SPL, and a busy road or loud machinery substantially higher, although distance, room reflections, frequency weighting, and measurement time all matter. A 10 dB increase means ten times the measured power ratio, not a universal tenfold subjective loudness increase. Human hearing is frequency-dependent, so A-weighted dB(A) readings and unweighted readings should not be compared casually.
Use Context Alongside the Number
Record the reference, weighting curve, bandwidth, microphone distance, and whether the number is an instantaneous peak or an averaged level. When sound is part of a communications system, convert the bit rate separately with the Data Transfer Converter; when studying a tone's physical frequency and wavelength, pair the acoustic level with the Frequency Wavelength Converter. These are related measurements, not interchangeable descriptions of loudness.