The scientific “unit” we call the decibel

Original Article ↗ Hacker News Discussion ↗

Usefulness of decibels / why experts like them

  • Many engineers (RF, radar, telecom, audio) defend dB as extremely practical:
    • Turn huge multiplicative ranges into small additive numbers.
    • Link budgets, cascaded filters, amplifiers, attenuators become “just add and subtract.”
    • A single log ratio framework spans sound pressure, voltage, power, digital full-scale, etc.
  • Some say dB is to engineering what aspect ratio is to images: a dimensionless ratio reused across contexts where the underlying units differ.

Core sources of confusion

  • dB is often treated as a unit instead of a ratio:
    • Specs and marketing write “94 dB” or “-45 dB” with no reference (dB SPL? dBV? dBu? dBFS? A-weighted?).
    • Even regulators and consumer datasheets omit weighting, reference levels, or measurement conditions.
  • Context-dependent bases:
    • For power-like quantities: 10·log₁₀(P₂/P₁).
    • For “root-power” quantities (voltage, pressure): 20·log₁₀(V₂/V₁).
    • Critics argue this is like milli- meaning different things per base unit; defenders say it keeps power and amplitude gains numerically aligned.

Perception vs physics (sound)

  • Frequent mix-ups between:
    • +3 dB ≈ double power (≈1.41× pressure), not double perceived loudness.
    • Many listeners report “about 10 dB” (sometimes 6–10 dB) as ~twice as loud.
  • Human hearing is roughly logarithmic, which justifies using a log scale, but not the casual “3 dB = twice as loud” rule of thumb.
  • Audio adds further layers: A‑weighting, B/C curves, SPL vs perceived loudness; proper loudness units like phons and sones exist but are rarely used in practice.

Domain-specific conventions and misuse

  • RF/telecom folks generally use dBm, dBV, dBu, dBFS, dB(SPL), dBi correctly and find them clean.
  • Audio and acoustics often drop suffixes or mix contexts, leading to real ambiguity for newcomers.
  • Some argue the people are the problem (“don’t understand or omit the reference”), not the dB concept; others reply that pervasive misuse is exactly what makes the system “ridiculous”.

Alternatives and reform ideas

  • Suggestions include:
    • Treating dB strictly as a “unit constructor” (e.g., dB(1 mW)), with mandatory suffixes.
    • Using explicit log-units like log₁₀(W) or base‑2 logs.
    • Better pedagogy and clearer standards (SI‑style guidance) rather than changing the entire ecosystem.