ISO 3382-1 Clarity (C50, C80) and Definition (D50)
TL;DR
Clarity C measures the ratio of early to late energy in the impulse response. C80 uses an 80 ms boundary (music clarity): C80 = 10·log10[∫₀⁸⁰h²dt / ∫₈₀^∞h²dt]. C50 uses 50 ms (speech clarity). Higher C values indicate better temporal clarity — more early energy relative to reverberant tail. Definition D50 is the early energy fraction: D50 = ∫₀⁵⁰h²dt / ∫₀^∞h²dt, expressed as percentage. Typical C80 values: -2 to +4 dB for concert halls, +2 to +8 dB for speech rooms. D50 above 0.50 generally indicates acceptable speech intelligibility.
Clarity C80 — Music (Clause 5.3)
C80 quantifies how clearly individual musical notes and rhythms can be perceived. It is defined as the logarithmic ratio of sound energy arriving within the first 80 ms to the energy arriving after 80 ms:
C80 = 10 · log₁₀ [ ∫₀⁰·⁰⁸ h²(t) dt / ∫₀.₀₈^∞ h²(t) dt ] (dB)
The 80 ms boundary reflects the temporal integration window for music perception. Energy arriving within this window fuses with the direct sound, supporting clarity. Energy after 80 ms smears temporal detail.
Clarity C50 — Speech (Clause 5.2)
C50 uses a 50 ms boundary, matching the shorter integration window for speech perception:
C50 = 10 · log₁₀ [ ∫₀⁰·⁰⁵ h²(t) dt / ∫₀.₀₅^∞ h²(t) dt ] (dB)
C50 correlates well with speech intelligibility (STI). Rooms with C50 > 0 dB generally have acceptable speech clarity; C50 > 3 dB indicates good clarity.
Definition D50 (Clause 5.4)
Definition expresses the early energy as a fraction of total energy:
D50 = ∫₀⁰·⁰⁵ h²(t) dt / ∫₀^∞ h²(t) dt
D50 ranges from 0 to 1 (often reported as percentage). Values above 0.50 (50%) indicate that more than half the sound energy arrives within the first 50 ms, supporting good speech perception. D50 is mathematically related to C50: C50 = 10·log₁₀[D50/(1-D50)].
Typical Values by Room Type
| Space | C80 (dB) | C50 (dB) | D50 |
|---|---|---|---|
| Concert hall (symphonic) | -2 to +2 | -4 to 0 | 0.30 – 0.50 |
| Concert hall (chamber) | +1 to +5 | -1 to +3 | 0.40 – 0.60 |
| Lecture hall | +3 to +8 | +1 to +6 | 0.55 – 0.75 |
| Classroom | +4 to +10 | +2 to +8 | 0.60 – 0.85 |
| Recording studio | >+10 | >+8 | >0.85 |
Relationship to Reverberation Time
In diffuse rooms, C80 and T30 are inversely correlated — longer reverberation means more late energy and lower clarity. However, early reflections can boost clarity without reducing reverberation time. This is why room shape (which controls early reflections) matters alongside absorption (which controls RT60).
SonaVyx Clarity Measurement
The RT60 tool computes C50, C80, and D50 automatically from the measured impulse response. Results are shown per octave band alongside Ts and G in the parameters panel. For speech-specific assessment, the STI tool provides a more comprehensive evaluation. For understanding INR requirements that affect C80 reliability, see INR requirements.
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Last updated: March 19, 2026