Clarity (C50/C80)
Definition
Clarity (C50/C80)
Clarity metrics C50 and C80 are acoustic parameters that compare the ratio of early sound energy to late sound energy, measured at 50 ms and 80 ms time boundaries respectively. Defined by ISO 3382-1:2009 Clause 4.3, C50 predicts speech clarity while C80 predicts music clarity. Higher values indicate better definition. SonaVyx computes C50 and C80 automatically from measured impulse responses.
C80 = 10 × log₁₀(∫₀⁸⁰ h²(t)dt / ∫₈₀∞ h²(t)dt) dB
How It Is Measured
C50 and C80 are computed from the room impulse response by dividing the energy before the time boundary (50 or 80 ms) by the energy after it. SonaVyx performs this calculation automatically after capturing an impulse response using sweep or MLS excitation. The result is computed per octave band from 125 Hz to 4 kHz and as a single weighted average across bands.
Practical Example
A multipurpose hall measures C80 of -1.5 dB at the rear seats. ISO 3382-1 suggests C80 between -2 and +2 dB for good musical clarity. The measured value is acceptable. However, C50 at the same position is -3.8 dB, indicating speech clarity is marginal. Adding a speech reinforcement system with directional speakers improves C50 to +1.2 dB by increasing early energy at listener positions.
The 50 ms and 80 ms Boundaries
The human auditory system integrates sound energy arriving within approximately 50 ms of the direct sound as reinforcement for speech perception. For music, the integration window extends to 80 ms because musical signals are less dependent on rapid temporal modulation than speech. Energy arriving after these boundaries is perceived as reverberance rather than clarity enhancement.
Target Values
For speech rooms, C50 should exceed +2 dB for good intelligibility. Concert halls for orchestral music target C80 between -2 and +2 dB. Values much above +5 dB indicate an overly dry, clinical acoustic that lacks warmth and envelopment. Values below -5 dB indicate excessive reverberance that washes out definition. SonaVyx reports C50 and C80 with color-coded quality indicators.
Relationship to RT60
Clarity and reverberation time are inversely related — longer RT60 generally produces lower clarity values because more energy arrives after the early boundary. However, the relationship is not strictly linear because early reflection patterns (direction, timing, level) strongly influence clarity independent of the late decay rate. EDT is a better predictor of clarity than T30 because it captures the early energy distribution.
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Measure clarity (C50/C80) — free impulse response analysis