TLDR
Late reverberation is the portion of a room's sound energy that arrives at a listener more than approximately 50 ms (speech) or 80 ms (music) after the direct sound. By this point, sound has bounced off so many surfaces that it arrives from all directions with roughly equal probability — this is the diffuse field. Late reverberation is what we commonly call "reverb": the wash of decaying energy that gives a cathedral its grandeur, makes a bathroom sound echoey, or turns a lecture hall into an intelligibility nightmare. RT60 primarily characterises this late decay. Too much late reverberation masks speech consonants and blurs musical passages. Too little makes a room feel sterile and disconnected. The art of acoustic design is getting this balance right for the intended use.
Real-World Analogy
Think of dropping a stone into a still pond. The initial splash is the direct sound. The first ring of ripples spreading outward are early reflections. But after those first rings bounce off the pond's edges and return, they collide with newer ripples going in different directions. Within seconds, the entire surface is a chaotic, disorganised shimmer with no discernible pattern — that is late reverberation. It carries energy from the original splash, but the information about where the stone landed is gone. You can tell something disturbed the water, but you cannot trace the ripples back to a single point. In a room, late reverberation similarly carries energy from the original sound source but has lost the directional and temporal information that makes speech understandable.
Technical Definition
Late reverberation begins when the sound field transitions from discrete, identifiable reflections to a statistically diffuse energy distribution. Per ISO 3382-1:2009, the conventional boundary is:
- 50 ms for speech applications (the D50 definition boundary)
- 80 ms for music applications (the C80 clarity boundary)
E(t) = E₀ × e^(−13.8t / RT60)
where E₀ is the energy density at the onset of the diffuse field and RT60 is the reverberation time. This exponential decay appears as a straight line on a logarithmic (dB) energy-time plot, and its slope is what RT60 measurement methods (T20 and T30 per ISO 3382-2:2008) extract.
Characteristics of the Late Field
- Spatial uniformity: In a fully diffuse field, the energy density is the same at every point in the room. In practice, full diffusivity is an approximation — real rooms deviate, especially at low frequencies where modes dominate.
- Temporal density: Late reflections arrive so rapidly (hundreds per second in a medium-sized room) that individual reflections are no longer distinguishable. They form a continuous, smooth decay.
- Frequency dependence: High frequencies decay faster than low frequencies because air absorption and typical surface materials absorb more energy at high frequencies. This is why the reverberant tail of a concert hall sounds progressively warmer (bass-heavy) as it fades.
The Sabine and Eyring Connection
Both the Sabine equation (RT60 = 0.161V/A) and the Eyring equation (RT60 = 0.161V / [−S × ln(1−ᾱ)]) model the late reverberant decay based on room volume, surface area, and average absorption. They are valid above the Schroeder frequency, where modal density is high enough for the diffuse-field assumption to hold.
Why It Matters for Design
- Speech intelligibility: Late reverberation is the primary enemy of speech clarity. Every syllable a speaker produces is followed by a reverberant tail that overlaps with the next syllable. If RT60 exceeds about 0.6 seconds in a classroom, consonant masking accelerates and STI drops below acceptable levels. ANSI S12.60 limits classroom RT60 to 0.6 s specifically to control late reverberation.
- Musical warmth and fullness: Concert halls thrive on controlled late reverberation. An RT60 of 1.8-2.2 seconds in a symphonic hall allows orchestral sound to bloom and sustain, creating the lush, immersive quality audiences prize. Cutting RT60 below 1.5 seconds would make the same hall sound dry and clinical.
- Background noise masking: In open-plan offices, moderate late reverberation contributes to the ambient sound level that masks conversational speech at neighbouring desks. Per ISO 3382-3, the balance between reverberation and sound masking systems determines speech privacy.
- Frequency balance: Since high frequencies decay faster, a room can have appropriate RT60 at 1 kHz but excessive bass reverberation. Frequency-specific treatment — bass traps for low frequencies, thin panels for mid/highs — shapes the reverberant tail across the spectrum.
- Acoustic comfort: Subjective studies consistently show that people rate rooms with excessively long late reverberation as "noisy," "fatiguing," and "stressful," even when the absolute sound level is moderate. The continuous wash of reflected energy demands cognitive effort to filter, leading to listening fatigue.
How AcousPlan Uses This
AcousPlan separates early and late energy in its simulation results, reporting RT60 (the late decay metric), EDT (which blends early and late behaviour), and C80/D50 (which quantify the early-to-late ratio). The frequency-dependent RT60 chart shows how the reverberant tail behaves across the 125-4000 Hz octave bands, highlighting bass build-up or treble over-damping. The auto-solve engine targets late reverberation reduction when STI is below threshold, recommending absorption placement on surfaces that contribute most to the diffuse field.
Related Concepts
- What is RT60? — The metric that directly measures late reverberant decay time
- What Are Early Reflections? — The structured energy that precedes the diffuse tail
- What is Clarity (C80, D50)? — Metrics quantifying the balance between early and late energy
- Room Acoustics Fundamentals — The complete picture of how rooms shape sound
- What Are Room Modes? — Low-frequency behaviour below the diffuse-field threshold
Calculate Now
See how late reverberation affects your space — run a simulation in AcousPlan and watch RT60, C80, and STI respond as you add absorption to tame the reverberant tail.