Reverberation Time (RT60) FAQ
Everything you need to know about RT60 — the most important single-number metric in room acoustics. Covers the Sabine and Eyring equations, measurement techniques, and target values for different room types.
Quick Navigation
- 1. What is RT60 in room acoustics?
- 2. What is a good RT60 for a classroom?
- 3. What is the Sabine equation and how does it work?
- 4. What is the difference between T20 and T30 in reverberation measurement?
- 5. Can RT60 be too low?
- 6. How does room volume affect RT60?
- 7. What are the best materials for reducing RT60?
- 8. How does furniture affect RT60 in a room?
- 9. How do you measure RT60 in an existing room?
- 10. Which acoustic standards specify RT60 requirements?
What is RT60 in room acoustics?
RT60 is the time in seconds for sound pressure level to decay by 60 decibels after the source stops. Defined in ISO 3382-2:2008 §3.1, it is the single most important metric for characterising room acoustics. A high RT60 means the room is reverberant — desirable for orchestral music but harmful for speech intelligibility. RT60 is measured across six octave bands (125–4000 Hz) and reported as a mid-frequency average (500–1000 Hz). The Sabine equation (RT60 = 0.161V/A) provides a first-order prediction, where V is volume in cubic metres and A is total absorption in sabins. AcousPlan calculates RT60 automatically from your room geometry and material selections, giving instant visual feedback as you adjust surfaces.
What is a good RT60 for a classroom?
A good RT60 for a classroom is between 0.4 and 0.6 seconds at mid-frequencies (500–1000 Hz). ANSI S12.60-2010 Part 1 specifies a maximum of 0.6 s for core learning spaces up to 283 m³, while BB93:2015 (the UK standard) requires 0.6–0.8 s depending on room type and whether the space serves children with special educational needs (where 0.4 s is recommended). Exceeding these targets degrades speech intelligibility — every 0.1 s increase above 0.6 s reduces the Speech Transmission Index by approximately 0.03. To achieve target RT60, install Class A absorptive ceiling tiles (NRC ≥ 0.90) across at least 80% of the ceiling area. Use AcousPlan's classroom calculator to model your room and verify compliance before specifying materials.
What is the Sabine equation and how does it work?
The Sabine equation is RT60 = 0.161V / A, where V is room volume in cubic metres and A is total absorption area in metric sabins (m²). Developed by Wallace Clement Sabine in 1898 and formalised in ISO 3382-2:2008 Annex A.1, it assumes a perfectly diffuse sound field with uniformly distributed absorption. A (total absorption) equals the sum of each surface area multiplied by its absorption coefficient at the relevant frequency band. The equation works well for rooms with average absorption coefficients below 0.3 — typical of untreated spaces. For heavily treated rooms (average α > 0.3), the Eyring equation gives more accurate results. AcousPlan applies both methods simultaneously and highlights which is more appropriate for your room configuration.
What is the difference between T20 and T30 in reverberation measurement?
T20 and T30 are extrapolated reverberation times derived from the decay curve, defined in ISO 3382-2:2008 §4.2. T20 evaluates the slope between −5 dB and −25 dB below the initial level, then extrapolates to a 60 dB decay. T30 uses the range from −5 dB to −35 dB. Both avoid the noisy tail of the decay curve where background noise contaminates results. T20 requires a lower signal-to-noise ratio (35 dB minimum) than T30 (45 dB minimum), making it easier to achieve in occupied rooms. In a perfectly diffuse field, T20 and T30 are equal; significant divergence (more than 10%) indicates non-diffuse conditions or coupled volumes. Professional measurement software reports both values — specify which you are using when documenting results.
Can RT60 be too low?
Yes, an excessively low RT60 creates an acoustically "dead" room that feels unnatural and fatiguing for occupants. Speech sounds dry and lacks projection, musicians lose ensemble awareness, and listeners report a claustrophobic sensation. For speech-primary rooms, RT60 below 0.3 s is generally considered too low — BS 8233:2014 Table 4 recommends 0.4–0.8 s for most occupied spaces. Concert halls target 1.8–2.2 s (ISO 3382-1:2009 §A.2) because reverberation provides warmth and envelopment. Recording studios deliberately create very low RT60 (0.2–0.3 s) in control rooms but pair this with near-field monitoring to compensate. The key is matching RT60 to the room's primary function. AcousPlan colour-codes your calculated RT60 as green (on target), amber (marginal), or red (non-compliant) for instant feedback.
How does room volume affect RT60?
Room volume has a directly proportional effect on RT60 — doubling the volume approximately doubles the reverberation time if absorption remains constant. This relationship is explicit in the Sabine equation: RT60 = 0.161V/A (ISO 3382-2:2008 Annex A.1). Larger rooms contain more air volume for sound energy to fill, so decay takes longer. This is why cathedrals (volumes exceeding 10,000 m³) can have RT60 values above 6 seconds, while a small meeting room (50 m³) might achieve 0.4 s with moderate treatment. When designing, remember that increasing ceiling height by 1 metre in a 100 m² room adds 100 m³ of volume — potentially increasing RT60 by 0.2–0.4 s. Compensate by adding proportionally more absorption. AcousPlan recalculates RT60 in real time as you adjust room dimensions.
What are the best materials for reducing RT60?
The most effective materials for reducing RT60 are porous absorbers with high NRC ratings: mineral wool ceiling tiles (NRC 0.85–0.95), open-cell acoustic foam (NRC 0.80–0.95), fabric-wrapped glass fibre panels (NRC 0.85–1.05), and polyester acoustic panels (NRC 0.70–0.90). Per ISO 354:2003, absorption coefficients are measured in a reverberation chamber across octave bands 125–4000 Hz. For broadband reduction, 50 mm mineral wool at least 100 mm from the backing surface provides excellent mid-to-high frequency absorption. Bass frequencies (125–250 Hz) require thicker treatments — 100 mm panels with a 200 mm air gap, or dedicated bass traps in corners. AcousPlan's material library includes over 5,000 products with manufacturer-verified absorption data, enabling accurate RT60 predictions before purchase.
How does furniture affect RT60 in a room?
Furniture provides meaningful acoustic absorption that can reduce RT60 by 0.1–0.4 seconds, depending on quantity and material. Upholstered seating is the most significant contributor — a single padded office chair provides approximately 0.3–0.5 m² of equivalent absorption area at mid-frequencies. Per ISO 3382-2:2008 §B.2, occupied rooms typically have 0.1–0.3 s lower RT60 than empty rooms due to audience and furniture absorption. Bookshelves act as low-frequency diffusers and mid-frequency absorbers. Hard furniture (metal desks, glass tables) adds negligible absorption but can create unwanted reflections. When modelling in AcousPlan, use the furniture adjustment option to account for typical furnishing levels — this avoids over-specifying ceiling absorption and wasting budget. Always verify RT60 targets under the expected furnishing condition.
How do you measure RT60 in an existing room?
RT60 is measured using the interrupted noise or impulse response method, as specified in ISO 3382-2:2008 §5. The impulse response method is preferred: an omnidirectional loudspeaker emits a known signal (exponential sine sweep or MLS), and a calibrated measurement microphone records the decay. Software processes the impulse response via Schroeder backward integration to extract T20 and T30 across octave bands. A minimum of two source positions and three receiver positions (six combinations) is required per ISO 3382-2 §5.3 for engineering-grade accuracy. Equipment needed includes a Class 1 sound level meter or measurement microphone, amplified omnidirectional speaker, and analysis software. AcousPlan's mobile measurement feature lets you capture RT60 using your phone's microphone as a screening tool, with results importable for comparison against predictions.
Which acoustic standards specify RT60 requirements?
Multiple international and national standards specify RT60 requirements. ISO 3382-2:2008 defines measurement methodology. ANSI S12.60-2010 sets 0.6 s maximum for classrooms. BB93:2015 provides RT60 targets for UK educational spaces. BS 8233:2014 Table 4 covers general building types (offices 0.5–1.0 s, lecture rooms 0.4–0.8 s). DIN 18041:2016 classifies German rooms into five groups (A1–A5) with specific RT60 targets. WELL v2 Feature S06 requires RT60 ≤ 0.6 s for enclosed offices. HTM 08-01 governs UK healthcare. AS/NZS 2107:2016 covers Australian buildings. Each standard defines room categories, volume thresholds, frequency ranges, and tolerance bands differently. AcousPlan supports all major standards and automatically checks your calculated RT60 against the applicable criteria, flagging non-compliance across each octave band.
Try AcousPlan Calculator
Put these answers into practice. Model your room, select materials, and get instant RT60, STI, and compliance results.
Open Free CalculatorRelated FAQ Topics
Explore More
Browse all 30 acoustic FAQ hubs by category or search across topics.
Definitions for every acoustic term, from RT60 and STI to NRC and flanking.
Acoustic targets and design guidance for every room type.
Free RT60, STI, and noise criteria calculator with 5,000+ materials.