ISO 3382 Complete Guide: Room Acoustics Measurement and Parameters

ISO 3382 is the foundation standard for room acoustics worldwide. Published and maintained by ISO Technical Committee TC 43 (Acoustics), Subcommittee SC 2 (Building Acoustics), it defines how to measure, calculate, and report the acoustic properties of enclosed spaces. If you work in architectural acoustics, every report you write references this standard. If you are a building physicist, facility manager, or architect specifying acoustic performance, your compliance targets trace back to ISO 3382.

The standard is not a single document. It is a three-part family, each addressing a fundamentally different type of space with different parameters, measurement procedures, and reporting requirements. Using the wrong part — or applying Part 2 methods in a Part 3 context — is one of the most common errors in acoustic consulting practice.

The Three Parts of ISO 3382

ISO 3382-1:2009 — Measurement of room acoustic parameters — Performance spaces

This part covers rooms designed for musical or theatrical performance: concert halls, opera houses, recital halls, multipurpose auditoria, and large churches used for music. It defines eight acoustic parameters derived from the room impulse response and specifies the measurement methodology to obtain them.

ISO 3382-2:2008 — Reverberation time in ordinary rooms

This part covers rooms where speech communication is the primary function: offices, classrooms, meeting rooms, hospitals, residential spaces, and other non-performance enclosures. The primary parameter is reverberation time (T20 or T30), and the standard includes prediction formulas in Annex A.

ISO 3382-3:2012 — Open plan offices

This part addresses the specific acoustic challenges of open plan work environments, where traditional reverberation time is insufficient to characterize acoustic quality. It introduces spatial decay metrics and privacy distances that are unique to open layouts.

Each part has its own scope, its own parameter set, and its own measurement requirements. They are not interchangeable. A compliance report that cites ISO 3382-2 for an open plan office is technically incorrect — that space falls under ISO 3382-3.

ISO 3382-1: Performance Spaces — The Full Parameter Set

ISO 3382-1 is the most technically demanding of the three parts. It defines eight acoustic parameters, all derived from the room impulse response measured between an omnidirectional source and one or more receivers. These parameters collectively characterize how a performance space supports music, speech, and the perception of spatial envelopment.

Parameters Defined in Section 4

What Each Parameter Tells You

EDT (Early Decay Time) is the parameter that best correlates with the subjective impression of reverberance — how reverberant a hall "sounds" to a listener. Unlike T30, which measures the full decay, EDT captures the first 10 dB of decay that dominates perceived reverberation during music. In a well-designed hall, EDT and T30 are similar. When EDT is significantly shorter than T30, the hall may feel drier than the reverberation time suggests.

T30 (Reverberation Time) is the classical Sabine parameter. ISO 3382-1 specifies the T30 evaluation range (-5 to -35 dB) rather than the full 60 dB decay because real measurements rarely achieve a 60 dB signal-to-noise ratio. The result is extrapolated to 60 dB by multiplying the measured decay time by 2. T20 (using -5 to -25 dB, multiplied by 3) is an alternative when background noise limits the usable dynamic range.

Ts (Centre Time) represents the "center of gravity" of the impulse response energy. Lower values indicate that most energy arrives early (good for speech clarity), while higher values indicate a more reverberant tail. Centre time is less commonly reported than C80 or D50, but it provides a single-number characterization of the balance between early and late energy without requiring an arbitrary time boundary.

C80 (Clarity for Music) quantifies the balance between early sound (within the first 80 milliseconds) and late reverberant sound. Positive values mean more early energy dominates — the music sounds clear and articulate. Negative values mean the reverberant tail dominates — the music may sound blurred. The 80 ms boundary corresponds roughly to the ear's temporal integration window for musical signals. Typical targets for concert halls range from -2 dB to +2 dB, depending on the repertoire the hall is designed to serve.

D50 (Definition for Speech) is the speech equivalent of C80. It uses a 50 ms boundary because speech perception integrates sound over a shorter window than music perception. D50 is expressed as a ratio (0 to 1) rather than in decibels. Values above 0.50 are generally considered adequate for speech intelligibility in performance contexts; values above 0.65 indicate excellent speech clarity.

G (Strength) measures how loud a hall makes a sound source appear, compared to the same source radiating in a free field (anechoic) measured at 10 m. It captures the combined effect of room size, absorption, and the reinforcement provided by reflections. Small, reverberant halls have high G values. Large, heavily absorbed halls have low G values. This parameter is essential for evaluating whether a concert hall provides sufficient loudness support for unamplified performance.

LF (Lateral Energy Fraction) quantifies the proportion of early energy arriving from lateral directions (the sides) rather than the median plane (above, in front, behind). Lateral early reflections are the primary physical correlate of "spaciousness" or "apparent source width" in concert halls. Higher LF values (above 0.15 to 0.25) indicate a wider perceived sound source. Rectangular halls tend to produce higher LF than fan-shaped or circular halls because side walls are closer and more parallel to the audience.

IACC (Interaural Cross-Correlation Coefficient) measures the similarity between the signals arriving at a listener's two ears. Low IACC values mean the signals are different — the listener perceives greater spatial impression and envelopment. High IACC values mean the signals are similar — the sound feels more "mono" or focused. IACC is typically reported as 1-IACC (so that higher values correspond to greater spaciousness).

Measurement Methodology (Section 5)

ISO 3382-1 §5 specifies the measurement procedure:

• Source: omnidirectional, meeting the directivity requirements of ISO 3382-1 §5.2. The source must radiate uniformly in all directions, with maximum deviation from omnidirectionality not exceeding the tolerances specified per octave band.
• Receiver: omnidirectional microphone for T30, EDT, C80, D50, Ts, and G. For LF, a figure-of-eight microphone oriented with its null axis pointing at the source is required. For IACC, a dummy head (binaural receiver) is required.
• Positions: the standard recommends a minimum of 3 source positions and sufficient receiver positions to characterize the spatial distribution of parameters across the audience area. The exact number depends on hall size and the desired statistical confidence.
• Frequency range: octave bands from 125 Hz to 4000 Hz as the minimum. Extension to 63 Hz and 8000 Hz is recommended.
• Condition: the hall should be unoccupied but with seats in their normal position (chairs down in a hall with tip-up seats).

Typical Target Values for Concert Halls

These are guideline values from acoustics literature, not mandated by ISO 3382-1 itself. The standard defines how to measure; it does not prescribe what values to achieve. Target values come from project briefs, client requirements, and design guides such as those published by Beranek and Long.

ISO 3382-2: Ordinary Rooms — Reverberation Time Measurement

ISO 3382-2 is the part most frequently cited in building codes and compliance frameworks. Its scope (§1) covers rooms where speech communication or acoustic comfort is important but where the space is not designed for musical performance. This includes classrooms, meeting rooms, private offices, hospital wards, residential rooms, corridors, lobbies, and worship spaces.

The Primary Parameter

The primary parameter in ISO 3382-2 is reverberation time, reported as either T20 or T30. The standard does not define EDT, C80, D50, or the other performance-space parameters — those belong to Part 1. If you need EDT or C80 for an ordinary room, you are free to calculate them, but you cannot claim ISO 3382-2 compliance for those parameters.

Frequency Range (Section 6.2)

Measurements must cover octave bands from 125 Hz to 4000 Hz at minimum. The standard octave band center frequencies are:

• 125 Hz
• 250 Hz
• 500 Hz
• 1000 Hz
• 2000 Hz
• 4000 Hz

Measurement Requirements (Section 5)

The measurement procedure specified in §5 includes the following requirements:

Source (§5.2): An omnidirectional sound source is required. The source must produce a sound pressure level sufficiently above the background noise in each octave band to permit reliable decay curve evaluation. Common sources include dodecahedron loudspeakers and starter pistols (impulsive sources), though the interrupted noise method is generally preferred for its superior repeatability.

Microphone (§5.3): An omnidirectional measurement microphone conforming to IEC 61672-1 Class 1.

Source positions: A minimum of 2 source positions is required (§5.4). These should represent typical sound source locations within the room — for example, where a teacher would stand in a classroom, or where a presenter would stand in a meeting room.

Receiver positions: A minimum of 3 receiver positions per source position, for a total of at least 6 source-receiver combinations.

Position constraints:

• At least 1.0 m from any reflecting surface (walls, large furniture)
• At least 1.2 m above the floor (or at seated ear height, approximately 1.2 m)
• At least 2.0 m between any two microphone positions
• At least 1.0 m between any source and any microphone

Prediction Formulas: Annex A

Annex A (informative) provides three prediction formulas for estimating reverberation time without measurement:

§A.1 — Sabine Equation:

T60 = 0.161 V / A

Where V is room volume (m3) and A is total absorption (m2 Sabine). This is the simplest formula and is valid when the average absorption coefficient is low (below approximately 0.20). At higher absorption levels, Sabine systematically overestimates reverberation time.

§A.2 — Eyring-Norris Equation:

T60 = 0.161 V / (-S ln(1 - alpha_mean))

Where S is total surface area (m2) and alpha_mean is the mean absorption coefficient. This formula accounts for the diminishing returns of absorption as alpha_mean increases. It converges to the Sabine formula when alpha_mean is small and predicts T60 = 0 when alpha_mean = 1.0 (a fully absorptive room), which is physically correct.

§A.3 — Millington-Sette Equation:

T60 = 0.161 V / (-sum of S_i ln(1 - alpha_i))

This formula applies the Eyring correction on a surface-by-surface basis rather than using the mean absorption coefficient. It is more accurate than Eyring-Norris when absorption is distributed very unevenly across surfaces — for example, when one surface has alpha = 0.90 (acoustic ceiling tile) and the others have alpha = 0.05 (concrete walls and floor).

When to Use Each Formula

• Sabine: Quick estimates, untreated rooms with alpha_mean below 0.15, early design stages.
• Eyring-Norris: Treated rooms with moderate to high absorption (alpha_mean above 0.15), rooms with reasonably uniform absorption distribution.
• Millington-Sette: Rooms with highly non-uniform absorption distribution, rooms where one surface is heavily treated and others are reflective.

ISO 3382-3: Open Plan Offices — Spatial Metrics

ISO 3382-3:2012 exists because reverberation time alone fails to characterize the acoustic quality of open plan offices. In a traditional enclosed room, RT60 tells you most of what you need to know about the acoustic environment. In an open plan office, RT60 may be 0.5 seconds everywhere, yet workers at different distances from a speaker experience radically different speech intelligibility. What matters in open plan is how quickly sound decays with distance and at what distance speech becomes unintelligible.

Parameters Defined in ISO 3382-3

How the Measurement Works

The measurement procedure is fundamentally different from Parts 1 and 2. Instead of placing source and receiver at discrete positions and measuring decay over time, ISO 3382-3 measures the spatial decay of sound along a line of workstations.

1. An omnidirectional loudspeaker is placed at a workstation position at seated head height (1.2 m).
2. Microphones are placed at successive workstation positions along a line, at the same height.
3. The loudspeaker emits a standardized speech signal (or equivalent broadband noise).
4. Sound pressure levels and STI values are recorded at each receiver position.
5. The results are plotted as a function of distance from the source, and the spatial decay curve is fitted.

Lp,A,S,4m indicates how loud speech is at a 4 m reference distance. This is influenced by ceiling height, ceiling absorption, and any screens or barriers between source and receiver. Lower values are better — they mean less speech energy reaches nearby colleagues.

rD (Distraction Distance) is the distance at which the Speech Transmission Index drops below 0.50. Inside this radius, overheard speech is intelligible enough to be distracting. Beyond rD, speech may be audible but not intelligible — it fades into background noise. The design goal is to minimize rD so that as few workstations as possible fall within the distraction zone.

rP (Privacy Distance) is the distance at which STI drops below 0.20. Beyond this distance, speech is effectively private — a listener cannot extract meaningful content even if some sound is audible. The design goal is to minimize rP for confidential work areas.

Target Values for Open Plan Offices

These quality categories are not prescribed by ISO 3382-3 itself but are derived from the research literature that informed the standard, including work by Hongisto, Virjonen, and Keranen.

Common Misapplications of ISO 3382

Using Part 2 for Open Plan Offices

The most frequent misapplication is measuring RT60 in an open plan office per ISO 3382-2 and concluding that the space is acoustically adequate because RT60 is below 0.6 seconds. Reverberation time in an open plan office is typically short — often between 0.3 and 0.5 seconds — because the large ceiling area provides substantial absorption. But a short RT60 tells you nothing about whether speech from a colleague 5 m away is distracting. That requires the spatial metrics of ISO 3382-3.

If your acoustic report for an open plan office cites ISO 3382-2 as the measurement standard, it is technically incomplete. WELL v2 Feature S06 (Sound Barriers) and the Leesman Index both expect spatial decay and privacy distance metrics that can only be obtained through Part 3 methods.

Reporting Only 500-1000 Hz Average

Many practitioners report reverberation time as a single number: the average of the 500 Hz and 1000 Hz octave bands. While this mid-frequency average is used by some building codes (such as BB93, which specifies the Tmf criterion), the ISO 3382-2 measurement itself must cover the full range from 125 Hz to 4000 Hz. The octave-band data must be reported alongside any single-number summary, because a room can pass the mid-frequency criterion while having excessive reverberation at 125 Hz — a common problem in rooms with insufficient low-frequency absorption.

Insufficient Measurement Positions

ISO 3382-2 requires a minimum of 2 source positions and 3 receiver positions per source (6 combinations total). In practice, many measurements are performed with a single source position and 2 or 3 receiver positions. This may be sufficient for a small, symmetric room, but it understates the spatial variation in larger rooms or rooms with asymmetric absorption distributions. For compliance documentation, adhering to the minimum position requirements in §5.4 is essential.

Measuring in Unfurnished Conditions

The standard specifies that measurements should be taken in furnished, unoccupied conditions. Measuring an unfurnished room during construction and using those results for compliance documentation is not valid under ISO 3382-2. Furniture (desks, chairs, bookshelves, curtains) adds significant absorption, particularly at mid and high frequencies. An unfurnished room will always have a longer RT60 than the same room furnished.

Ignoring Background Noise Requirements

The 45 dB signal-to-noise requirement is often overlooked. If background noise from HVAC, traffic, or other sources is too high, the lower portion of the decay curve is contaminated, and the T20 or T30 evaluation will be unreliable. In noisy environments, it may be necessary to turn off HVAC during measurement — but this must be documented, and the report should note that the measured RT60 reflects conditions without mechanical noise.

How Building Codes Reference ISO 3382

ISO 3382 does not prescribe target values for any room type. It defines measurement and calculation methods. The actual performance targets come from national building codes, certification schemes, and project-specific requirements that reference ISO 3382 as the measurement methodology.

WELL v2 (IWBI)

WELL Building Standard v2 references ISO 3382-2 for reverberation time measurement in enclosed rooms (Feature S01 — Sound Mapping, Part 1). The RT60 targets depend on room type and volume. WELL also references ISO 3382-3 implicitly through its open plan acoustic requirements in Feature S06 (Sound Barriers), which require speech privacy assessments that align with Part 3 methodology.

BB93:2015 (UK)

Building Bulletin 93, the UK Department for Education's acoustic design standard for schools, references ISO 3382-2 for RT60 measurement. BB93 specifies Tmf (mid-frequency reverberation time, average of 500 Hz and 1000 Hz) targets for different school room types, ranging from 0.4 s for small interview rooms to 0.8 s for large lecture halls.

DIN 18041:2016 (Germany)

The German standard for acoustic quality in rooms references ISO 3382-2 for measurement methodology but adds its own classification system. DIN 18041 categorizes rooms by use (speech intelligibility spaces vs. music performance spaces) and specifies target reverberation times as a function of room volume and use category.

ANSI S12.60-2010 (United States)

The American standard for classroom acoustics does not directly reference ISO 3382. Instead, it references ASTM E2235-04, which is the US equivalent measurement procedure for reverberation time in rooms. However, the underlying methodology is technically compatible with ISO 3382-2, and results obtained per either standard are generally accepted interchangeably.

NCC 2022 / AS 2107 (Australia)

The National Construction Code of Australia references AS 2107 (Acoustics — Recommended design sound levels and reverberation times for building interiors) for reverberation time targets. AS 2107 specifies measurement per ISO 3382-2 or its Australian adoption, AS ISO 3382.2.

NRA 2000 (France)

The French acoustic regulation (Nouvelle Reglementation Acoustique) references the ISO 3382 series for measurement methodology and specifies target values for DnT,A (standardized level difference) and L'nT,w (standardized impact sound pressure level) in residential buildings. Reverberation time in common areas must be measured per ISO 3382-2.

The Relationship Between ISO 3382 and Other Standards

ISO 3382 does not exist in isolation. It forms part of a network of acoustic standards that together define the measurement, prediction, and assessment of room acoustics:

• ISO 354:2003 — Laboratory measurement of sound absorption coefficients. The absorption data used in ISO 3382-2 Annex A formulas (Sabine, Eyring, Millington-Sette) are obtained per ISO 354.
• IEC 60268-16:2020 — Speech Transmission Index. STI, used in ISO 3382-3 to determine rD and rP, is defined and calculated per IEC 60268-16.
• ISO 9613-1/2 — Sound attenuation during outdoor propagation. Air absorption corrections applied to high-frequency RT60 predictions reference ISO 9613.
• ISO 18233:2006 — Application of new measurement methods in building and room acoustics. Provides guidance on impulse response measurement techniques used to obtain ISO 3382 parameters.
• ISO 3741/3745 — Sound power measurement. The reference sound power used to calculate Strength (G) in ISO 3382-1 is determined per these standards.

Practical Implementation Checklist

For practitioners preparing an ISO 3382-2 measurement campaign, the following checklist summarizes the standard's requirements:

1. Confirm the correct part of ISO 3382 for the space type (Part 1, 2, or 3).
2. Verify that the room is furnished and unoccupied.
3. Measure background noise levels in each octave band (125 Hz to 4000 Hz).
4. Confirm that the sound source achieves at least 45 dB above background noise in each band.
5. Set up a minimum of 2 source positions at typical source locations.
6. Set up a minimum of 3 receiver positions per source, each at least 1.0 m from walls, 1.2 m above floor, and 2.0 m apart.
7. Perform at least 6 source-receiver measurements.
8. Calculate T20 or T30 per octave band using the least-squares fit to the decay curve.
9. Report results per octave band, including the spatial average and standard deviation.
10. State the evaluation method (interrupted noise or integrated impulse response).
11. Document background noise levels, room conditions, furnishing state, HVAC status, and temperature.

How AcousPlan Implements ISO 3382

AcousPlan's calculation engine implements the full parameter set defined in ISO 3382-1: RT60 (T20, T30), EDT, C80, D50, Ts, G, and LF. Prediction uses the Sabine, Eyring-Norris, and Millington-Sette formulas from ISO 3382-2 Annex A, with automatic selection based on the room's average absorption coefficient.

For open plan offices, AcousPlan calculates the spatial decay metrics defined in ISO 3382-3 — D2,S, Lp,A,S,4m, rD, and rP — using the room geometry, ceiling absorption, screen heights, and background noise level you specify.

Compliance checking is built in. Select a building code — BB93, DIN 4109, NCC, NRA, or IBC — and AcousPlan evaluates your design against the applicable RT60, background noise, and speech intelligibility targets, generating a report that cites the specific ISO 3382 clauses referenced by each code.

Every calculation includes the standard citation, the formula used, and the intermediate values, so your compliance documentation traces directly from the design model to the standard requirement.

Run your first ISO 3382 compliance check in AcousPlan — free for single rooms.