What This Procedure Card Is
The ISO 3382 measurement procedure card is a one-page field reference that walks you through an RT60 site measurement in the correct sequence, with the acceptance criteria for each step printed alongside it. It is designed for acoustic consultants, building physicists, and site engineers who carry out RT60 measurements as part of compliance verification, post-occupancy evaluation, or design validation.
The card covers ISO 3382-2:2008 — the revision that governs ordinary rooms (offices, classrooms, meeting rooms). For performance spaces (concert halls, theatres), ISO 3382-1:2009 applies and introduces additional parameters including EDT, C80, C50, and lateral energy fraction. The procedure card focuses on the -2 revision because it applies to the vast majority of building projects.
Use AcousPlan's RT60 calculator to compare your measured values against standard-specific targets and identify compliance gaps before writing the final report.
Who Needs This Card
Junior consultants on their first site measurement — ISO 3382-2 is not long, but it assumes familiarity with acoustic measurement principles that junior staff may not have internalised. A procedure card that condenses the standard's measurement requirements into a checklist format prevents common errors: wrong source positions, too few microphone positions, insufficient dynamic range.
Contractors verifying acoustic performance at practical completion — Building contracts increasingly include acoustic KPIs (RT60 ≤ X seconds, STI ≥ Y). A site engineer responsible for commissioning verification needs a practical procedure reference, not a 30-page standard document.
Acoustic consultants as a client-handover reference — When instructing a testing subcontractor, providing this card along with the measurement specification ensures the subcontractor follows the correct procedure and does not produce non-compliant data that has to be re-collected.
Equipment Checklist
Before arriving on site, confirm you have the following equipment:
| Item | Specification | Notes |
|---|---|---|
| Omnidirectional sound source | Dodecahedron loudspeaker, ≥ 110 dB(A) at 1 m in octave bands 125–4,000 Hz | Class 1 source for T30; class 2 acceptable for T20 only |
| Power amplifier | Rated for source at full output | Check impedance match |
| Signal generator / laptop | ISO 3382-2 §5.3: pink noise or MLS/ESS signal capability | MLS/ESS preferred for impulse response method |
| Calibrated measurement microphone | Class 1 (IEC 61672), omnidirectional, flat ±2 dB from 63 Hz to 8 kHz | Calibrate on site before first measurement |
| Sound level meter or DAQ system | Real-time octave band analysis, 125 Hz to 4,000 Hz minimum | 63 Hz octave band recommended for low-frequency rooms |
| Acoustic calibrator | 94 dB or 114 dB, IEC 60942 Class 1 | Calibrate before and after measurement session |
| Microphone stand | Height adjustable to 1.2–1.5 m | 1.2 m for seated audience; avoid tables and reflective surfaces |
| Tape measure or laser distance meter | ± 10 mm accuracy | For recording source and microphone positions |
| Background noise meter | Measure L_A before and after measurement | Document background noise level per ISO 3382-2 §5.5 |
Site conditions: ISO 3382-2 §4 requires measurements to be taken with the room in its normal operational condition. This means: furniture and fittings in place, HVAC at its normal operational setting (document the noise level), windows and doors closed. Do not measure in an empty room if the room will be normally furnished — the RT60 will be significantly longer and non-representative.
Source and Microphone Positioning Rules
Source Positions (ISO 3382-2 §5.4.1)
- Minimum 2 source positions, ideally 3 or more
- Source must be at least 1.0 m from any surface (wall, floor, ceiling, furniture)
- Source positions should represent typical speaker locations (e.g., teaching position, presentation position)
- In rectangular rooms, avoid placing the source at the geometric centre — this creates standing wave symmetry that produces non-representative decay curves
- Source height: 1.5 m above finished floor level (representing a standing speaker)
Microphone Positions (ISO 3382-2 §5.4.2)
- Minimum 2 microphone positions per source position (6 total for a 3-source setup)
- Microphone must be at least 1.0 m from any surface
- Microphone must be at least 2.0 m from the source (far enough to be in the reverberant field)
- Microphone must be at least 1.0 m from other microphones if measuring simultaneously
- Microphone height: 1.2 m above finished floor level (representing a seated listener)
- For audience areas, distribute microphone positions across the full seating area, not just the front rows
Minimum Combinations Required
| Room Type | Source Positions | Microphone Positions | Min. Combinations |
|---|---|---|---|
| Small rooms (< 100 m²) | 2 | 3 | 6 |
| Medium rooms (100–500 m²) | 3 | 4 | 12 |
| Large rooms (> 500 m²) | 4 | 6 | 24 |
| Irregular geometry / balconies | +1 per zone | +2 per zone | As above, plus zones |
Measurement Procedure — Interrupted Noise Method
The interrupted noise method (ISO 3382-2 §5.3.1) is the most widely used field procedure because it requires standard loudspeakers and measurement systems without specialist impulse response software.
Step 1 — Background noise measurement: Measure background noise level in all octave bands (125–4,000 Hz) with the source switched off and the room in its operational state. Record L_background for each octave band. This determines the available dynamic range.
Step 2 — Source level check: Switch on the source at the intended level. Verify that the steady-state level exceeds the background noise by at least 45 dB (for T20) or 55 dB (for T30) in each octave band. If this margin is not achieved, increase source power or accept that T20 rather than T30 will be the result.
Step 3 — Steady-state excitation: Excite the room with the source signal (pink noise or band-limited white noise) until the sound field is fully established — typically 5–10 seconds, or at least twice the expected RT60.
Step 4 — Source interruption and decay capture: Switch off the source abruptly (hard off, not faded) and capture the subsequent decay curve. The measurement system records the level in each octave band as a function of time. Minimum capture duration: 2 × RT60 after source-off.
Step 5 — Repeat: Make at least 3 repetitions at each source–microphone combination. Use the average decay curve for T20/T30 evaluation, or evaluate each repetition and check for consistency.
Measurement Procedure — Impulse Response Method
The impulse response method (ISO 3382-2 §5.3.2) uses MLS (maximum-length sequences) or ESS (exponential sine sweep) signals to measure the room impulse response, from which the decay curve is derived by Schroeder backward integration. This method provides better dynamic range and allows post-measurement analysis.
Step 1 — MLS/ESS signal emission: Play the test signal through the omnidirectional source. The signal must be long enough to capture the full reverberant decay (typically 4–8 × RT60).
Step 2 — Impulse response computation: Deconvolve the recorded response with the test signal to obtain the room impulse response h(t). Commercial software (WinMLS, ARTA, Dirac, Room EQ Wizard) handles this automatically.
Step 3 — Schroeder integration: Integrate the squared impulse response backwards in time to produce the energy decay curve (EDC). The EDC is equivalent to the ensemble-average decay curve from the interrupted noise method.
Step 4 — T20/T30 evaluation: Fit a regression line to the EDC between −5 dB and −25 dB (T20) or −5 dB and −35 dB (T30). Extrapolate to −60 dB to obtain T20 or T30.
Advantage of impulse response method: Single measurement captures all source–receiver combinations simultaneously if the signal-to-noise ratio is sufficient. Better suited for large rooms. The Schroeder integration eliminates the variance associated with random noise decay curves.
T20 and T30 Evaluation
| Parameter | Evaluation Range | Extrapolation | Best Used When |
|---|---|---|---|
| T20 | −5 dB to −25 dB | × 3 to give 60 dB equivalent | Dynamic range limited; short RT60 rooms |
| T30 | −5 dB to −35 dB | × 2 to give 60 dB equivalent | Long RT60 rooms; good dynamic range |
| EDT | 0 dB to −10 dB | × 6 | Performance spaces (ISO 3382-1 only) |
Which to report: ISO 3382-2 §6 requires that you report T20 or T30 — specify which one in the measurement report. For compliance against standards such as DIN 18041, BB93, or WELL v2 F74, T30 is generally preferred when dynamic range permits. Do not mix T20 and T30 values across different measurement positions in the same room.
Data Quality Checks
Run these checks on each decay curve before accepting the result:
1. Linearity check: The decay curve should be approximately linear (straight line on a dB vs. time plot) from the start of decay to the noise floor. A curved or non-linear decay indicates non-diffuse sound field conditions, double slopes (coupled rooms), or HVAC interference. Investigate before reporting.
2. Dynamic range check: The regression line for T30 must begin above the noise floor by at least 10 dB. If it does not, the T30 result is unreliable — report T20 instead or increase source level.
3. Inter-position consistency: If T20 or T30 values vary by more than 15% between measurement positions in the same room, the room has significant spatial variation. Report the range and the arithmetic mean. Investigate whether the variation is due to measurement error or genuine room geometry effects.
4. Low-frequency validity: 125 Hz octave band measurements are susceptible to standing waves, particularly in rooms smaller than 200 m³. A 125 Hz T30 with high inter-position variance (>25%) should be flagged in the measurement report. ISO 3382-2 Annex A provides guidance on lower frequency limits.
5. Background noise change check: Re-measure background noise after the measurement session and compare with the pre-session reading. If the background noise level has increased significantly (e.g., HVAC cycling, adjacent activity), measurements taken during the high-noise period may be invalid.
Reporting Requirements
ISO 3382-2 §6 specifies the minimum content of a compliant measurement report:
- Room identification (name, function, floor area, volume)
- Date and time of measurements
- Room condition (furnished/unfurnished, HVAC state, occupancy)
- Source and microphone positions (plan drawing with distances from walls)
- Equipment list with calibration dates
- Background noise levels per octave band
- T20 or T30 per octave band at each source–microphone combination
- Arithmetic mean T20/T30 per octave band across all combinations
- Uncertainty estimate (inter-position standard deviation × coverage factor)
- Compliance assessment against specified target (DIN 18041, BB93, WELL, etc.)
Related Resources
- What Is RT60? — fundamental explanation of reverberation time
- RT60 Measurement Methods Compared — interrupted noise vs. impulse response vs. mobile apps
- RT60 Calculator — Quick — compare your measured RT60 against target values for your room type and standard
- DIN 18041 Complete Guide — target RT60 values by German room category
Measurement procedure references ISO 3382-2:2008 §§4–6. AcousPlan's RT60 calculator applies standard-specific compliance targets to your measured or predicted values. Measurement results should be reported by a suitably qualified acoustic professional.