Acoustic Measurement FAQ
Comprehensive guide to acoustic measurement — from RT60 measurement per ISO 3382-2 to equipment selection, measurement positions, uncertainty analysis, and reporting requirements.
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- 1. How do you measure RT60 in a room?
- 2. What equipment is needed for acoustic measurements?
- 3. What measurement method does ISO 3382 specify?
- 4. What is the difference between impulse response and interrupted noise methods?
- 5. How many measurement positions are required?
- 6. What is measurement uncertainty in acoustic testing?
- 7. How often should acoustic measurement equipment be calibrated?
- 8. What software is used for acoustic measurement analysis?
- 9. What format should an acoustic measurement report follow?
- 10. What conditions should be present during acoustic measurements?
How do you measure RT60 in a room?
RT60 is measured using either the interrupted noise method or the impulse response method per ISO 3382-2:2008 §5. The impulse response method is preferred: an omnidirectional loudspeaker emits an excitation signal (exponential sine sweep or MLS sequence), and a calibrated microphone records the room's impulse response. Software processes the impulse response using Schroeder backward integration to extract the energy decay curve. T20 (slope from −5 to −25 dB, extrapolated to 60 dB) and T30 (slope from −5 to −35 dB) are derived per octave band. The interrupted noise method uses broadband noise from a loudspeaker, which is abruptly stopped; the ensuing decay is recorded and averaged over multiple repetitions. Measurement positions: per ISO 3382-2 §5.3, use minimum 2 source positions and 3 receiver positions (6 combinations) for engineering-grade accuracy, with at least 2 m between positions. Report results per octave band (125–4000 Hz) and as a mid-frequency average.
What equipment is needed for acoustic measurements?
Professional acoustic measurement requires: (1) Sound level metre — Class 1 per IEC 61672-1:2013 with octave and third-octave band analysis. Leading models: Brüel & Kjær 2270 (£6,000+), NTi XL2 (£3,000+), Svantek 977 (£4,000+). (2) Measurement microphone — 1/2-inch free-field condenser microphone, factory calibrated (sensitivity traceable to national standards). (3) Acoustic calibrator — Class 1 per IEC 60942:2017 (94 dB or 114 dB at 1000 Hz). (4) Omnidirectional loudspeaker — dodecahedron design for RT60 measurement, capable of 100+ dB at 1 m across 100–10000 Hz. (5) Power amplifier — sufficient to drive the loudspeaker to achieve adequate signal-to-noise ratio (≥ 45 dB for T30). (6) Software — for signal generation (sine sweep, MLS), impulse response capture, and analysis (DIRAC, EASERA, REW). (7) Tapping machine — for impact sound testing per ISO 16283-2. (8) Calibration records — all equipment calibrated within 12 months. AcousPlan's mobile measurement provides screening-level assessment with consumer hardware.
What measurement method does ISO 3382 specify?
ISO 3382 specifies different methods depending on the part. Part 2 (ordinary rooms) defines two methods: the interrupted noise method (§5.2.1) and the integrated impulse response method (§5.2.2). The interrupted noise method generates broadband noise via a loudspeaker, which is stopped abruptly; the decay is recorded and ensemble-averaged over 3+ repetitions. The impulse response method captures the room's impulse response using a sine sweep, MLS, or impulsive source (blank pistol, balloon burst), then applies Schroeder backward integration. Part 1 (performance spaces) additionally defines parameters EDT, C80, D50, lateral fraction (LF), and strength (G), all derived from the impulse response using specific time windows. Part 3 (open plan offices) specifies the spatial decay method: a loudspeaker emits pink noise at a fixed position while measurements are taken at distances from 2 m to 16+ m along defined paths. AcousPlan's calculation engine uses the mathematical framework from all three parts.
What is the difference between impulse response and interrupted noise methods?
The impulse response method captures the complete temporal response of the room to an impulsive excitation, from which all acoustic parameters (RT60, EDT, C80, D50, STI) can be derived through post-processing. It uses a deterministic excitation signal (exponential sine sweep or MLS) deconvolved from the recorded response. Advantages: single measurement yields all parameters, better signal-to-noise ratio (45+ dB achievable), and repeatable. The interrupted noise method generates broadband noise via a loudspeaker, stops it abruptly, and records the subsequent decay. Advantages: simpler equipment (no signal processing required), directly shows the decay curve. Disadvantages: only yields RT60 (not EDT, C80, etc.), requires multiple repetitions for statistical averaging (minimum 3 per position per ISO 3382-2 §5.3), and lower signal-to-noise ratio in noisy environments. Modern practice strongly favours the impulse response method. ISO 3382-2:2008 accepts both methods as equally valid for RT60 determination.
How many measurement positions are required?
The number of measurement positions depends on the required accuracy grade per ISO 3382-2:2008 §5.3. Survey grade: minimum 1 source position, 2 receiver positions — suitable for quick assessments with uncertainty of approximately ±10%. Engineering grade: minimum 2 source positions, 3 receiver positions (6 source-receiver combinations) — the standard requirement for compliance verification, with uncertainty approximately ±5%. Precision grade: minimum 2 source positions, 6+ receiver positions (12+ combinations) — for research or detailed spatial characterisation. Source positions should represent typical talker/music positions. Receiver positions must be at least 2 m apart, at least 1 m from any surface, and at 1.2 m height (seated ear height) per ISO 3382-2 §5.3.2. Exclude positions within the critical distance of the source (approximately √(A/50) metres). For large rooms or rooms with complex geometry, additional positions may be needed to capture spatial variation. AcousPlan's measurement import feature accepts data from multiple positions.
What is measurement uncertainty in acoustic testing?
Measurement uncertainty quantifies the range within which the true value lies with a stated confidence level. Per ISO 3382-2:2008 Annex D, typical uncertainty for engineering-grade RT60 measurement is ±5% (relative, 95% confidence) for mid-frequencies (500–2000 Hz) and ±10% for low frequencies (125–250 Hz) where modal behaviour causes greater spatial variation. Sources of uncertainty include: spatial variation between measurement positions (typically the largest contributor), equipment accuracy (Class 1 metre: ±0.7 dB per IEC 61672-1), calibration uncertainty (±0.3 dB), environmental conditions (temperature and humidity affect air absorption at high frequencies), and post-processing choices (curve fitting method, evaluation range). Report uncertainty as: "RT60 (500 Hz) = 0.65 ± 0.05 s (k=2)" where k=2 represents 95% confidence. Reducing uncertainty requires more source-receiver combinations and careful position selection.
How often should acoustic measurement equipment be calibrated?
Acoustic measurement equipment requires two levels of calibration. Field calibration: check the sound level metre against the acoustic calibrator before and after every measurement session per IEC 61672-3:2013 — the reading should be within ±0.3 dB of the calibrator reference. If the drift exceeds ±0.5 dB, the measurement data is invalid. Laboratory calibration: the full measurement chain (microphone + preamplifier + sound level metre) must undergo traceable calibration at an accredited laboratory (UKAS in the UK, NIST-traceable in the US) at intervals not exceeding 24 months per IEC 61672-3. Many acoustic consultancies recalibrate annually. The acoustic calibrator itself requires laboratory calibration every 12–24 months per IEC 60942:2017. Maintain a calibration register recording: equipment serial numbers, calibration dates, certificate numbers, and next due dates. Include calibration status in measurement reports to demonstrate traceability and validity. AcousPlan's measurement import includes a calibration status field.
What software is used for acoustic measurement analysis?
Professional acoustic measurement software spans several categories. Room acoustic analysis: DIRAC (Brüel & Kjær, industry standard, ISO 3382 compliant), EASERA (AFMG, comprehensive analysis suite), REW (Room EQ Wizard, free, excellent for smaller projects), and ARTA (free, impulse response analysis). Sound insulation testing: NorBuild (Norsonic, pre-completion testing with ISO 16283 compliance), insul (Marshall Day Acoustics, prediction), and BASTIAN (DataKustik, ISO 12354 prediction). Environmental noise: CadnaA (DataKustik, noise mapping per ISO 9613), SoundPLAN (noise prediction), and NoiseMap. Building services noise: ASHRAE duct calculator, CIBSE noise prediction spreadsheets. General-purpose: MATLAB with Signal Processing Toolbox, Python with acoustics libraries. AcousPlan provides design-stage prediction and post-measurement comparison in a single platform — import measured data and compare against your acoustic model to validate predictions and identify discrepancies.
What format should an acoustic measurement report follow?
An acoustic measurement report should follow the structure defined in ISO 3382-2:2008 §7 (for RT60) or ISO 16283-1:2014 §10 (for sound insulation). Essential contents: (1) Project details — building address, room identification, purpose of measurement. (2) Standard reference — which standard and clauses were followed. (3) Equipment — make, model, serial number, calibration date, and certificate reference for all equipment. (4) Environmental conditions — temperature, relative humidity (affects air absorption), and external noise conditions. (5) Measurement positions — plan showing source and receiver locations with distances. (6) Results — octave-band data (125–4000 Hz), derived single-number ratings (T20/T30, DnT,w + Ctr, etc.), and spatial statistics (mean, standard deviation, minimum). (7) Assessment — comparison against applicable criteria with pass/fail statement. (8) Uncertainty — reported per ISO/IEC Guide 98-3. (9) Photographs — room condition and equipment setup. AcousPlan generates PDF reports compatible with this structure.
What conditions should be present during acoustic measurements?
Proper measurement conditions are essential for valid, comparable results per ISO 3382-2:2008 §5.1. Room condition: unoccupied but fully furnished (or empty if measuring the unfurnished condition — state clearly in report). Doors and windows closed. All fixed building services (HVAC, lighting) operating at design condition for background noise measurements; switched off for RT60 measurements if they generate audible noise. Temperature: 15–25°C (affects speed of sound and air absorption). Humidity: 30–70% RH (significant effect on air absorption above 2000 Hz in large rooms — record and correct per ISO 9613-1). External noise: should be at typical levels; note any unusual sources (construction, events). Signal-to-noise ratio: ≥ 35 dB for T20, ≥ 45 dB for T30 measurements per ISO 3382-2. Avoid measurements during extreme weather (high wind affects low-frequency background). Document all conditions in the measurement report with photographs.
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