What is the minimum INR required for T30 measurement per ISO 3382?

ISO 3382-1 clause C.6 requires a minimum impulse-to-noise ratio (INR) of 45 dB for T30 extraction. For T20, the minimum INR is 35 dB. Insufficient INR produces unreliable decay curve fitting and should trigger a warning in compliant measurement software.

How many measurement positions does ISO 3382-1 require?

ISO 3382-1 clause 6 recommends at least three source positions and six receiver positions for spatial averaging. The minimum source-receiver distance must exceed the critical distance. Positions should cover the audience area uniformly while avoiding symmetry axes of the room.

What is the difference between T20, T30, and EDT?

T20 extrapolates from the -5 to -25 dB decay range, T30 from -5 to -35 dB, and EDT from 0 to -10 dB. T30 is the most reliable for characterizing physical room decay. EDT better correlates with perceived reverberance because it emphasizes the early sound field.

Can I measure room acoustics with a phone or laptop microphone?

Phone and laptop MEMS microphones can provide useful relative measurements but lack the flat frequency response and omnidirectional polar pattern required by ISO 3382-1 clause 5. For calibrated results, use a measurement microphone with known frequency response and apply correction curves.

What RT60 target should I aim for in a classroom?

Classrooms typically target RT60 of 0.4 to 0.6 seconds per ANSI S12.60 and BB93 standards. Background noise should remain below 35 dBA. Higher RT60 values reduce speech clarity and STI scores, making it difficult for students to understand the teacher from the back of the room.

What is the minimum INR required for T30 measurement per ISO 3382?

ISO 3382-1 clause C.6 requires a minimum impulse-to-noise ratio (INR) of 45 dB for T30 extraction. For T20, the minimum INR is 35 dB. Insufficient INR produces unreliable decay curve fitting and should trigger a warning in compliant measurement software.

How many measurement positions does ISO 3382-1 require?

ISO 3382-1 clause 6 recommends at least three source positions and six receiver positions for spatial averaging. The minimum source-receiver distance must exceed the critical distance. Positions should cover the audience area uniformly while avoiding symmetry axes of the room.

What is the difference between T20, T30, and EDT?

T20 extrapolates from the -5 to -25 dB decay range, T30 from -5 to -35 dB, and EDT from 0 to -10 dB. T30 is the most reliable for characterizing physical room decay. EDT better correlates with perceived reverberance because it emphasizes the early sound field.

Can I measure room acoustics with a phone or laptop microphone?

Phone and laptop MEMS microphones can provide useful relative measurements but lack the flat frequency response and omnidirectional polar pattern required by ISO 3382-1 clause 5. For calibrated results, use a measurement microphone with known frequency response and apply correction curves.

What RT60 target should I aim for in a classroom?

Classrooms typically target RT60 of 0.4 to 0.6 seconds per ANSI S12.60 and BB93 standards. Background noise should remain below 35 dBA. Higher RT60 values reduce speech clarity and STI scores, making it difficult for students to understand the teacher from the back of the room.

Complete Guide to Room Acoustics Measurement — ISO 3382

Why Measure Room Acoustics?

Every room shapes sound. A concert hall needs controlled reverberation for musical warmth, while a classroom demands short decay times for speech clarity. Without measurement, acoustic design relies on subjective guessing. ISO 3382-1 provides objective parameters that quantify exactly how a room performs acoustically.

Key Parameters Defined by ISO 3382-1

Reverberation Time (RT60)

RT60 measures the time for sound to decay by 60 dB after the source stops. ISO 3382-1 clause 3.1 defines three extraction methods: T20 (extrapolated from -5 to -25 dB), T30 (from -5 to -35 dB), and EDT (from 0 to -10 dB). T30 requires a minimum impulse-to-noise ratio (INR) of 45 dB per clause C.6.

Clarity and Definition

Clarity C80 compares early energy (first 80 ms) to late energy, critical for music. C50 uses a 50 ms window for speech. Definition D50 expresses the early-to-total energy ratio as a percentage. ISO 3382-1 clause 3.5 specifies these parameters. Values of C50 above 2 dB typically indicate good speech clarity.

Centre Time and Sound Strength

Centre time Ts, defined in ISO 3382-1 clause 3.6, represents the time at which half the total impulse response energy has arrived. Lower Ts values correlate with better intelligibility. Sound strength G measures SPL relative to a reference measurement in a free field at 10 metres distance.

Measurement Procedure

ISO 3382-1 clause 6 requires omnidirectional source and receiver positions. Minimum source-receiver distance must exceed the critical distance. At least three source positions and six receiver positions are recommended for spatial averaging. The excitation signal should be a logarithmic sine sweep or MLS sequence providing sufficient SNR.

Octave Band Analysis

Room acoustic parameters are typically reported in octave bands from 125 Hz to 4 kHz. ISO 3382-1 Table 1 specifies just-noticeable differences (JND) for each parameter: 5% for RT60, 1 dB for C80, and 5% for D50. These JNDs help determine measurement uncertainty requirements.

Interpreting Results

Optimal RT60 depends on venue type and volume. Concert halls target 1.6-2.2 seconds, lecture halls 0.6-0.8 seconds, recording studios 0.2-0.4 seconds, and worship spaces 1.2-2.5 seconds. EDT often differs from T30 due to early reflection density, and the EDT/T30 ratio indicates whether the room sounds more reverberant or controlled than the decay curve suggests.

Common Measurement Mistakes

The most frequent errors include insufficient source level leading to poor INR, measuring too close to walls where direct-to-reverberant ratio is skewed, and using inadequate averaging. ISO 3382-1 Annex A recommends checking linearity of the Schroeder decay curve with an r-squared value exceeding 0.995 for T30 extractions.

Complete Guide to Room Acoustics Measurement