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IS 17351:2020 is the Indian Standard (BIS) for test methods for determining sound absorption coefficients of materials - reverberation room method. This standard specifies the method for measuring the random incidence sound absorption coefficient of materials in a reverberation room. It details the requirements for the test facility, specimen preparation, measurement procedure, and the calculation using Sabine's formula. This allows for standardized evaluation and comparison of acoustic material performance for noise control applications in buildings.
Describes the reverberation room method for determining sound absorption coefficients of materials.
BIM-relevant code. See the BIM Hub for ISO 19650, IFC, and LOD/LOIN frameworks used alongside it.
Practical Notes
! The mounting condition of the test specimen significantly impacts results. Ensure the mounting type specified in Annex A (e.g., Type A for direct mount, Type E-xxx for mounting with an air gap) matches the intended real-world installation.
! Results are reported as the Sound Absorption Coefficient (αs) per frequency band. A single-number rating like Noise Reduction Coefficient (NRC) is an average of mid-frequencies (250, 500, 1000, 2000 Hz) and may not fully represent the material's performance, especially at low or high frequencies.
! This method measures random-incidence absorption, simulating real-world diffuse sound fields. For quality control or material science, the normal-incidence method (IS 9901 / ISO 10534) using an impedance tube is often used, but yields different values.
ISO 354:2003International Organization for Standardization (ISO), International
HighCurrent
Acoustics — Measurement of sound absorption in a reverberation room
Defines the method of measuring sound absorption coefficient of materials using a reverberation room, which IS 17351 is based on.
ASTM C423-17American Society for Testing and Materials (ASTM), USA
HighCurrent
Standard Test Method for Sound Absorption and Sound Absorption Coefficients by the Reverberation Room Method
Specifies the US standard reverberation room method, which is technically similar but differs in key parameters like specimen size and room volume.
EN ISO 354:2003European Committee for Standardization (CEN), Europe
HighCurrent
Acoustics - Measurement of sound absorption in a reverberation room (ISO 354:2003)
The European adoption of ISO 354, making it technically identical to the international standard and very similar to the Indian standard.
JIS A 1409:2016Japanese Industrial Standards (JIS), Japan
HighCurrent
Acoustics — Measurement of sound absorption in a reverberation room
The Japanese national standard for the reverberation room method, also closely aligned with ISO 354.
Key Differences
≠IS 17351 requires a reverberation room volume of 180 m³ to 500 m³, while the US equivalent ASTM C423 allows a smaller minimum volume of 125 m³.
≠The standard test specimen area in IS 17351 (and ISO 354) is 10 m² to 12 m², which is significantly larger than the typical 6.69 m² (72 sq ft) required by ASTM C423.
≠IS 17351 specifies the use of the 'weighted sound absorption coefficient' (αw) as per ISO 11654 for single-number rating, whereas ASTM C423 uses the 'Noise Reduction Coefficient' (NRC) and 'Sound Absorption Average' (SAA).
≠The minimum required decay range for reverberation time measurement is 20 dB in IS 17351, whereas ISO 354 specifies a stricter minimum of 25 dB for the evaluation range.
Key Similarities
≈The fundamental principle is identical across all standards, using the Sabine equation to calculate absorption from the change in a room's reverberation time with and without the test specimen.
≈All standards mandate measuring and correcting for sound absorption by air, using formulas based on ISO 9613-1, which accounts for temperature and humidity.
≈The procedure requires measurements to be made in one-third octave frequency bands, typically from 100 Hz to 5000 Hz.
≈All standards require spatial averaging using multiple loudspeaker and microphone positions to ensure a reliable measurement of the room's average reverberation time.
Parameter Comparison
Parameter
IS Value
International
Source
Reverberation Room Volume
180 m³ to 500 m³
≥ 125 m³
ASTM C423-17
Test Specimen Area
10 m² to 12 m²
6.69 m² (72 ft²) standard
ASTM C423-17
Test Specimen Area
10 m² to 12 m²
10 m² to 12 m²
ISO 354:2003
Single Number Rating Method
Weighted Sound Absorption Coefficient (αw) per ISO 11654
Noise Reduction Coefficient (NRC) and Sound Absorption Average (SAA)
ASTM C423-17
Frequency Range of Measurement
100 Hz to 5000 Hz
100 Hz to 5000 Hz
ISO 354:2003
Minimum Decay Range for T60 Evaluation
20 dB (35 dB preferred)
25 dB (35 dB preferred)
ISO 354:2003
Correction for Air Absorption
Required (based on ISO 9613-1)
Required (based on ISO 9613-1)
ISO 354:2003
⚠ Verify details from original standards before use
Key Values7
Quick Reference Values
Minimum reverberation room volume180 m³
Recommended reverberation room volume200 m³
Standard test specimen area10 m² to 12 m²
Typical measurement frequency range100 Hz to 5000 Hz in 1/3-octave bands
Minimum number of microphone positions6
Minimum number of sound source positions2
Minimum decay range for reverberation time measurement20 dB
Key Formulas
αs = (55.3 * V / c * S) * (1/T2 - 1/T1) — Sabine's formula for calculating sound absorption coefficient
Tables & Referenced Sections
Key Tables
Table A.1 - Mounting descriptions for test specimens (e.g., Type A, Type E)
Key Clauses
Clause 4 - Apparatus
Clause 5 - Test Specimen
Clause 6 - Test Procedure (Measurement of Reverberation Time)
Clause 7 - Calculation of sound absorption coefficient
Annex A - Mountings for Test Specimens
Frequently Asked Questions4
What is the main result obtained from this test?+
The sound absorption coefficient (αs) as a function of frequency, typically in one-third octave bands from 125 Hz to 4000 Hz.
What is a reverberation room?+
A room with very hard, reflective, and often non-parallel surfaces designed to create a diffuse sound field where sound persists for a long time. (Clause 4.1)
Why is the test specimen mounting so important?+
The way a material is mounted (e.g., directly on a wall or with an air gap behind it) drastically changes its sound absorption characteristics, especially at low frequencies. (Annex A)
What is the difference between αs (sound absorption coefficient) and NRC (Noise Reduction Coefficient)?+
αs is the absorption value at a specific frequency band. NRC is a single-number average of the αs values at 250, 500, 1000, and 2000 Hz, rounded to the nearest 0.05.