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.

IS 17351

: 2020

Test methods for determining sound absorption coefficients of materials - Reverberation room method

CurrentSpecializedTesting MethodBIMArchitectural · Insulation Materials

Download PDF BIS Portal Google

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.

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

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.