ISO 354 Acoustics - Measurement of Sound Absorption in a Reverberation Chamber

The International Organization for Standardization (ISO) standard "ISO 354 Acoustics - Measurement of sound absorption in a reverberation chamber" describes a method for measuring the sound absorption coefficient of acoustic materials used as wall or ceiling applications in a reverberation chamber, or the equivalent sound absorption area of ​​objects such as furniture, people, or space absorbers. This method is not intended to measure the absorption characteristics of poorly damped resonators. The results obtained from this method can be used for comparison purposes and for design calculations regarding room acoustics and noise control.

The average reverberation time in the reverberation chamber is measured with and without the test specimen attached. From these reverberation times, the equivalent sound absorption area of ​​the test specimen is calculated using the Sabine equation. In the case of a test specimen uniformly covering a surface (a flat absorber or a specific array of test objects), it is obtained by dividing the sound absorption coefficient by the treated surface area. If the test specimen consists of several identical objects, the equivalent sound absorption area of ​​a single object is found by dividing the sound absorption area by the number of objects.

The Sabine equation is a formula used in acoustics to calculate the reverberation time of a room, the time it takes for a sound to decay by 60 decibels after the source stops. The Sabine equation is fundamental to room acoustics and helps design conference halls, concert venues, and studios for optimal sound quality. It is most accurate for spaces with moderate absorption (not highly reflective or absorbent).

The reverberation chamber volume should be at least 150 m³. For new construction, a volume of at least 200 m³ is strongly recommended. When the room volume is greater than approximately 500 m³, it may not be possible to accurately measure high-frequency sound absorption due to air absorption.

The shape of the reverberation chamber must be such that the requirements specified in the standard are met. To achieve a uniform distribution of natural frequencies, particularly in the low-frequency bands, no two dimensions of the room should be in the ratio of small integers. The reverberating sound field within the room must be sufficiently diffuse. Regardless of the room shape, fixed or suspended diffusers or rotating vanes are generally used to achieve satisfactory diffusion. The equivalent sound absorption area of ​​an empty room calculated in accordance with this standard, determined in one-third-octave bands, should not exceed the values ​​given in the table. The graph of the equivalent sound absorption area of ​​the empty room against frequency should be a smooth curve and should not contain any dips or peaks that differ by more than 15 percent from the average of the values ​​in two adjacent one-third-octave bands.

The test sample should have an area between 10 m² and 12 m². If the room volume is greater than 200 m², the upper limit for the test sample area should be increased by a factor of 2/3. The area selected depends on the room volume and the absorbency of the test sample. The larger the room, the larger the test area should be. For samples with a low absorbency coefficient, the upper limit area should be selected.

The test specimen should be rectangular with a width-to-length ratio between 0,7 and 1. It should be positioned so that no part of it is closer than 1 m to any edge of the room. The distance should be at least 0,75 m. The edges of the specimen should preferably not be parallel to the nearest edge of the room. If necessary, heavy test specimens can be mounted vertically along the walls of the room, resting directly on the floor. In this case, the minimum distance requirement of 0,75 m is not required.

The test specimen shall be mounted in one of the mountings specified in the annex to the standard (Annex B), unless the relevant specifications provided by the manufacturer or application details provided by the user require a different mounting. Measurements of the reverberation time of an empty room shall be made without the frame or side walls of the test specimen, except for the barrier around the Type J mounting.

Rectangular unit sound absorbing pads or deflectors should be installed in the Type J installation as specified in the annex to the standard (Annex B).

Individual objects (e.g., chairs, free-standing screens, or people) should be set up for testing as they are typically set up in practice. For example, chairs or free-standing screens should be placed on the floor but no closer than 1 m to any other boundary. Space absorbers should be installed at least 1 m from any boundary or room diffuser and at least 1 m from any microphone. Office screens should be installed as individual objects.

A test sample should consist of a sufficient number of discrete objects (generally a minimum of three) to provide a measurable change in the equivalent sound absorption area of ​​the room, greater than 1 m² but not more than 12 m². If the room volume is greater than 200 m³, these values ​​should be increased by a factor of 2/3. Objects normally treated as discrete objects should be arranged randomly, at least 2 m apart. If the test sample consists of only one object, it should be tested at least three locations, at least 2 m apart, and the results averaged.

Changes in temperature and relative humidity during a measurement can have a significant impact on the measured reverberation time, particularly at high frequencies and low relative humidities. These changes are quantitatively described in ISO 9613-1. Measurements should be carried out in an unoccupied room and in the room containing the test specimen, with temperature and relative humidity conditions being approximately the same, so that adjustments due to air absorption do not differ significantly. In all cases, the relative humidity in the chamber should be at least 30 percent and no more than 90 percent, and the temperature should be at least 15 degrees Celsius throughout the entire test. Corrections for changes in air absorption should be applied to all measurements. The test specimen should be allowed to reach equilibrium with respect to temperature and relative humidity in the chamber before performing the tests.

This standard describes two methods for measuring distortion curves: the interrupted noise method and the integrated impulse response method. The distortion curve measured by the interrupted noise method is the result of a statistical process, and to achieve adequate reproducibility, it is necessary to average several distortion curves or reverberation times measured at a microphone/loudspeaker location. The integrated impulse response of a room is a precise function and is not prone to statistical drift, so averaging is not necessary. However, it requires more complex instrumentation and data processing than the interrupted noise method.

In short, this standard is an important international standard as it defines the method of measuring the sound absorption of materials in the reverberation chamber.

Our organization, which has been supporting businesses across all sectors for years through a wide range of testing, measurement, analysis, and evaluation activities, has a strong team of employees who closely follow global developments in science and technology and are constantly improving themselves. In this context, we also provide testing services in accordance with the "ISO 354 Acoustics - Measurement of sound absorption in an echo chamber" standard.