Sound absorption

Sound absorbing materials are used within rooms and enclosed working areas to reduce reflected sound and thereby to reduce the noise levels in the room, reduce or control reverberation, and improve listening environments for speech and music.  Common sound absorbing materials include acoustic ceiling tiles and soft furnishings and screens in offices and call centres.  These and/or more specialist sound absorbing materials are used in cinemas, theatres, sound recording and broadcast studios, factories, workshops, indoor firing ranges, vehicles, etc.

The sound absorption properties of a material are quantified by its sound absorption coefficient.  The sound absorption coefficient of a material can have a value between 0 and 1, with 0 representing no absorption and total reflection, and 1 representing total absorption of all the incident sound.  The sound absorption coefficient varies with the frequency of sound.

The sound absorption properties of objects such as chairs or padded screens are quantified by the equivalent sound absorption area.  This is the area of a perfectly absorbing surface which would absorb the same amount of incident sound as the real object.

There are two main ways of measuring sound absorption.  One method involves the use of a reverberation room and is suitable for large objects, furniture, panels, etc.  The other method involves the use of an impedance tube and is suitable for small samples. ISVR Consulting is equipped to carry out measurements using both methods.

Using a reverberation room to measure sound absorption

The test method is defined in BS EN ISO 354:2003 Acoustics - Measurement of sound absorption in a reverberation room.  The method is used for testing plane absorbers i.e. flat areas of sound absorptive material such as carpeting or acoustic tiling, or for testing discrete sound absorbers, i.e. pads, baffles, chairs or free-standing screens.

The method requires a diffuse reverberant sound field in a reverberation room.  The reverberation time is measured in the empty room when the sound is switched off.  The reverberation time is the time taken for the sound level in the room to decay by 60 dB after the sound source has stopped.  The test specimen or test items are then placed in the room and the reverberation time is measured again. Because of the sound absorption, the reverberation time is now shorter.  From these two reverberation times, the equivalent sound absorption area of the test specimen, AT, is calculated by using Sabine's equation.

In the case of a test specimen that uniformly covers a surface (a plane absorber or a specified array of test objects), the sound absorption coefficient is obtained by dividing AT by the treated surface area S.

When the test specimen comprises several identical objects, the equivalent sound absorption area Aobj of an individual object is found by dividing AT by the number of objects, n;    Aobj = AT/n

Plane absorbers

The specimen supplied for testing should have an area between 14.5 m2 and 17.4 m2. The test specimen should be rectangular with a ratio of width to length of between 0.7 and 1. No part of the tests specimen should be closer than 1 metre to any edge of the room and preferably with the edges of the specimen not parallel to the edges of the room.

The test specimen is placed directly against a room surface, usually the floor. The perimeter edge of the test specimen is sealed or covered to prevent the edges from absorbing sound. If the edges of the test specimen are exposed when the material is normally installed in an actual application, then the edges of the test specimen are left uncovered. The perimeter edges of the test specimen may be sealed with an acoustically reflective frame made of steel, gypsum board or wood. The frame is tightly butted to the specimen to ensure there is no air gap between it and the test specimen. If two or more pieces of material are butted together to form the test specimen, it may be necessary to cover the joints between the adjacent pieces with tape, caulking compound, or other material that is not absorbing. This will prevent the side edges form absorbing sound.

Discrete sound absorbers

Discrete objects (e.g. chairs, free-standing screens or persons) are installed for the test in the same manner as they are installed in practice. A test specimen comprises a sufficient number of individual objects (in general, at least three) to provide a measurable change in the equivalent sound absorption area of the room. Objects normally treated as individual objects are arranged randomly, spaced at least 2 metres apart and at least 1 metre from any room edge.

The two-microphone transfer-function (impedance tube) method

The two-microphone transfer-function (impedance tube) method is described in BS EN ISO 10534-2:2001 Acoustics. Determination of sound absorption coefficient and impedance in impedance tubes. Part 2: Transfer-function method and in ASTM E1050-98 Standard Test Method for Impedance and Absorption of Acoustical Materials Using a Tube, Two Microphones, and a Digital Frequency Analysis System.

This method is designed for measurements of absorption coefficient and specific acoustic impedance of small samples of circular-cut sound absorbing materials normally in the frequency range 150 Hz to 6000 Hz.

A loudspeaker is mounted at one end of an impedance tube and a small sample of the material is placed at the other end. The loudspeaker generates broadband, stationary random sound waves. These propagate as plane waves in the tube, hit the sample and are reflected resulting in a standing wave interference pattern. The sound pressure at two or more locations is measured and the complex transfer function calculated. It is then possible to determine the complex reflection coefficient, the sound absorption coefficient and the normal acoustic impedance of the material.

Two different diameter tubes are used to cover the full frequency range. A 100 mm diameter tube is used to cover measurements over the frequency range 150 Hz to 1600 Hz, and a 30 mm diameter tube is used to cover measurements in the frequency range 1200 Hz to 6000 Hz.