Sound Absorption

What is Absorption?

Absorption refers to the process by which a material, structure, or object takes in energy when waves are encountered, as opposed to reflecting the energy. Part of the absorbed energy is transformed into heat and part is transmitted through the absorbing body. The energy transformed into heat is said to have been ‘lost’. (e.g. spring, damper etc.)

What is Sound Absorption?

When the sound waves encounter the surface of the material: part of them reflects; part of them penetrate, and the rest are absorbed by the material itself.

Formula for Sound Absorption: –

The ratio of absorbed sound energy (E) to incident sound energy (Eo) is called sound absorption coefficient (α). This ratio is the main indicator used to evaluate the sound-absorbing property of the material. A formula can be used to demonstrate this.

α (absorption coefficient) =E (absorbed sound energy)/ Eo (Incident sound energy)

In this formula: α is the sound absorption coefficient;

E is the absorbed sound energy (including the permeating part);

Eo is the incident sound energy.

Generally, the sound absorption coefficient of the materials is between 0 to 1. The larger the numeral is, the better the sound absorbing property. The sound absorption coefficient of suspended absorber may be more than one because its effective sound-absorbing area is larger than its calculated area.

Example: If a wall is absorbed 63% of incident energy and 37% of energy is reflected then the absorption coefficient of wall is 0.63.

How can we measure Absorption Coefficient?

The absorption coefficient and impedance are determined by two different methods according to the type of incident wave field.

1. Kundt’s tube (ISO 10534-2)
2. Reverberation room (ISO 354)

Kundt’s Tube Measurement Method: (ISO 10543-2)

For measurement of small specimen use Kundt’s tube or Impedance tube also called as Standing wave tube.  The result from measurement of absorption factor and acoustic impedance, using the standing wave method, obviously are meaningful only when assuming these to be independent of the size of the specimen, which is normally quite small.  The absorption factor for normal incidence is determined by measuring the measuring the maximum and minimum pressure amplitude in the standing wave set up in the tube by a loudspeaker.

This basic technique is, an mentioned in the introduction, considered a little outdated in comparison with more modern methods based on transfer was implemented relatively late (1993) in an international standard, ISO 10534-1, after being used for al least 50 years.  Commercial equipment has also been available for many decades.  However, there exists a second part of the mentioned standard, ISO 10534-2, based on using broadband signals and measurement of the pressure transfer function between different positions in the tube.  ISO 10543-2, which implies the specified two microphone method is extended to spherical wave fields.

Normally Placid Impedance tube is used for absorption coefficient and transmission loss measurement.

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The above fig shows Impedance tube

Reverberation Room: (ISO 354)

Reverberation Room method is traditional method, measurement of the absorption factor of larger specimens is performed in a reverberation room.  One then determines the average value over all angles of incidence under diffuse field conditions.  The product data normally supplied by producers of absorbers are determined according to the international standard ISO 354, required for measurement is 10-12 square meters and there are requirements as to shape of the area.  The reason of these requirements is that the absorption factor determined this method always includes an additional amount due to the edge effect, which is a diffraction phenomenon along the edge of the specimen.  This effect makes the specimen acoustically larger the geometric area, which may result in obtaining absorption factors larger than 1.0.  Certainly, this does not imply that the energy absorbed is larger than the incident energy.

Sound Absorption coefficient of different materials:

The sound absorption of the material is not only related to its other properties, its thickness, and the surface conditions (the air layer and thickness), but also related to the incident angle and frequency of the sound waves. The sound absorption coefficient will change according to high, middle, and low frequencies. In order to reflect the sound-absorbing property of one material comprehensively, six frequencies (125Hz, 250Hz, 500Hz, 1000Hz, 2000Hz, 4000Hz) are set to show the changes of the sound absorption coefficient. If the average ratio of the six frequencies is more than 0.2, the material can be classified as sound-absorbing material.

Application of Sound Absorber:

These materials can be used for sound insulation of walls, floors, and ceilings of concert hall, cinema, auditorium, and broadcasting studio. By using the sound absorbing material properly, the indoor transmittance of sound waves can be enhanced to create better sound effects.

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