Absorption materials

One of the properties that have different sound absorption materials is, defined as the ability to transform sound energy into other energy.

This property is used as materials for insulation and acoustic enclosures for conditioning.

It defines the sound absorption coefficient as the ratio of energy incident on the surface and the energy absorbed by it. A totally reflecting surface would have a sound abortionists coefficient equal to 0 and a totally absorbing surface equal to 1.

The sound absorption properties depend on the frequency of the incident sound wave, usually given as curves of absorption:

We distinguish the following types of materials according to its absorption:

• Resonant materials present their maximum absorption at a given frequency (natural frequency of the material).
• Porous materials: have a higher absorption coefficient with increasing frequency.
• Absorbents as a membrane or panel: convert mechanical sound energy in the deformed wave to be excited by sound. The maximum absorbance for low frequencies.
• Helmholtz resonance: dissipate only a few certain frequencies for which they were designed.

Materials resonant

Are often used as plates and are used in cases where special treatment should be performed at low frequencies and there is a small space.

Its configuration is in the form of sheet or plate which vibrates on a cushion of air.

The absorption coefficient depends on the internal losses of the plate material and frictional losses in mounts. The absorption can be increased by filling the air cavity with absorbent materials.

Porous materials

Such materials have a structure formed by a series of air cavities linked together.

At inicidir the sound wave on the material, a significant portion of its energy penetrates into the interstices, causing the fiber movement and converting the sound energy into kinetic energy. The air enters the cavities occupied by brushing motion with the moving fiber and converting the kinetic energy into heat energy.

Rock wool and glass wool are examples of such materials. Used in combination with rigid materials.

The optimal values of absorption (of the order of 99%) occur for thicknesses that match with 1 / 4 wavelength.

The thicknesses used in practice are constrained by the limitations of space and cost. Usually used thicknesses of 3-4 cm at densities of 70-80 kg/m3.

Hole resonators of Helmholtz

They come in plates as described above, except that it presents a series of perforations in its surface.

The cavities are filled with air in the enclosure through a narrow opening that is  resonator neck. By influencing the sound wave in the air in the cavity causes continuous compressions and rarefactions so that dissipates the energy of the sound wave.

The resonators have high sound absorption values in a narrow range of frequencies, which are used when you want to fix the sound absorption of a compound for these frequencies. In the case of filling the cavity with porous absorption materials lose some of its effectiveness to expand their design frequency range of effectiveness.