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.

Thermoacoustic insulation: open-cell polyurethane

The polyurethane foam is used for its low density and good properties such as thermal and acoustic insulation.

The foam of open cell polyurethane materials are very porous and has an excellent performance regarding sound insulation.

Detail porosity open-cell polyurethane

Its main advantages are the following:

• Low birth weight (10-12 kg/m3).
• Low thermal conductivity (0,035-0.040 W / m K)
• High sealability.
• Easy implementation, possibility of application by projection.
• Good behavior in case of fire.
• Increased durability over other insulation systems.
• Good adhesion to different substrates: brick, wood, concrete, plaster laminate.

Laboratory testing shows that its acoustic features are similar to rock wool and glass wool commonly used, being easier to install and ensuring a better seal cracks and holes that form the bridge acoustic seal encounters with carpentry metal boxes and shutters.

Thermal insulation foam closed-cell polyurethane has better properties than open cell (conductivity 0.028 W / m K) being able to combine both to achieve better performance or Thermoacoustic.

In the next picture can be seen the mode of application:

Implementation of open cell polyurethane projection

But should not make the mistake of applying it to improve soundproofing enclosures already implemented without first analyzing transmisón pathways of sound. It is common to confuse the information provided by the manufacturer on possible improvements in sound reduction index with real improvement when applied in a run in a building enclosure.

In architectural acoustics applications may be employed in improving the sound insulation of facades and interior partitions as in improving the impact sound insulation.

Local soundproofing: acoustic ceilings

The first element of a place that should be considered when trying toimprove the sound insulation of a room is the roof of it.




The first step is to perform a repair sealed it for the purpose of filling the cracks and pores that can deposit the forged like the rest of the room walls to be treated.

Avoid tight junctions between the conduits and the various walls of the room, for elements that can be used as fasteners elastic rubber sheets ...

When performing a local acoustic conditioning must begin with the roof of it. The installation of an acoustic ceiling prevents the transmission of airborne noise through forged.

Good acoustic performance of these roofs is due to the following mechanisms of action:

• Law of mass: the soundproofing of a homogeneous element depends directly on its surface mass. In an acoustic ceiling elements that give mass to all gypsum boards are laminated.

• Elasticity: the sound waves cause vibration of the acoustic ceiling. Rigid connections between roof and structure contribute to the transmission of vibration to other enclosures. The vibrations cause the generation of sound waves in adjacent enclosures. To avoid this effect used so-called roof insulators, these elements are dampers that dissipate the energy of the incident sound waves allowing some movement of the roof.

• Sealing: The junction between the acoustic ceiling and walls of the enclosure should not be rigid, as it conveys the vibrations of the walls to the acoustic ceiling. To resolve this union used rubber bands around the perimeter. Thus ensuring flexible watertight joint between the different elements.

Among the plasterboard is usually placed bituminous foil which provides better insulation at low frequencies and avoids the effect of resonance effects of the plates.

In the cavity between the plasterboard and forged which is based should be placed sound-absorbing mineral wool to improve the sound insulation of the whole. These mineral wool favor the dissipation of waves that form in the air, transforming sound energy into heat energy by friction with the pores of the material.

The following details can be seen the various elements of acoustic ceiling:






The installation of the roof is done through a series of metal sections that are attached to supporting elements through the roof insulators. Plasterboard fixing these metal sections.

In cases that require very high performance can use a cladding for the roof of the local boards as plasterboard.

For the proper design of an acoustic ceiling special attention to the mass of all elements of it. Depending on the mass of the assembly and the type of roof insulators indicates the number of insulators. They must withstand a load as close as possible to its optimal design load provided by the manufacturer.

Insulators should be distributed evenly to avoid supporting various loads, and to prevent the occurrence of buckling of the plasterboard and metal structures that support them.

Once installed, the local acoustic ceiling will proceed to install the cladding on the walls of the enclosure and decorative ceiling is suspended acoustic ceiling of the room. The cavity between the two roofs will pass facilities (air conditioning ducts, electrical ...)