Acoustic isolation

Acoustic isolation. Isolating means preventing a sound from entering or leaving a medium. Therefore, to insulate, both absorbent materials and insulating materials are used. When the acoustic wave impinges on a construction element, part of the energy is reflected, another is absorbed and another is transmitted to the other side as the vibration spreads to all the structural or construction elements with which it is in mechanical contact. In this way, the vibration reaches the adjacent rooms where it becomes pressure waves that generate the sound.


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  • 1 General
  • 2 Characterization
  • 3 Structures
  • 4 Good insulation
  • 5 Sound absorber
    • 1 Types of materials in terms of absorption
  • 6 Source


The isolation offered by the element is the difference between the incident energy and the transmitted energy, that is, it is equivalent to the sum of the reflected part and the absorbed part. There are several basic factors that go into achieving good sound insulation:

  • Mass factor. Acoustic insulation is mainly achieved by the mass of the construction elements: the higher the mass, the greater resistance it opposes to the shock of the sound wave and the greater the attenuation. For this reason, it is not convenient to speak of specific acoustic insulation, since they are normal materials and not as with thermal insulation.
  • Multilayer factor. When it comes to constructive elements consisting of several layers, a suitable arrangement of them can improve acoustic insulation to levels higher than the sum of the individual insulation of each layer could achieve.

Each element or layer has a resonance frequency that depends on the material it is made of and its thickness. If the sound (or noise) that reaches the element has that frequency it will produce the resonance and when vibrating the element, it will produce sound that will be added to the transmitted one. Therefore, if two layers of the same material and different thickness are arranged, and therefore will have a different resonance frequency, the frequency that the first layer passes in excess will be absorbed by the second.

  • Dissipation factor. It also improves insulation if an absorbent material is placed between the two layers. These materials are usually of low density and with a large number of pores and are normally placed because they are also usually good thermal insulators. Thus, an absorbent material placed in the closed space between two parallel partitions improves the insulation that these partitions alone would offer.

The reflection of the sound can also be attenuated by placing a layer of absorbent material on the walls of the construction elements, although these techniques belong more properly to the field of acoustics .


The function of insulating materials, depending on where they are, can either be to reflect most of the energy they receive (outside), or, on the contrary, to absorb it.

Despite this, it is necessary to differentiate between acoustic insulation and acoustic absorption:

  • The acoustic insulation allows to provide a protection to the enclosure against the penetration of noise, at the same time, which prevents the sound from going out.
  • On the other hand, acoustic absorption, which aims to improve the acoustics of the room itself, controlling the reverberation time, etc.

Therefore, insulating materials are generally poor absorbers. It is a logical fact, the mission of an insulator, if it is placed inside it can be to absorb the sound that reaches it, however, placed outside, it will have the mission of reflecting the greatest amount of sound energy it receives, to prevent enter the enclosure.


Regarding the structures, an absorbent material placed in the closed space between two parallel partitions improves the insulation that these partitions alone would offer.

It cannot be said that there are specific acoustic insulation, as there are specific thermal insulation.

The acoustic insulation capacity of a certain construction element , made with one or more Materials, is its ability to attenuate the sound that passes through it. The [attenuation]] or loss of sound transmission of a certain material is defined as the difference between the incident sound power and the level of sound power that the material passes through.

The loss of sound transmission depends on the frequency, the size of the partition or wall and the absorption of the receiving room. The fact that sound attenuation depends on multiple factors means that acoustic insulation materials cannot be properly said to exist.

The acoustic insulation of a flat element is determined in the laboratory, producing a sound on one of its faces and measuring the sound transmitted on the other. The result is expressed in decibels. This result, if it appears reflected in the technical specifications of the material, does so under the nomenclature of insulating capacity and must refer to a specific thickness / thickness.

It is mainly achieved by the mass of the construction elements, although an adequate arrangement of materials can improve the acoustic insulation to levels higher than the sum of the individual insulation of each element could achieve.

Good insulation

Materials that are hard, heavy, nonporous, and, if possible, flexible, are required to achieve good sound insulation. That is, it is preferable that the insulating materials are heavy and soft at the same time.

The lead is the best insulator of all because it insulates sound and vibration. Other insulating materials are materials such as concrete , terrazzo, steel , etc. they are rigid enough and not porous enough to be good insulators.

They also act as a great and effective acoustic insulator, the air chambers (a hermetic air space) between walls. If, in addition, absorbent material is added in the space between the partitions (for example, glass wool ), the insulation improves even more. When performing acoustic conditioning, you have to pay attention not only to the walls and floors of the room, but also to the small details. A joint between two badly sealed panels, a door that does not fit, etc., can reduce the effectiveness of the insulation.

Acoustic absorbent

They are materials used in the acoustic conditioning of rooms, due to their ability to absorb most of the energy they receive. Therefore, by reflecting a very small percentage of the incident sound, unwanted reflections are avoided, which can damage the acoustics of the premises, by introducing distortions, etc.

In the professional field, the absorption capacity of these materials will have been calculated in laboratories and in the technical specifications of each material, it will be given its absorption coefficient and the critical frequency for each determined thickness.

Types of materials in terms of absorption

  1. Resonant materials, which have the maximum absorption at a certain frequency: the material’s own frequency.
  2. Porous materials, which absorb more sound as the frequency increases. That is, they more effectively absorb high frequencies (the treble). The more porous the material, the greater the absorption. The denser this material is, the greater the absorption, up to a certain limit where it would behave as a reflective, the average densities of these materials are around 80 kg / m3. Another factor to consider is the thickness used, which the greater the absorption at less frequency is effective (theoretically a porous absorbent begins to be effective at the frequency that 1/4 of its wavelength coincides with its thickness). Even its placement, by separating it from the rigid surface (wall) where it is located, improves its absorption at a lower frequency.
  3. Panel or membrane absorbers absorb low frequencies (bass) more effectively than high frequencies.
  4. Helmholtz Absorbent A type of artificially created absorbent that specifically removes (absorbs) a certain frequency range.


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