WHAT IS SOUND ABSORPTION?
Some materials have an easy time absorbing sound. The greater or lesser ease with which they absorb it, is measured by a quality called acoustic absorption, and, in particular, it is parameterized by a coefficient called "Sound absorption coefficient":
From the very definition of the coefficient á, and from experience, it follows:
- It is dimensionless.
- It is always less than unity.
- Porous materials of open cell, or consisting of fibers (mineral wool, sheep's wool, cotton, etc.) have high values of the absorption coefficient.
- Materials with reflective surfaces (metal, for example) have very low absorption coefficients.
ON WHAT FACTORS DOES SOUND ABSORPTION DEPEND?
Although there are several factors that influence the capacity of acoustic absorption by a material, we will focus on three, related to air ducts:
a) The type of material
Those materials with an open cell structure, or constituted by fibers, have high values of acoustic absorption. As far as air ducts are concerned, sound absorption will be favoured:
In ducts made with absorbent materials, such as glass wool ducts, versus solutions based on metal ducts.
Within the glass wool ducts, the interior coatings that allow the absorption of sound towards the glass wool panel will favor the absorption.
Differences in absorption can be very high, depending on the type of material. Table 1 shows how glass wool ducts with net fabric or glass veil inner lining have high absorption coefficients.
b) Geometry
For the same material and the same frequency, the absorption will be greater at greater thicknesses.
The high acoustic absorption of glass wool motivates its use in anechoic chambers, where maximum absorption is sought.
(c) Frequency
All materials are more likely to absorb high-frequency (treble) sounds than low-frequency (bass) sounds. On the other hand, in air conditioning installations, noise problems are generated at low frequencies (usually associated with vibrations). Therefore, in air conditioning, materials with high sound absorption must be selected, especially at low frequencies (See table Absorption index).
In short
As a summary, we can conclude that, acoustic absorption is the ability of some materials to absorb sound. This capacity, which some products present, such as Climaver panels, is measured by means of the absorption coefficient, and has the following characteristics:
- It is very high in glass wool.
- It is higher in high frequencies than low (although, in air conditioning, higher values are of interest at low frequencies).
- Increases with thickness.
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