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Sound Absorption

Sound Absorption Materials

Quantifying Absorption: The Sound Absorption Coefficient

The Practical Application of Sound Absorption Materials

sound absorption

Advanced Sound Absorption Evaluation at XEFRA Laboratory

sound absorption

Sound absorption is a critical factor in the acoustic treatment of rooms, particularly ceilings, to mitigate reverberated sound energy. The use of sound absorbing materials helps to control reverberation time and, depending on their proximity to the sound source, can significantly reduce the overall sound pressure level in the room.

Absorption of emitted sound energy is a highly effective method of reducing noise within enclosed spaces, such as ducts or insulated enclosures designed to isolate sound sources. Absorbent materials are also used in the construction of sound barriers to reduce the reflection of sound from their surface.

The effectiveness of sound absorption materials is quantified by the sound absorption coefficient (α), which is the ratio of absorbed sound energy to incident energy. The α value indicates the fraction of sound energy a material can absorb and can vary between zero, in the case where all energy is reflected (total reflection), and one, in the case where all energy is absorbed (total absorption).

For example, a material with an sound absorption coefficient of 0.65 absorbs 65% of the sound energy. This coefficient varies with the frequency and angle of sound wave incidence, necessitating thorough testing for accurate evaluation.

In the laboratory, the absorption coefficient at normal incidence, by the standing wave method in a tube, is measured on small samples, and the absorption coefficient by random incidence, in a reverberation chamber, on large samples.

Since under real-life conditions sound waves incidence at different angles, the random incidence absorption coefficient is more representative. A method such as reverberation chamber measurement is suitable for evaluating absorbing structures or elements. In particular, experimental absorption coefficient values greater than 1, sometimes measured in a reverberation chamber, indicate non-ideal test conditions, as they were obtained under conditions of an insufficiently diffuse sound field.

Sound Absorption in Materials: Mechanisms and Selection

The physical process behind sound absorption involves converting part of the incident sound energy into heat, a mechanism that varies with the type and structure of the absorbing element. Three basic absorption mechanisms can be identified: porosity, membrane resonance, and cavity resonance. The choice of a specific sound absorbing material is influenced not only by its acoustic properties but also by factors like mechanical strength, appearance, the possibility of being coated or painted, cost, installation method, and fire resistance.

At XEFRA Laboratory, we specialise in determining the sound absorption of materials or composite structures using standardized methods: diffuse incidence in a reverberation chamber and normal incidence. For the second, it is a matter of measuring the reverberation time in a reverberation chamber with and without the sample under test, which the standard prescribes must have a surface area of no less than 10 m2.

The ISO 354:2003 standard also specifies, quite appropriately, how the different types of structure under test are to be assembled in the laboratory. The results of the two reverberation time measurements are then processed using Sebine’s formula to obtain the sound absorption coefficient (α) values as a function of frequency.

These measurements, integral in determining the sound absorption coefficient of materials, can be conducted using various instruments and methodologies. Our current procedure employs the interrupted steady-state noise decay method.

Based on the definition of reverberation time, the measurement must allow for the assessment of the decay of interrupted stationary noise from a steady state condition. The phenomenon is analogous to the feeding, by means of a jet of water at a constant flow rate, of a vessel with filter walls. In fact, the water fed in a constant and continuous manner will partly pass outside the vessel, and partly increase the internal liquid level. Once the steady state conditions are reached, the poured water will filter entirely through the walls, while the level will remain constant. In the cases which will be considered for the measurements in this procedure, the energy density will reach values which will differ from the steady state energy density by insignificant amounts, after intervals of time which will be of the order of a few seconds at most.

However, Sebine’s definition doesn’t explicitly state the exact moment at which reverberation time begins to be measured. The sound energy, in fact, density doesn’t start to decrease immediately after the source is switched off, but after a delay – the time it takes for the direct sound wave to reach the listening point. It is often not possible to derive the reverberation time from Sabine’s definition, since a decay of 60 dB cannot be recorded if there is too much background noise present, so an extrapolation of the first part of the decay curve is used.

Theoretically, if the energy density decay were exactly exponential, as predicted by statistical acoustics, the level curve would display a consistent slope. Extrapolation, therefore, would not lead to any error. However, real-world conditions often show decay curves that deviate from this ideal, because they are anything but straight, with double slopes or non-negligible curvatures.

This is where the result is influenced by the extension in dB or ms of the initial section of the curve used for extrapolation. In this regard, the reference standard recognises as the classic reverberation time, directly related to Sebine, the value obtained by extrapolating from -5 dB to – 25 dB below the steady state level indicated as TR20.

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