2. Sound absorption

The acoustic absorption is the phenomenon by which the acoustic energy is transformed into another type of energy: thermal, mechanical, or deformation energy. The acoustic absorption is, then, an energy dissipation phenomenon.

The acoustic absorption coefficient α of a surface is defined as the relation between the acoustic energy that it can absorb and the incident energy that affects it. It is dimensionless:

$$\mathbf{x} = \frac{E\_o}{E\_i} \tag{4}$$

Then, the absorbed energy turns out to be Ea = α Ei.

There are three families of absorbent materials, which fulfill their function according to different phenomena: porous/fibrous materials, membrane absorbers, and resonators. Each one has its best performance in different frequency ranges, as shown in Figure 2.

#### 2.1 Porous or fibrous sound absorbers

Absorbent materials are usually elastic, not very dense and permeable; in fact, they are formed mostly by air. These are soft or fibrous materials containing fine

different conditions has been studied from the point of view of their behavior as

It should be taken into account that for a fibrous absorbent material to work well, its pores must not be clogged. When they are mounted in a polluted atmosphere place, their performance will decline if the pores become saturated with particles. Sometimes to protect them, they are covered with fabric or even with a grid or perforated plate, although strictly some acoustically active surface is lost. They should not be covered with rigid materials that hide them completely, since it is necessary that the porous/fibrous surface be kept available to absorb the sound. The thickness of an absorbent material is very important for its performance: the greater the thickness, the more the opportunities to lose energy for the incident

If the absorbent material had an infinite thickness, it could be considered a perfect absorber, and all the energy of the incident wave could be absorbed. But the actual thickness is limited, and there is usually a reflecting surface behind the absorber. The sound waves will be reflected in this material, but they will have to cross twice (round trip) the thickness of the acoustic absorbent to return to the emitting room. They suffer a lot of reflections to get through, and they lose energy in each of them, mainly by friction or deformation. These energy loses are those that, in short, "spend" the acoustic energy and reduce the amplitude of the reflected

In porous or fibrous absorbent materials, it is valid to assume that the sound

An empirical result is that the best performance is given for a material thickness equal to or greater than a quarter of the wavelength (λ/4), so these materials will be effective for medium to high frequencies. For example, for a frequency of 1.000 Hz, the corresponding wavelength is 34 cm, and then λ/4 is 8.5 cm, which would be a reasonable thickness of fibrous absorbent material to be placed. Sometimes, when it is desired to extend the operating range, faceted foams or anechoic wedges are placed, which may have surface irregularities of several centimeters in height

To improve performance at low frequencies, an air chamber can be left between

the absorbent material and the rigid face to be treated. Although in theory the minimum distance between the absorber and the facing should also be at least λ/4 of the main frequency to be absorbed, it is empirically recommended that the separa-

As the thickness of the air chamber increases, the best performance of the material moves to lower frequencies. The same occurs when a protection is applied

The density of the material improves the performance as it increases to an optimal point, but if it continues to increase even more, the material will gain rigidity, and its performance will worsen, as it will start to perform more like a solid than a fibrous material (it will no longer be composed mostly of air). In general, it is assumed that the density of a fibrous sound absorbent material should not exceed

There are many ways to mount the absorbent materials on different surfaces (walls, ceilings, floors); they can also be installed as suspended panels. The value of α could vary not only with the frequency of the incident wave but also with the way

Figure 3 shows acoustic absorbers made with waste from the textile industry (weaving). These panels were made with agglomerated wool remnants that were then surface-patterned to improve their absorption and diffusion characteristics. They are low-cost panels developed at the University of the Republic (School of

wave that returns to the emitting environment.

pressure level decays linearly with the thickness.

(15 cm and even more).

tion be at least λ/10.

100 kg/m<sup>3</sup>

7

.

the material is installed.

over the absorbent material.

acoustic absorbers.

How Do Acoustic Materials Work?

DOI: http://dx.doi.org/10.5772/intechopen.82380

wave.

Figure 2. Types of acoustic absorbers and their performance curves. Adapted from [2].

channels interconnected with each other. Although they are considered the absorbent materials par excellence, they are not the only ones. They can absorb acoustic energy through two mechanisms:


The performance of these absorbers improves the smaller the length of the incident wave in relation to the dimensions of the irregularities of their surfaces. Therefore, their performance improves with increasing frequency, and it is usually at least good at most conversational frequencies. Because of their high air content, their acoustic impedance Z is very close to that of the air (Zair). Then, most of the energy of the incident wave will tend to penetrate the material, and only a small fraction will be reflected.

The fibrous absorbent materials par excellence are glass wool and rock wool. However, the performance of many materials, everyday objects, and even people in
