1. Introduction

### 1.1 Acoustic quality

Under the term "acoustic quality," a set of characteristics (as sound pressure levels, spectral composition, and duration of the sounds perceived) is integrated, as well as others related to the space itself—for example, its reverberation time which allow to qualify how valuable this space is regarding its aptitudes or potentialities for the desired use.

One place can have good acoustic quality for a certain use but not for another. For example, the high reverberation time of Catholic churches is part of the characteristics of the space of meditation that is desired to be generated there and is suitable for interpreting/listening to sacred music; however, it conspires against the understanding of the spoken word.

#### 1.2 Noise control and acoustic project

The concepts of acoustic quality and noise control are often closely related. Noise control refers to a set of methods, techniques, and technologies that allow obtaining acceptable noise levels in a certain place, according to economic and operational considerations [1].

Noise control does not necessarily imply reduction of noise emissions; it refers to making acceptable the sound level in immission (i.e., the signal that reaches the receiver). To know if it is, some objectives and valid criteria must be selected and applied to compare with, in order to answer the question of "acceptable for what" or "for whom". There are different ways to attack a wide range of cases in order to achieve the desired acoustic quality at the receiver.

The acoustic project of enclosures involves the selection of materials to determine the type and quality of walls, surfaces, etc. so that a certain location is apt for one use. It involves avoiding an undesired level of incidence of external noise and making the internal reverberation characteristics adequate for the desired use. To achieve the acoustic quality objectives, working harmonically on insulation, absorption, and diffusion of sound is needed.

#### 1.3 Sound insulation and absorption

When a sound wave reaches one surface, part of its energy (incident energy Ei) is reflected toward the same half space from where it comes (reflected energy Er). According to Snell's law, the angles of Ei and Er with the surface of incidence are equal. When the propagation media changes from media 1 to media 2, the incident and refracted angles should fulfill next relation (also according to Snell's law):

$$\mathbf{n}\_1 \sin \theta\_1 = \mathbf{n}\_2 \sin \theta\_2 \tag{1}$$

When a good insulation is reached, a great amount of the incident energy is retained in the emission source room. If the room has low sound absorption, the sound pressure levels inside could eventually increase. To avoid this result, a suitable absorption treatment should be done on the surfaces of the room in order to

The so-called absorbed energy is dissipated in the surface on which the sound waves impinge. It is related to the characteristics of the surface material, both in terms of its internal structure and its elasticity and texture: the more elastic or rough the surface is, the more energy will be absorbed, as it will deform more or the sound path will be increased the more through multiple reflections. The dissipation that occurs in the pores and the microstructure of the material will be higher, and less energy will be reflected toward the room. Anyway, the amount of energy involved in these phenomena is very little to cause a perceptible change of the surface

However, if acoustic energy is emitted in a low absorption room, the sound pressure levels inside can be increased by successive sound reflections. Consequently, the insulation required to protect the contiguous rooms against sound

Regarding what happens at the emission room, the acoustic energy distribution is strongly related to its acoustic quality. If a homogeneous distribution is achieved, every people at the room will have the same sound quality experience. In fact, there are some common acoustic defects that can occur, for example, stationary waves due to normal modes. To work on the acoustic energy distribution into a room, attention on sound diffusion is needed. Sound diffusion materials are those that contribute to scatter sound waves in different directions to reach a homogeneous (or diffuse) sound field. The main property of sound diffusion materials is their surface design: they have irregular surfaces with cavities and protuberances whose dimensions are calculated according to the sound frequencies they are expected to

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

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

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

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

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reduce the reflections (the reverberation).

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

How Do Acoustic Materials Work?

temperature or shape.

correct.

2. Sound absorption

it. It is dimensionless:

shown in Figure 2.

5

2.1 Porous or fibrous sound absorbers

transmission may be higher.

where n1 and n2 are the relations with sound speed c1 and c2 in the considered media and they fulfill the relation (c0 is the reference sound speed in air):

$$\mathbf{c}\_0 = \mathbf{n}\_1 \; \mathbf{c}\_1 = \mathbf{n}\_2 \; \mathbf{c}\_2 \tag{2}$$

The non-reflected energy is usually expressed as the sum of two terms: transmitted energy Et and absorbed energy Ea. The first one is the part of the energy that passes through the surface or wall and generates another acoustic wave at the other side of the wall; the second one is the part of the incident energy that is dissipated at the surface (Figure 1).

Let α be the absorption coefficient (α = Ea/Ei) and τ the transmission coefficient (τ = Et/Ei). The part of the incident energy that is not reflected should accomplish:

$$\mathbf{E\_i - E\_r = aE\_i + \tau} \mathbf{E\_i = (\alpha + \tau)} \to\_i \tag{3}$$

The more energy is absorbed, the less energy should be transmitted and vice versa. In other words, a good insulating material is bad for acoustic absorption, as a high value of α is related to a low value of τ. The more porous is a material, the less it insulates; the more resistance to the flow of air a material presents, the better insulation performance it has. Intuitively (but not strictly), the heavier the material is, the better insulation performance is expected.

Figure 1. Possible destinations of the acoustic energy that reaches a surface.

### How Do Acoustic Materials Work? DOI: http://dx.doi.org/10.5772/intechopen.82380

The acoustic project of enclosures involves the selection of materials to determine the type and quality of walls, surfaces, etc. so that a certain location is apt for one use. It involves avoiding an undesired level of incidence of external noise and making the internal reverberation characteristics adequate for the desired use. To achieve the acoustic quality objectives, working harmonically on insulation,

When a sound wave reaches one surface, part of its energy (incident energy Ei) is reflected toward the same half space from where it comes (reflected energy Er). According to Snell's law, the angles of Ei and Er with the surface of incidence are equal. When the propagation media changes from media 1 to media 2, the incident and refracted angles should fulfill next relation (also according to Snell's law):

where n1 and n2 are the relations with sound speed c1 and c2 in the considered

The non-reflected energy is usually expressed as the sum of two terms: transmitted energy Et and absorbed energy Ea. The first one is the part of the energy that passes through the surface or wall and generates another acoustic wave at the other side of the wall; the second one is the part of the incident energy that is dissipated at

Let α be the absorption coefficient (α = Ea/Ei) and τ the transmission coefficient (τ = Et/Ei). The part of the incident energy that is not reflected should accomplish:

The more energy is absorbed, the less energy should be transmitted and vice versa. In other words, a good insulating material is bad for acoustic absorption, as a high value of α is related to a low value of τ. The more porous is a material, the less it insulates; the more resistance to the flow of air a material presents, the better insulation performance it has. Intuitively (but not strictly), the heavier the material

media and they fulfill the relation (c0 is the reference sound speed in air):

n1 sin θ<sup>1</sup> ¼ n2 sin θ<sup>2</sup> (1)

c0 ¼ n1 c1 ¼ n2 c2 (2)

Ei � Er ¼ αE<sup>i</sup> þ τ Ei ¼ ð Þ α þ τ Ei (3)

absorption, and diffusion of sound is needed.

1.3 Sound insulation and absorption

Acoustics of Materials

the surface (Figure 1).

Figure 1.

4

is, the better insulation performance is expected.

Possible destinations of the acoustic energy that reaches a surface.

When a good insulation is reached, a great amount of the incident energy is retained in the emission source room. If the room has low sound absorption, the sound pressure levels inside could eventually increase. To avoid this result, a suitable absorption treatment should be done on the surfaces of the room in order to reduce the reflections (the reverberation).

The so-called absorbed energy is dissipated in the surface on which the sound waves impinge. It is related to the characteristics of the surface material, both in terms of its internal structure and its elasticity and texture: the more elastic or rough the surface is, the more energy will be absorbed, as it will deform more or the sound path will be increased the more through multiple reflections. The dissipation that occurs in the pores and the microstructure of the material will be higher, and less energy will be reflected toward the room. Anyway, the amount of energy involved in these phenomena is very little to cause a perceptible change of the surface temperature or shape.

However, if acoustic energy is emitted in a low absorption room, the sound pressure levels inside can be increased by successive sound reflections. Consequently, the insulation required to protect the contiguous rooms against sound transmission may be higher.

Regarding what happens at the emission room, the acoustic energy distribution is strongly related to its acoustic quality. If a homogeneous distribution is achieved, every people at the room will have the same sound quality experience. In fact, there are some common acoustic defects that can occur, for example, stationary waves due to normal modes. To work on the acoustic energy distribution into a room, attention on sound diffusion is needed. Sound diffusion materials are those that contribute to scatter sound waves in different directions to reach a homogeneous (or diffuse) sound field. The main property of sound diffusion materials is their surface design: they have irregular surfaces with cavities and protuberances whose dimensions are calculated according to the sound frequencies they are expected to correct.
