*Overview of Noise Control Techniques and Methods DOI: http://dx.doi.org/10.5772/intechopen.104608*


#### **Table 4.**

*Acoustic properties of some materials (values from various sources).*

#### **Figure 4.**

*Acoustic phenomena that occur in a barrier (adapted from Ref. [16]).*

Reflection and refraction follow Snell's law; the fraction of the acoustic energy that will be refracted and reflected can be estimated with the coefficients *Fr* and *Ft* presented in the previous section. Please consider that the refracted energy involves both the transmitted and the absorbed at the surface of the screen, that is, the refracted energy is all the energy that is not reflected.

The requirements for the materials for building acoustic barriers are not acoustically challenging: a minimum surface density of 10 kg/m<sup>2</sup> is sufficient to obtain an adequate transmission loss (TL) value. Tightness is important: a percentage of openings of more than 1.5% in the screen surface causes a reduction of 3 dB in its transmission loss TL.

The most important feature to consider for designing a noise barrier is diffraction. Both the upper and the side edges of a noise barrier become diffracted noise sources. Hence, for calculating the SPL at a receiver without the direct vision of the main noise source, the contributions of each one of these new sources are to be added. If the barrier has a special heading, its reduction may be also considered.

A barrier can be considered thin or thick. A thin barrier can be thought of as a flat surface with mass but without thickness; thus, its edges would act as lines where diffraction can occur. An acoustic barrier is said to be thick when it has more than one point where diffraction can occur. The most frequent case is that of embankments, although the buildings of a city are very important to control noise pollution, as they have many edges for diffraction.

When the wavelength to be controlled is less than 20% of the top width (*λ* < *e*/5), the barrier will be considered thick. If the top width exceeds 3 m, the barrier will behave as thick for all frequencies. Otherwise, the barrier will behave as it is thin and it should be designed in such a way. In the case of thick screens, the thickness *e* may be added to the smallest distance *a* or *b* (**Figure 5**) to get the new values *a*<sup>0</sup> or *b*<sup>0</sup> . All calculations may be computed using the new values *a*<sup>0</sup> or *b*<sup>0</sup> .

A thin barrier can turn into a thick one simply by adding a proper header for having multi-edge diffraction and enhancing its performance and effectiveness. Using absorbing materials on the surface exposed to the noise source is not mandatory; nevertheless, it can be useful for reducing noise reflections, that is, when there are screens on both sides of a highway or railway.

The best geometry for using absorbing materials is when the height *H* of the screen is greater than 20% of its length *L* (*H* > *L*/5). In addition, the best solution for avoiding reflections when *L*/10 > *H* > *L*/20, is by positioning one of the two screens at a minimum angle of 15° vertical. When *L*/5 > *H* > *L*/10, cases should be studied individually. If *H* < *L*/20, no simple action will significantly improve its performance [18].

*Overview of Noise Control Techniques and Methods DOI: http://dx.doi.org/10.5772/intechopen.104608*

#### **Figure 5.**

*Cross section of a thick screen (adapted from Ref. [17]).*

Many natural fibers can contribute to acoustic absorption; the best performance was achieved by fibers, such as kapok, pineapple-leaf, and hemp [19].

Substituting a reflective barrier placed 1 m far from a railway with an absorptive one could improve IL up to 6–10 dB [20]. Placing a Schroeder diffuser on the surface of an extruded-PVC acoustic barrier could improve the IL in almost all frequencies between 315 Hz and 8000 Hz [21].

When a transparent or a reflective material is selected for a noise barrier, the birds will not be able to visually recognize it. Thus, to minimize bird collisions onto barriers, the use of opaque stripes or dots should be taken into account in the terms of reference [22]. The old usage of painting raptors onto transparent barriers [20] can also be substituted with an ultraviolet "A" reflecting coating (UV-A), that is visible only to birds.

#### *3.2.2 Types of acoustic barriers*

When selecting the type of acoustic barrier to construct, not only the acoustic performance must be taken into account, but also the available space, characteristics of topsoil and subsoil conditions, the possibilities of landscaping integration, and the cost per m or per km, both for construction/mounting and maintenance.

The most common types of barriers are thin screens, soil embankments, partitions or enclosures, and green curtains:


• Green barriers: They produce the best esthetic result. On the other hand, their acoustic performance is poor and they need constant maintenance.

Both thin and green barriers can be designed as modular infrastructure, to build different configurations [23]. The "A-frames" built out of Corten steel facilitate plants growth, protection, irrigation, and maintenance [20].

The costs of different materials of noise barriers by life-cycle cost analysis are compared in Ref. [24]. The present net worth is considered by adding the initial construction cost and the annual cost of maintenance and replacement during a life of *n* years, with an interest rate *i* of 5.5%. The service life of embankments and concrete is considered of *n* = 50 years, and of *n* = 25 years for steel-, wood- and aluminumbased materials. In this framework, the most cost-efficient option was earth embankments; the least cost-efficient was the aluminum-based materials (four times more expensive than earth embankments).

#### *3.2.3 Basic acoustic design of an infinite acoustic barrier*

Although there are several explicit calculation methods, they are only approximations. If an accurate value is needed, applying numerical modeling techniques is mandatory. Possibly, one of the earliest methods to calculate the IL of an acoustic barrier is Maekawa's [20]. It considers frequencies between 100 Hz and 5000 Hz. The depletion of SPL due to diffraction on the top edge can be calculated as IL = 10 log(20 *N*).

*N* is the Fresnel number and it can be obtained with basis on the wavelength λ and the difference between the direct path *d* (the geometric distance between source and receiver) and the path over the barrier, or diffracted path (*a* + *b*), as indicated in **Figure 4**:

$$N = \frac{2}{\lambda} \left( a + b - d \right) = \frac{2\delta}{\lambda} \tag{3}$$

Best results of Maekawa's expression are obtained when the height of the barrier is significantly less than its length, *a* is less than 5 m, the height of the source is greater than *a* and the height of the receiver is greater than *b*.

Other expressions for estimating IL are in use, aiming to improve the accuracy of the results [20, 25]:


Kurze-Anderson [20]:

$$
\Delta L = \text{IL} = 20 \text{ log } \frac{\sqrt{2\pi N}}{\tanh\sqrt{2\pi N}} + 5 \text{ for } -0.2 < N < 12.5 \tag{4}
$$

or Δ*L* = IL = 24 dB for *N* > 12.5 (only for point source).

Taking into account that *tanh X* <sup>¼</sup> *senh X* cosh *<sup>X</sup>* <sup>¼</sup> *eX*�*e*�*<sup>X</sup> eX*þ*e*�*<sup>X</sup>*.

When ground absorption is to be considered, please use the next expression, either for thin or thick barriers:

$$\text{IL} = \text{10 } \log \left( \text{3} + \text{10 } N \, K \right) \text{-} A\_{\text{ground}} \tag{5}$$

The ground absorption *Aground* can be computed by any proper method. The value of *K* is related to meteorological conditions, according to: *abd* p

$$\text{When } 100 \text{ } m < a + b < 300 \text{ } m: K = e^{-0.0005 \sqrt{\frac{ab}{M}}} \text{; otherwise, } K = 1.$$

Pita Olalla [26] proposes the following expression for a linear source:

$$\text{IL} = \Delta L = 15 \log \frac{\sqrt{2\pi N}}{\tanh \sqrt{2\pi N}} + 5 - 10 \log \left( 2e^{-h/2\lambda} + 1 \right) \tag{6}$$

where *h* = height of the acoustic barrier.
