**3.3 Green barriers**

A green barrier designed to have an acoustic function should not be confused with an esthetic one. In fact, guidelines for greening acoustic barriers can be found, for example [29].

A study carried out in Virginia (US) concluded that "*there was minimal noise attenuation that could be attributed to the coniferous trees at the 15 study sites examined. Attenuation was not correlated with tree stand age, height, species, or density for these sites*." [30]

Shrubs and trees should meet certain characteristics to take part in a noise barrier. The depth of the green curtain may be at least 15–40 m. Even if its best performance is between 250 and 1000 Hz, an attenuation greater than 3 dB in *LAeq* should not be expected. The best attenuation performance of a green barrier occurs at a wavelength twice the size of the leaves of the tree species [20].

The expression for estimating the attenuation of a dense green barrier, proposed by Kurze, Beranek and Hoover, as presented in Ref. [31]:

$$
\Delta L \,\left[dB\right] = \frac{d \,\left[m\right]}{100} f^\ddagger \tag{8}
$$

Even if direct relations between the IL and the length, depth and diameter of the branches of urban green curtains have been reported and drawn [32], analytic expressions for predictive purposes are not easy to develop.

It is usually assumed that the attenuation of green barriers is almost negligible for frequencies below 400 Hz and that they may be useful only for frequencies above 1000 Hz, provided that the curtain is dense and its thickness is several tens of meters in depth. The improvement in the attenuation in low-frequencies is mostly related to fallen leaves and branches, which increases the surface porosity and thus, the ground absorption. Martens' work shows the best performance is reached when the leaf dimension is similar to half of the wavelength to absorb [20].

In a study analyzed in Ref. [31], the acoustic attenuation of two green barriers of cedar trees 4–4.5 m in height is compared. In the first case, the trees were aligned by rows and columns, in an area of 30 m � 5 m, very close to one another, with branches overlapping; in the other case, The trees were planted in a diagonal pattern, on the vertices of equilater triangles of 1.20 m side (quincunx pattern)., occupying an area of 25 � 9.20 m. The results are shown in **Table 5**.

The absorption coefficient of leaves and plants seems to be low. Low absorption coefficients were found up to 1600 Hz, most of them no greater than 0.30. The best performance was that of Winter *Primula vulgaris*, with values between 0.6 and 0.7 for frequencies of 500–1600 Hz; and the worst, *Hedera Helix's*, with all values below 0.20. *Overview of Noise Control Techniques and Methods DOI: http://dx.doi.org/10.5772/intechopen.104608*


#### **Table 5.**

*Attenuation of a green barrier of cedar trees in dB/m of curtain depthness in two different arrangements (from Ref. [31]).*

The study states that "*the leaf area density and dominant angle of leaf orientation are two key morphological characteristics that can be used to predict accurately the effective flow resistivity and tortuosity of plants*" [33]. Two other studies obtained similar results [34, 35]. The main conclusion of Asdrubali et al. [34] was that "*the main absorber is the substrate soil ( … ). The presence of the plants becomes useful only when a large number of them is installed on the sample, otherwise is even pejorative within some frequency ranges."* Just the opposite, Azkorra et al. [36] obtained a weighted sound absorption coefficient of 0.40, but the best absorption coefficients were at frequencies of 125 and 4000 Hz, and the worst ones, were at 500 and 1000 Hz.

#### **3.4 Sonic crystal (acoustic open barriers)**

Sonic crystals (SC) are periodic structures that have been studied in recent decades to learn about their possibilities of behaving as "acoustic open barriers". They have been also been studied for using them for sound diffusion [37].

The idea of an open barrier seems incompatible with the tightness and continuity of the surface of a "conventional" barrier. A SC is a periodic structure that can be triangular, squared, rectangular, or hexagonal; the best geometry is triangular [16]. The essential point is that the structure is the same regardless of the orientation of the material. Depending on the number of dimensions in which the pattern is repeated, the SC will be one-dimensional, two-dimensional, or three-dimensional. The most useful structure for acoustic barriers is two-dimensional with Cermet's topology (i.e., there is no contact between one scatterer to the other, each scatterer is totally surrounded by the external fluid—the air in this case) [38]. Thus, the key is that the operation principle is different in nature—the SC does not attenuate noise by diffraction, but by Bragg interference, widely used in optics—when a wave reaches a crystal structure surrounded by a fluid, the energy of some frequencies cannot be transmitted. These frequencies are the so-called "*Bragg frequencies*." This occurs due to a destructive interference caused by a multiple scattering process [16, 37–39]. If *a* is the distance between two scatterers, the main controlled frequency will be *f* = *c*/2*a* (*c* is the sound velocity and *a* is the so-called *constant of the lattice*). Due to the destructive interference caused by the SC structure, all the acoustic energy at that frequency is reflected. This frequency itself is a *bandgap*; the width of the bandgaps is linked to the *filling factor*, which represents the fraction of the lattice area that is occupied by scatterers. The greater the filling factor—without losing the crystal structure—the best IL is achieved; but if the filling factor is close to 1, the SC will become a continuous barrier [38]. It is worth mentioning that the height of the barrier is not a critical factor in the design [16].

The main advantages of acoustic open barriers are that they are lighter, easier to install and they do not need to have a significant additional height above the source. Research on this technology in the last years has allowed the development of new metamaterials, for example, a scatterer consisting of an absorbent split ring resonator, which has the structure of a SC with a cavity to act as a Helmholtz resonator and an absorbing material to reduce the acoustic energy at high frequencies [38].
