# Recent Developments in Sonic Crystals as Barriers for Road Traffic Noise Mitigation

^{1}

^{2}

^{*}

## Abstract

**:**

## 1. Introduction

_{den}, and 32 million are exposed to a noise level higher than 65 dB(A) of L

_{den}. Even if not considered the most annoying, road traffic is the most diffused noise source, to the point that it is used as a reference for estimating other sources’ limits [3]. The continuous growth of vehicular traffic and the large number of people exposed to it have made sleep disturbance [4,5] and annoyance [6] caused by road traffic noise important issues observed both by citizens and control bodies. Studies have shown that exposure to road traffic noise can induce further adverse health effects, including cardiovascular effects [7,8], learning impairment [9,10], and hypertension ischemic heart disease [11].

## 2. Sonic Crystals as Acoustic Barriers

_{BG}is determined by the lattice constant α, that is, the distance between two lattice scatterers and the speed of sound in the medium c: ${f}_{BG}=\frac{c}{2\alpha \mathrm{sin}\theta}$. The size of the band gap is influenced by the following parameters:

- the density ratio M, that is, the ratio between the densities of the scatterers’ material and that of the medium in which they are immersed;
- the filling factor ff, expressing the ratio between the volume occupied by the scatterers and the total volume of the crystal;
- the lattice designs.

## 3. Parameters Influencing Insertion Loss and Band Gap

#### 3.1. Shape of Scatterers

#### 3.2. Diameter of Scatterers

#### 3.3. Number of Scatterers

#### 3.4. Filling Factor

## 4. Recent Applications

#### 4.1. Hollow Scatterers

#### 4.2. Scatterers Coated with Porous Material

#### 4.3. Coupled Barriers

#### 4.4. Low-Height Barriers

#### 4.5. Green Barriers

## 5. Discussion

## 6. Conclusions

## Author Contributions

## Funding

## Conflicts of Interest

## References

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**Figure 1.**Sound pressure levels at 1.5 kHz for triangular and square lattice, modified from [39].

**Figure 2.**Sound pressure levels (SPL) and insertion loss (IL) for different scatterer shapes, modified from [39].

**Figure 3.**IL for square and triangular lattices, double- or triple-row, varying the scatterer diameter and IL for triangular lattices, double- or triple-row, for diameter variations in some random elements, modified from [35].

**Figure 4.**IL for various sonic crystals design. ✲ D = 0.15 m and ff = 1; ◻ D = 0.30 m and ff = 0.225; O D = 0.15 m and ff = 2; △ D = 0.30 m and ff = 1, modified from [36].

**Figure 6.**Rows of scatterers or random scatterers repositioned from rectangular lattice, modified from [51].

**Figure 7.**Rows of scatterers or random scatterers repositioned from triangular lattice, modified from [51].

**Figure 8.**Insertion loss for a sonic crystal with 54 scatterers in three rows under hollow scatterer or solid scatterer conditions, modified from [39].

**Figure 9.**Insertion loss for a standard barrier (dashed line) and sonic crystals with a porous shell of different thicknesses, modified from [59].

**Figure 10.**Insertion loss relative to a standard barrier for a coupled barrier in three different cylinder settings, modified from [52].

**Figure 11.**Insertion loss of low-height barrier in different cylinder settings, modified from [65].

**Table 1.**Comparative analysis of insertion loss (IL) obtained in different studies. d

_{r}is the distance of IL evaluation from the barrier, D is the scatterer diameter (or side), and α is the lattice constant.

Authors | IL (dB) | d_{r} (m) | Scatterer’s Shape and Material | D (m) | Lattice’s Shape and Depth (m) | α (m) | Hollow | Porous | Note | |||
---|---|---|---|---|---|---|---|---|---|---|---|---|

250 Hz | 500 Hz | 1 kHz | 2 kHz | |||||||||

Morandi et al. [49] | - | 9 | 15 | 0 | 0.4 | Polyvinyl chloride cylinders | 0.2 | Square 0.8 | 0.2 | No | No | Real dimension |

Morandi et al. [80] | - | 9 | 15 | 18 | 0.25 | Polyvinyl chloride cylinders | 0.08 | Square 0.96 | 0.2 | Yes | No | Different configurations |

Martins et al. [35] | - | 5 | 9 | 9 | 1 | Rigid cylinders | 0.2 | Triangular 1.4 | 0.4 | No | Yes outside | Numerical simulations |

Santos et al. [51] | 0 | 12 | 10 | 10 | 0.65 | Polyvinyl chloride cylinders | 0.2 | 1.4 | 0.2 | No | No | In scale; various shapes |

Amado-Mendes et al. [38] | 7 | 5 | 15 | - | 0.5 | Wooden cylinders | 0.1 0.2 | Square 1.0 | 0.1 | No | No | Real dimension |

Jiang et al. [50] | 11 | 2 | 0 | 15 | 0.8 | Steel cylinders | 0.04 | Square 0.75 | 0.08 | No | No | Non-real scale |

Chong [39] | 3 | 0 | 20 | - | 0.05 | Polyvinyl chloride cylinders | 0.11 | Square 0.7 | 0.16 | Yes | Yes | Non-real scale, resonant cavities |

Jean and Defrance [36] | 7 | 10 | 9 | 9 | 10 | Wooden cylinders | 0.3 0.7 | Rectangular 2.1 | 0.40 | No | No | Cylinders of 2 different diameters |

Sánchez-Dehesa et al. [59] | 3 | 5 | 16 | 0 | 1 | Steel cylinders covered by porous material | 0.04 0.08 | Square 0.58 | 0.11 | No | Yes | Porous material outside |

Koussa et al. [52] | 15 | 20 | 25 | 30 | 0.4 | Aluminum cylinders | 0.05 0.13 | Rectangular, 1st section 0.3 and 2nd section 0.5 | 0.08 0.17 | No | No | 2 noise crystals combined to a conventional noise barrier |

Koussa et al. [52] | 15 | 23 | 25 | 33 | 0.4 | Aluminum cylinders | 0.05 0.13 | Rectangular, 1st section 0.3 and 2nd section 0.5 | 0.08 0.17 | Yes | No | 2 noise crystals combined to a conventional noise barrier |

Koussa et al. [52] | 15 | 26 | 30 | 33 | 0.4 | Aluminum cylinders | 0.05 0.13 | Rectangular, 1st section 0.3 and 2nd section 0.5 | 0.08 0.17 | Yes | Rock wool | 2 noise crystals combined to a conventional noise barrier |

Koussa et al. [65] | 14 | 10 | 13 | 14 | 0.4 | Aluminum cylinders | 0.05 0.13 | Rectangular, 1st section 0.3 and 2nd section 0.5 | 0.08 0.17 | Yes | No | Low-height barrier of 3 different sections |

Koussa et al. [65] | 15 | 10 | 13 | 14 | 0.4 | Aluminum cylinders | 0.05 0.13 | Rectangular, 1st section 0.3 and 2nd section 0.5 | 0.08 0.17 | Yes | Rock wool inside | Low-height barrier of 3 different sections |

Lee et al. [57] | 1.5 | 8 | 10 | 3 | 1 | Aluminum parallelepiped | 0.04 | Square 0.37 | 0.1225 | Yes | No | Outdoor measurements, Helmholtz resonator |

Godinho et al. [66] | 4 | 5 | 15 | 18 | 0.5 | Maritime pine timber logs | 0.1 0.2 | Square 1.0 | 0.1 0.2 | No | No | On field measurements |

Cavalieri et al. [58] | 2 | 20 | 12 | 18 | 0.45 | Wooden rods of square cross-section | 0.05 | Square 0.3 | 0.05 | Yes | No | Helmholtz resonator |

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**MDPI and ACS Style**

Fredianelli, L.; Del Pizzo, L.G.; Licitra, G.
Recent Developments in Sonic Crystals as Barriers for Road Traffic Noise Mitigation. *Environments* **2019**, *6*, 14.
https://doi.org/10.3390/environments6020014

**AMA Style**

Fredianelli L, Del Pizzo LG, Licitra G.
Recent Developments in Sonic Crystals as Barriers for Road Traffic Noise Mitigation. *Environments*. 2019; 6(2):14.
https://doi.org/10.3390/environments6020014

**Chicago/Turabian Style**

Fredianelli, Luca, Lara Ginevra Del Pizzo, and Gaetano Licitra.
2019. "Recent Developments in Sonic Crystals as Barriers for Road Traffic Noise Mitigation" *Environments* 6, no. 2: 14.
https://doi.org/10.3390/environments6020014