*2.2.2 Resistive or dissipative silencers*

These are the most suitable silencers to control medium and high frequencies sectors of the broadband noise spectrum. These consist of devices coated with a porous acoustic absorbing material, in contact with the airflow where noise control is needed. The most usual acoustic absorbing materials for the coats are mineral wool or fiberglass. Their effectiveness is based on dissipating the acoustic energy of sound waves as heat, by forcing the flow to lose its energy through friction.

The performance of acoustic dissipative silencers depends on the features of the coating, the thickness and length of the acoustic absorbing materials, the features of

#### **Table 2.**

*Sketch and TL of two designs of reactive silencers (adapted from Ref. [7]).*

**Figure 2.** *Sketch of a side Helmholtz silencer (adapted from Ref. [6]).*

the flow passage section, and the pressure drop in the system (remember that the pressure drop is proportional to the squared velocity of the flow *u*<sup>2</sup> ).

### **2.3 Encapsulations**

Encapsulations can be suitable options for noise control for many types of equipment. The most used enclosures are small buildings where the noisy machines are enclosed. They must be designed by taking into account not only the acoustic characteristics but the ventilation needed for proper operation. Even when each machine complies with noise emission standards, the need of reducing SPL in areas close to public or private works is a major concern. Giraldo Arango states that "Empresas Públicas de Medellín E.S.P." (Public Companies of Medellín, Colombia) successfully incorporated some environmental management requirements in its work specification, especially for Public Works [8]:

*… the use of modern machinery and equipment with soundproofed engines; preventive and corrective maintenance measures must be taken to keep equipment in proper working conditions; if not possible to insulate the sound emission of, that is, jackhammers or disc cutters, then a maximum of two continued hours of noisy equipment operation should be taken, with breaks of the same duration; coordinating and scheduling with authorities of schools and health institutions, in order to conduct noisy operations during class breaks or shift changes …*

He also states: " *… people from public and private sectors mention it is not possible to control noise. … it has been proven that it is possible in practice by using other methods as those normally used, and which are environmentally and economically more attractive.*" In his opinion, stricter control of environmental management is needed; control activities should be handled by the contractor [8].

Some machines have the possibility of building a customized enclosure, either removable or fixed. They must have both good acoustic insulation and absorption properties. **Table 3** presents the comparison of the SPL measured at 1 m from two twin well-point pumps [9]. One of them was enclosed in a customized acoustic removable capsule. Both measurements were done with a class 1 sound level meter during the same morning when the pumps were working in similar conditions in the same area of the city. Drastic reduction in the values of some parameters was experienced for A-weighted sound pressure levels.

#### **2.4 Traffic noise control: special pavements**

The noise of road vehicles is generated from three main sources—engine acoustic emissions, which represent the main source at low velocity; tire-pavement noise, which is dominant from 60 to 100 km/h (approximately); and aerodynamic noise, which is more significant at high speeds (greater than 100 km/h).

Acoustic pavements are designed for reducing noise due to contact with the tires. They have a high sound absorption coefficient in the frequency range where rolling emissions are greater. Thus, the acoustic energy is dissipated as heat, rather than reflected on the ground. Smooth pavements will be less noisy at high speeds if they are dry; in wet conditions, an increase of about 4 dB can occur.

There are different responses of pavements according to the frequency. The results depend on the mass percentage of the asphalt, the air pockets in mineral aggregates, and


LAFmax *= A‐weighted maximum sound pressure level, measured in fast time weighting;* LAFmin *= A‐weighted minimum sound pressure level, measured in fast time weighting;* LAIeq *= A‐weighted equivalent sound pressure level, measured in impulse time weighting;* LAFeq *= A‐weighted equivalent sound pressure level, measured in fast time weighting;* LCFeq *= C‐ weighted equivalent sound pressure level, measured in fast time weighting;* LAF10 *= A‐weighted 10% exceedance sound pressure level, measured in fast time weighting;* LAF50 *= A‐weighted 50% exceedance sound pressure level, measured in fast time weighting;* LAF90 *= A‐weighted 90% exceedance sound pressure level, measured in fast time weighting.*

#### **Table 3.**

*Comparison of the acoustic performances of two well-point pumps, one of them into a customized acoustic enclosure (from Ref. [9]).*

the aggregates grading curve [10]. The most silent pavements are the so-called draining or porous pavements. They can reduce up to 5 dB, especially in high frequencies. They have a very high percentage of holes in their structure, which absorb part of the sound energy emitted by vehicles (both by the tires and by the engine) and drain rainwater as well. The high porosity is obtained by using uniform-sized aggregates.

Acoustic pavements are not only expensive but also they age as their pores clog; then, they lose their acoustic properties. Highways and high-speed roads are less vulnerable to aging, due to a self-cleaning effect caused by the movement of vehicles at high speeds. In urban areas where circulation speeds are slower, permeability is quickly lost. The cleaning procedures are expensive, difficult and of limited efficiency, so the construction of these special pavements is restricted to high-speed traffic lanes.

There are currently promising developments in acoustic pavements that use reclaimed materials. The aging phenomenon was studied at three pavement sections of rubberized asphalt (i.e., asphalt containing crumb rubber from tires) [11]. The best analytic relation to link acoustic properties and aging is not linear but logarithmic. The main variables that correlate in a direct sense are the temperature of air and pavement, and the hardness of the rubber of the tires; an inverse relation was found with other variables, such as heavy traffic flow, age of the pavement, and climate variables [11].

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

#### **Figure 3.**

*Measured sound pressure level spectrum of a wind-induced annoying noise caused by airflow through small holes (from Ref. [12]).*

#### **2.5 Aerodynamic noise**

The interaction between wind and constructions can produce acoustic emissions. Sometimes they are caused by the detachment of the boundary layer developed on a surface, for example, the blades of a wind turbine, the hood of a vehicle, or the wings of an aircraft. The use of a slitted-sawtooth serrated trailing edge to control the trailing edge noise in large wind turbines reduces SPL by about 5 dB [13]. When the noise is related to the airflow through holes or slots, the noise spectrum usually presents strong pure tones, as shown in **Figure 3**.

The emission of noise can be related to a constriction in the flow that causes an increase in the velocity of the airflow; it is the case of the passage of air through holes, slots, or openings that are part of the design of a building. Analyzing the geometry of the problem and the statistics of wind velocity and direction, the most suitable solutions for preventing the phenomenon are—modifying the dimensions of holes and slots; selectively blocking or covering the openings [14]; or modifying the geometry of the wind passage to act on the degree of turbulence of the incoming flow [12].
