**1.1 Noise barriers: the ubiquitous solution to the road noise problem**

Chronic exposure to environmental noise is a widespread problem around the world, causing significant impacts on human health and well-being. Road traffic is the predominant source of noise in urban areas and represents the second most important health risk factor after air pollution [1]. In Europe, it is estimated that about 20% of the total population is exposed to road traffic noise levels considered harmful to health [2]. Moreover, the problem is expected to become more severe in the next decades. In the European Union, the population exposed to high road noise levels is projected to rise both inside and outside urban areas over the next years due to urban growth and increased demand for mobility [3].

In the last decades, the introduction of more stringent environmental noise legislation has resulted in a series of noise abatement measures of varied nature. These included urban planning measures (such as the designation of

noise-sensitive areas, or regulations on vehicle speed limits or traffic restrictions), measures to improve the acoustic performance of vehicles, pavements and buildings, and the construction of noise barriers. Currently, noise barriers have become frequent features along many roads and railways.

The history of noise barriers precedes the appearance of the first generation of environmental regulations in the World. The first documented noise barrier installed on a road was built in 1963 [4]. In the following years, several new design criteria and new materials for barriers were rapidly introduced. And so, the first lightweight barriers with an absorptive treatment on the panel surface date back to the early 1970s [5]. By 1975, Japan had already built noise barriers along 79 km of new highways [6], and the USA had installed about 57 km of barriers at certain types of highway projects [4].

In the 1960s and 1970s, the first research studies were initiated to analyze the acoustic properties of barriers and calculate noise attenuation levels. Probably the most famous of these studies was the Maekawa empirical chart of 1968 [7], as well as the formulations developed by other authors based on Maekawa's original proposal [8–12]. It was also during this period that the first regulations for noise management and abatement were adopted in countries such as the USA (1972), Canada (1973), Germany (1974) and Japan (1974).

Since the 1980s, many countries have adopted Environmental Impact Assessment (EIA) legislation that requires the evaluation of, among others, proposed road projects that are likely to have significant environmental impacts. As part of the EIA process, the project developer is required to evaluate road traffic noise and must determine appropriate mitigation measures to minimize its effects. Constructing a noise barrier is probably the most mentioned mitigation measure in EIAs conducted around the world [13]. As an example of the extensive use of these devices, it was estimated that the global production of noise barriers reached approximately 370 million m2 in 2014 [14]. In the European Union, these devices have become the most prominent noise mitigation measure applied to major roads located outside residential areas [15]; in the USA, about 5700 km of barriers have been built to date [4].

Noise barriers have been made of many different materials and have taken many different forms over time. In the past, simple reflecting barriers made of concrete, masonry blocks, or earth berms were often used, but modern barriers tend to have absorptive treatments which minimize the level of reflected noise. In recent years, a number of innovative barriers are being developed, such as combined noise and safety barriers, lowheight barriers, photovoltaic barriers, noise walls with titanium dioxide (TiO2) coating, inox/corten steel barriers, or acoustic devices based on sonic crystals [16].

The growth in the use of noise barriers has also been coupled with a growing interest in their effectiveness as a tool to reduce noise pollution. The evaluation of this effectiveness is, however, a difficult task, given that these devices are placed outdoors under very varied conditions, with diverse barrier designs and locations, fluctuating noise sources, and changing environmental conditions. This chapter outlines the fundamentals of the acoustic performance of barriers, describes the main approaches for the evaluation of their effectiveness, as well as the main findings obtained in the studies conducted on the attenuation levels measured.

#### **1.2 Acoustic performance of noise barriers**

A noise barrier is a structure that obstructs the direct transmission of airborne noise produced by a source, such as road traffic, and redistributes the sound energy into several paths (**Figure 1**):

*Approaches for Noise Barrier Effectiveness Evaluation Based on* In Situ *"Insertion Loss"… DOI: http://dx.doi.org/10.5772/intechopen.104397*

#### **Figure 1.**

*The acoustic performance of a noise barrier (based on [17]).*


Noise barriers cause an area of decreased sound energy behind the barrier (also called shadow zone) which is a combination of reflection, diffraction, and transmission losses. Due to the nature of sound, diffraction does not bend all frequencies uniformly: higher frequencies are diffracted to a lesser degree; lower frequencies are, by contrast, diffracted deeper into the shadow zone behind the barrier. As a result, noise barriers are generally more effective in attenuating the higher frequencies.

The acoustic performance of a noise barrier depends on a set of intrinsic and extrinsic characteristics [18]. Intrinsic characteristics refer to the properties of individual components of the barrier, such as the type, thickness, and design of the materials used. Extrinsic characteristics consider the attenuation of the barrier once it has been installed. These characteristics are mainly determined by a set of contextspecific conditions, such as:

• The position of the barrier relative to the source and the receiver, and its effective height and length to block propagation paths.


These contextual properties largely determine the diffraction characteristics of the barrier and the global noise attenuation that can be achieved. The noise diffracted on the top and around the ends of the barrier is the most important factor limiting its acoustic performance [18].
