**1. Introduction**

When faced with a situation in which the sound pressure levels (SPL) are too high according to the intended use of a certain place, there are several options for enhancing its acoustic quality. Noise control solutions can be passive or active. Passive control is the most used solution; it aims to dissipate the excess acoustic energy through absorption, transmission, or diffusion. On the other hand, active noise control (ANC) solutions imply introducing a specific acoustic signal to counteract the sound waves to be controlled. As in most environmental issues, the possible measures must be analyzed seeking first to improve the performance of the source, that is, to reduce its emissions; then the possibilities of acting on the propagation path are to be studied; taking protection measures at the potentially affected receivers will be considered only as a last option for noise control.

Even with the latest technology to meet a requirement or to satisfy an objective, noise emissions can occur due to friction, impact, turbulence, imbalance of moving parts, and cavitation in fluids, among many other causes. In addition, the lack of maintenance or operating in an incorrect manner can increase noise emissions. Measures in the propagation pathway become necessary in many cases, for example, when working with high-power or high-speed machinery. The most common options are silencers, either reactive or dissipative; plenum chambers; acoustic coatings; enclosures; noise barriers; vibration dumping or insulation. When reducing the sound

pressure levels at the receiver is still needed, the use of personal hearing protection is considered, although sometimes control booths or cabins can be very good options.

First, having an accurate diagnosis of the problem is essential. Some issues that should be considered before making a decision are ventilation requirements of the equipment; characteristics of location; availability of space; energy requirements; operation and maintenance; and costs.

This chapter presents the main features of passive control systems, which are usually the most common; and then, ANC solutions, which have various applications, especially for low-frequency noise.
