**2. Passive noise control**

Passive noise control includes a wide set of techniques aimed to make the immission sound pressure level admissible to the receivers.

There are some general measures that can make a difference. They are low cost and based on common sense. It is rather common to find these improvement opportunities, for example, rearranging the work process to avoid unnecessary exposure to high noise levels to the public, changing the direction of loudspeakers at music venues, or encouraging the public to disperse at the end of recreation activities.

There is a wide spectrum of control measures to be considered, depending on the type of source. The sooner control measures are taken, the faster the results.

#### **2.1 Engines noise control**

Periodic preventive maintenance of devices, engine mobile parts, and machinery is essential—checking the condition of gears, bearings, and proper lubrication.

Noise emitted from internal combustion engines is the result of airflow processes into the engine (aerodynamic noise), from the mechanical movement of the engine (mechanical noise), and from pressure increases associated with the combustion process (combustion noise). Combustion noise is normally the predominant source in diesel engines, due to the rapid rise of pressure inside the cylinders. The noise emitted by a combustion engine is related to the efficiency of conversion of chemical energy into mechanical energy; the acoustic energy related to the roar of the engine is usually between 10<sup>8</sup> and 10<sup>5</sup> of its power. For two devices of the same power, the noise emitted by a diesel generator is greater than the one emitted by a natural gas one [1].

The acoustic power of engines is related to their maintenance—acoustic emissions are an indicator of waste of energy. For continuous combustion systems, the combustion noise can be expressed in terms of the thermo-acoustic efficiency, which is the ratio between the total energy of a sound pulse and the heat release rate. An engine in good operating conditions should normally emit between 10<sup>6</sup> and 10<sup>5</sup> of its power through acoustic energy; if the system is in improper condition, the acoustic emissions could rise to 10<sup>4</sup> of its thermal power [2]. The maximum thermo-acoustic efficiency expected for unconfined hydrocarbon flames is 10<sup>6</sup> . When a combustion engine is not in proper operating conditions, the frequencies that denote higher sound pressure levels are the harmonics of the rotational frequency. This also happens in other rotating or reciprocating machines. Malfunctions in electric motors are usually related to excessive noise in harmonics of the synchronous frequency. In other electrical devices, the noise appears in harmonics of the line frequency. A catalog of engine problems and the frequencies where they appear is presented in Ref. [3].

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

Heat recovery boilers are recommended as noise control devices for large engines. They act as passive silencers, but when installed, of course, they can provide other services too (e.g., heating for decreasing the fuel viscosity).

A heat recovery steam generator performs "*a secondary function as an in-line silencer for combustion turbine noise emissions"* [4]*.* The recovery boiler can be considered part of the noise emission control system to comply with the immission level regulations. Hence, it must be specified as such.

Sometimes dedicated passive silencers can be avoided by the installation of recovery boilers with a secondary function as exhaust silencers. **Table 1** presents the reduction in SPL measured between up-flow and down-flow of a recovery boiler. The reduction is from 10 dB (at 125 Hz) to 35 dB (at 8000 Hz). The reduction in A-weighted SPL is also high: 25 dB [5].

#### **2.2 Passive silencers**

Passive systems, whose generic designation is silencers, can act either through their geometric characteristics or through the incorporation of acoustic absorbent materials. According to their principle of action, silencers can be classified into two families: reactive or reflective silencers and dissipative or resistive silencers.

### *2.2.1 Reactive or reflective silencers*

The principle operation of reactive silencers is based on generating sound reflections from geometric properties of the propagation medium, for example, section changes. They are usually solved with coupled tubes. **Figure 1** shows a sketch of a reactive silencer; how to calculate its transmission loss (TL) is also presented.

For a discontinuous area reactive silencer as sketched:

When an acoustic wave of amplitude *Ai* propagates with velocity *v1* in a tube of section *S1* and it changes abruptly to a section *S2*, a reflected wave of amplitude *Ar* goes back to the source and a transmitted wave of amplitude *At* continues propagating with velocity *v2*. According to Snell's Law, the amplitudes of these waves (*Ar* and *At*) can be written as [6]:

$$A\_l = \frac{2\,\upsilon\_2}{\upsilon\_1 + \upsilon\_2} A\_i; A\_r = \frac{\upsilon\_2 - \upsilon\_1}{\upsilon\_1 + \upsilon\_2} A\_i \tag{1}$$


#### **Table 1.**

*SPL reduction achieved by a recovery boiler in a paper mill (from Ref. [5]).*

**Figure 1.**

*Sketch of operation principles of reactive silencers (adapted from Ref. [7]).*

The greater the difference between *S1* and *S2*, the better attenuation is obtained; the shorter the transition between *S1* and *S2*, the better attenuation is also obtained. According to Ref. [6], the SPL reduction in an abrupt expansion as sketched in **Figure 1** could be between 2 dB (for *S2*/*S1* = 4) to more than 8 dB (for *S2*/*S1* = 25).

These silencers do not require absorbent materials. Their best application is to control narrow band noise or tones at discrete frequencies, especially low frequencies. The most common designs are sketched in **Table 2**. The highest frequency of plane (or bidimensional) waves to be controlled by this type of silencer can be estimated as [6]: *f* < *c*/2*a*, where *c* is the velocity of sound and *a* is the diameter of the tube, or the minor dimension of its section if it is rectangular. It is important to consider that the wavelength λ of the waves to be controlled should also satisfy: λ ≪ *a*.

Also, the well-known Helmholtz resonator can be designed to serve as a silencer, for example, mounting a Helmholtz device designed according to the frequencies to control one side of the tube/duct, where the noise control is needed. A reduction up to 8 dB can be obtained [7]. The volume *V* of the cavity and the radio *r* and length *l* of the neck are to be determined according to the frequency *f* to control, using the equation of the resonance frequency of a Helmholtz presented in **Figure 2**.
