**5.3 Control strategies in active noise control systems**

There are two basic control strategies in ANC systems: anticipatory or feedforward control and feedback control.

The applicability of *feedforward control* relates to cases whether the propagation time between the reference microphone (the primary sensor) and the cancelation loudspeaker (the secondary source) is sufficient—in electrical terms—so that the canceling signal could be reintroduced downstream when the primary noise signal "passes" the loudspeaker. This is why active control was originally reserved for low*Overview of Noise Control Techniques and Methods DOI: http://dx.doi.org/10.5772/intechopen.104608*

frequency sounds since at lower frequencies there was more time (or distance traveled) to insert the secondary signal. The objective of the system is to minimize the error signal so that the residual acoustic noise error (i.e., the sum of the original and input signals) results as low as possible.

The *feedback system* is simpler than the feedforward one, but it can only be applied if the signal to be controlled is well known and very stable, that is, with very few variations. Thus, the need to know the signal to be controlled at all times is avoided since the system assumes it is a fixed and known input.

A single microphone is used as an error microphone, which registers the sum of the signal to be canceled plus the cancelation signal. The information is sent to a simpler control system, which sends the signal to be emitted to the actuator aiming to minimize the error between the current signal and the predefined one.

A combination of both systems is also possible to be used ("hybrid control"). *Hybrid control* (FF/FB) refers to a two-degree-of-freedom controller, which enables robust performance over a larger range of frequencies. The design of each controller depends on the design of the other, for example, DEUS control structure output (two inputs, one output). In 2020, another structure for ANC systems was proposed [42]; it modifies the path for measuring the error.

Adaptive controllers may be mentioned as well. They allow good performance to be achieved even when the information of the signal to be processed is incomplete. When working under stationary conditions, an adaptive controller quickly converges to Wiener's optimal solution; when working in nonstationary conditions, the algorithm allows tracking time variations in the input data, as long as these variations are slow enough to track. Controllers can apply many possible algorithms, including the classic "least mean squares."

#### **5.4 Some applications of active control**

Possibly the most successful case in using ANC is that of active hearing protectors, which generate a counterwave to protect workers exposed to high SPL. ANC-ear protectors were one of the first applications of massive use of ANC; they met great commercial interest. Its use for the protection of workers is not the only application: not considering its dangers, it may be said that there are also motorcycle helmets with ANC, due to the occupational exposure of those who work on motorcycles (deliverymen, police officers [43]). Other nonpassive protectors are those that allow selective filtering of some frequency bands, such as those which are especially of interest to musicians; and those that allow masking the source by emitting a different signal (e.g., white noise, or even turning a radio on).

The applications of active control in the propagation medium are diverse and some of them are novel. A modern system for indoor urban noise control is through ANC. A 10 dB reduction in traffic and railway noise was achieved on frequencies from 100 Hz to 1000 Hz for canceling the incoming signals in a real window of 1 m 1 m [44]. There are ANC systems for controlling SPL in the interior of a vehicle [45], a "noisecanceling office chair" [46] and a "capsule" for home devices, to control both noise and vibrations [47].

An interesting application of ANC is that proposed for a real case of thermoacoustic instabilities in large combustion engines. Shortly explained, thermoacoustic instabilities appear when working with machinery and equipment on such a scale that the main phenomena, such as mixing or ignition, cannot be considered punctual neither instantaneous nor homogeneous throughout the combustion

#### **Figure 7.**

*Installation of thermoacoustic instability in the 25 Hz third-band octave from a large engine (from Ref. [49]).*

chamber. Thus, the system can frequently enter into resonance if there is coupling with some geometric dimension.

When preparing this entry into resonance, a process of "loss of chaos" is observed [48], which reaches the condition of thermoacoustic instability from which, after some time that is not fixed or preestablished, the system recovers and returns to normal.

In this case study, a detailed analysis of the problem showed that fluctuations in the pressure of the fuel inlet line were occurring, leading to the occurrence of the abovementioned instabilities. One of the three ways in which these were manifested was by the loss of chaos in sound emissions, as described in Ref. [48]. The phenomenon appears especially in the 25 Hz third-octave band (blue line in **Figure 7**). The transition lasts several minutes, and it can be seen that the loss of chaos is the preamble for the emission in that band to present an abrupt increase of SPL about 5 min later. The suggested solution was a typical feedforward ANC system since there was a reaction gap of several minutes.
