**5. Active noise control (ANC)**

ANC refers to a technique that aims to reduce the SPL through the emission of sound signals in the counter phase with those that occur in the system to control. Then, destructive interference is generated between both signals. This method involves adding acoustic energy to a system that already has an excess of acoustic energy; thus, it should be applied very carefully. On the other hand, active silencers are more accurate and versatile than passive ones. ANC can be used in the source, during sound propagation or at the receiver. Best results should be expected when the signal is periodic, low frequency, and narrow band.

In a similar way to ANC techniques, there are active vibration control techniques. They act with analogous principles, but in this case, they introduce oscillations to counteract the original vibrations.

#### **5.1 Main components of an active noise control system**

An ANC system consists of the following components:

• A primary acoustic field, which is the acoustic field to be controlled.


The system works as follows:

