Active noise control

What is it?

Active noise control is a technique that seeks to cancel unwanted noise through destructive interference by using an additional acoustic field (secondary) generated by an electronic control system.

 

How does it work?

Sound propagates through the air in the form of waves, usually caused by the vibration of a body (sound source).  In the case of two sound sources that emit the same frequency, the resulting sound field is the combination of the two wave motions generated by each of the sources.  If at a specific point the kinematics of the two movements is opposite (one wave is 180° out of phase with the other), a destructive interference occurs which cancels the total movement and, consequently, the sound field.

Main components of an active noise control system

 

The complexity of acoustic fields and emission phenomena have up until now limited the commercial application of this procedure.

 

Applications

 

There are basically two fields of research and development for active noise control: the optimization of control systems and the understanding and application of the physical cancellation mechanisms. At the LEAM, we work especially in the latter field, while we collaborate on the former with the ESAII Department of the Terrassa Campus.

In general, the application of active control can be grouped into two broad categories: active noise control (ANC) and active structural acoustic control (ASAC).  In ANC, the actuators are speakers which produce an inverted phase signal to cancel the unwanted noise.  However, if the noise is caused by the vibration of a flexible structure, then ASAC is more appropriate.  In ASAC, actuators are vibrating sources (shakers, etc...) that can change how a structure vibrates, altering the way in which it radiates sound.

 

 

Simulation of flight conditions and the application of ANC

 

The LEAM is working on both areas. For active noise control we have worked on optimizing the application of this technique in duct systems [1], even though this is a potential commercial applications (for installation of air conditioning), the reality is that the proper placement of the different transducers on the acoustic field inside the duct is very important to achieve a stable and operative system.  We are also working on the active window concept, which aims to maintain the sound insulation of walls despite the presence of openings[2,3].  We began studying this concept in the facades of homes subjected to noise from aviation through our participation in a European project [4] entitled "L’intégration environnementale des aéroports sur le territoire" EU INTERREG IIIB-MEDOCC (2003-06) .Currently, the study is active through a project funded by the Ministry of Science and Innovation entitled "Ventana activa: control activo de transmission de ruido a través de aperturas".

 

Achieving attenuation in an entire area (global control) is complicated due to the complex acoustic field that is formed. For this reason, at the LEAM we are working on the optimization and implementation of local noise control techniques [5,6,7].  This technique allows for reductions in specific areas, without affecting the rest of the acoustic field. For example, this technique is being developed for application in modes of transport.

 

[1]    Romeu J,  Jiménez S, Salueña X, Capdevila R, Coll Ll., Active noise control in ducts in presence of standing waves. Its influence on feedback effect, Applied Acoustics (2001).

[2]    Romeu J,  Jiménez S, Salueña X, Capdevila R, Díaz N., Active noise transmission through an aperture using active feedforward control, J. Acoust. Soc. Am. (2002).

[3]    T. Pàmies, J. Romeu, M. Genescà, A. Balastegui , Sound radiation from an aperture in a rectangular enclosure under low modal conditions, J. Acoust. Soc. Am. (2011).

[4]    Evaluation and monitoring of the acoustic impact of airports on the management of the territory. F. Bianchi, G.A. Bravo, C. Bruguera, S. Brustia, D. Danielli, J. Romeu, M. Genescà, F. Guyon, N. Lanese, R. Ledys, J. Maillard, M. R. Martinez, G. Moschioni, A. Norcini, T. Pàmies, P.J. Reneé, J. Roland, M. Rossi, A. Roure, M. Ruiz, M. Sassi, F. Scamoni, J.M. Seguí, M. Tarabini, F. Valentini, M. Villot. Ed. RegioneLombardia (2006).

[5]    J. Romeu, T. Pamies, S. Jimenez, P. Llibre, In flight experiments of local active noise attenuation for passengers in small aircraft. Internoise 2004, Prague, the Czech Republic.

[6]    J. Romeu, T. Pàmies, M. Genescà, J. Cardona,  Local active noise attenuation in a small piston engine aircraft cabin. Noise Control Engineering Journal (2007).

[7]    J.Ignacio Palacios, J. Romeu, A. Balastegui, Two step optimization of transducer locations in single input single output tonal global active noise control in enclosures. Journal of Vibration and Acoustics (2010).