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Outdoors Active Noise Control Technologies

Exploitable results

To reduce population exposure to aircraft noise near airports, a new protection technique is proposed. Based on wave structure identification for aircraft noise (so called anticipation technique), this technique is different from standard auto adaptative approaches, generally used in confined spaces. Auto adaptative techniques operate in open free space for moving sources. From an intuitive idea, where a counter source system can reproduce a wave identical to the incident wave in a simple representation (plane), the full theory has been developed, to investigate the possibilities and design the system elements (mechanical, electronics, computing, electro-acoustical). It has been demonstrated that: -Acoustic behaviour was, at the most, identical to a passive screen of equivalent surface below the cut off frequency f0 (function of the distance between loud-speakers). The screening effect is obtained when: -Time-space characteristics of incident and rebuilt waves are identical: the discrepancy limits the proposed performance (with an energy accuracy of 5%, the attenuation is 20dB max). -Loud-speakers driving is done with an acoustical flow rate equal to twice incident acoustical flow rate. -This screen plays a reflecting role similar to a vacuum layer (soft screen). Building two loud-speaker sources, it is possible to get a reflection identical to a passive screen (hard screen) or even to a non reflecting, but dissipative screen (absorbing screen). -The loud-speakers number goes up with f0 (ex : 10 sources/m2 for f0 = 1 000 Hz): The number of counter sources, made of several loud-speakers can be reduced with loud-speakers in clusters, linking loud-speakers individually driven. -A computation program "OMBRE-ECRAN" (shadow screen) has been written to simulate performance of such screens. -Construction of a full scale prototype (a street light type with 15 counter sources at 1,2 m distance) has proven the feasibility of the technologies with the requested accuracy (around 2,5% -13dB). -Despite major problems following the arson and destruction of the on site lab facilities, and a major dysfunction within the electronic units, partial evaluation of the prototype including tests in a Kundt tube has confirmed the global feasibility. -When the electronic difficulties are processed, the final phase of full scale validation will be carried out to confirm the global process feasibility. Economic and societal feasibilities will be assessed through a pilot system in situation. Further in line with the possibilities open by the OMBRE-ECRAN software, a procedure extension will be undertaken towards motorway and railway applications. (These two fields of application deal with more independent waves).