The paper presents recent progress in the control of active wave absorbers. A simple theoretical model of an ideal piston wave absorber is derived in the framework of 2D linearized potential theory. Due to a lack of causality, this optimal device cannot be physically realized in the time-domain, but sub-optimal realizable approximations were derived. Because they work better when the incident wave frequency is known, a self-adaptive tuner is developed. It is based on a frequency tracking extended Kalman filter algorithm. The absorption properties of such a self-tuning system are shown to equal those of sub-optimal frequency adapted systems. INTRODUCTION The principle of active absorption of free-surface waves lies in the generation of waves in counterphase with regard to the waves they are intended to cancel (Milgram 1970). Thus, an active wave absorber, also referred to as : dynamic wave absorber, is nothing but a wave generator driven in close reaction to the incident wave. The problem to be solved is to determine the reaction law giving the best absorption coefficient whatever the incident waves may be. In the present study, the wave absorber consists in a simple piston at the end of a two-dimensional wave tank. The single degree of freedom of the device is the horizontal translation, but the method could be extended straightforwardly to hinged paddle wave absorbers. Systems with more than one degree of freedom were investigated in our first studies (Clement and Chapuis 1985), but we concluded at that time that the associated mechanical constraints were not sufficiently balanced by the gain in hydrodynamic (absorption) efficiency. The theoretical modelling of the piston wave-absorber is based on free-surface potential flow theory. A perfect absorption may easily be achieved in the linear frequency domain approach by determining the piston motion from either the hydrodynamic force signal or the water wave elevation (Skourup 1996).

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