where sO(n,k) is the output of the omni microphone for sample n of block k and sD(n,k) is the output of the compensation filter 109 for sample n of block k, and ??(k)=C*??(k???1)+(1???C)*P(k), and where C is a constant between 0 and 1 ??(k) is an output from the instantaneous mode value for block k.