Method of transfer function generation and active noise cancellation in a vibrating system

DETAILED DESCRIPTION I claim: 1.
A method for the active cancellation of an incident vibration field (N(i.
omega.
)) comprising the steps of: (i) superposing a cancelling vibration field (C(i.
omega.
)) on the incident field to create a residual vibration field (R(i.
omega.
)); (ii) operating on the residual field with a transfer function to obtain an updated cancelling field, the transfer function being divided by a reference point into an upstream part (Fi(i.
omega.
)) and a downstream part (Fo(i.
omega.
)); and (iii) periodically updating the downstream part (Fo(i.
omega.
)) of the transfer function by multiplying the last obtained value (Fo.
sub.
n (i.
omega.
)) by a factor which is the ratio of a computational value of the last cancelling field (C.
sub.
n (i.
omega.
)) and a computational value for the sum of previous residual fields (R(i.
omega.
)).
2.
The method according to claim 1, wherein the reference point is chosen at a position in which the upstream transfer function approximates to unity.
3.
The method according to claim 1, wherein the reference point is chosen at a position in which the upstream transfer function has known characteristics which can be included in the computation.
4.
The method according to claim 1, wherein the updating factor is deduced using the expression ##EQU4## where R.
sub.
n (i.
omega.
) is the computational value of the residual field on the nth update, and for the special case where n=1, N(i.
omega.
)-R.
sub.
n (i.
omega.
) is replaced by N(i.
omega.
).
5.
The method according to claim 4, wherein the updating factor is taken to be ##EQU5## where C.
sub.
n (i.
omega.
) is the computational value of the cancelling field on the nth update.
6.
A method of updating the transfer function used in a transformed domain to determine a cancelling vibration field (C(T)) which when superposed on an incident vibration field (N(T)) will produce a residual vibration field (R(T)), the updating being effected so as to decrease the residual vibration field, the method comprising: multiplying the existing value of the transfer function in the transformed domain (Fo