The wn contain very low frequencies. Is this really wanted?
You cannot filter scalars. Try to omit the loop:
z = filter(B,A,a);
If you really have a good reason to filter the data element by element, you have to consider the current status of the filter:
s = [];
z = zeros(1, length(t)); % pre-allocate!
for ii = 1:length(t) % Not 0:length(t) !
[z(ii), s] = filter(B, A, a(ii), s);
end
