The following Matlab command in the filter design toolbox allows
one to design digital IIR filters with constraints on the pole radii:
[NUM,DEN] = IIRLPNORMC(N,D,F,EDGES,A,W,RADIUS)
Also, the Matlab filter design and analysis tool fdatool
in the filter design toolbox allows the interactive design of a
wide variety of digital FIR and IIR filters.
Discussion of how string instruments are synthesized
(1984 Ph.D. Thesis by Julius O. Smith)