Characteristic impedance of a tube

Hi all,

I need to calculate the characteristic (or specific) impedance of a tube. Generally this tube has a varing cross-sectional area (diameter changes and it has tube is not cylidrical). However for a very simplified case, let's assume it is a perfect cylender terminated at a finite length. The walls and termination are not rigid so it will have interaction with acoustic pressure inside.

According to the text books, characteristic impedance of a tube is Zc=rho*c/A (rho: density of air, c: sound velocity and A: cross-sectional area). the tube has a finit length so at high frequencies, standing waves will be generated. To calculate the reflectance, I need to have the characteristic impedance. Is Zc always constat or it is a function of wall and termination properies and it will vary at different frequencies?

Thanks for your inputs