# Homework Help: Surface impedance in porous rigid materials!

1. May 29, 2017

### plibe

Hi guys,

I've got an exam in a few days and my lecturer is unresponsive, so wondering if you can help me. The question I am solving is the following:

Calculate the impedance at the surface of a layer of spruce wood of thickness d equal to 2mm, and of density rho equal to 0.47 Mg/m3 at 1000Hz. Assume the porosity of wood is 0.4 and the bulk modulus of the wood is 1.5GPa.

(Hint: first calculate the wave number by using k=w*sqrt(rho/K), secondly calculate the characteristic impedace using Zc= sqrt(K*rho), then calculate the surface impedance using Z(M)=-j*(Zc/porosity)*cot(kd))

Am I supposed to convert the K=1.5GPa to Pa? My lecture notes say to use GPa in the Zc formula, but I don't know if it's right.

If I use 1.5GPa=1500000000Pa, then I get Zc= 839642.7812 and Z(M) comes to -298418017.3j

If my impedance values are correct, what units are they in? And how could Z(M) be a negative value?

Thanks in advance for any help! X

2. Jun 3, 2017

### PF_Help_Bot

Thanks for the thread! This is an automated courtesy bump. Sorry you aren't generating responses at the moment. Do you have any further information, come to any new conclusions or is it possible to reword the post? The more details the better.