# Acoustic impedance in Materials

#### sandon

Acoustic impedance of material is defined by density multiply by the velocity of waves within the base material.

My question is if i have a tubular or hollow part with a base material, does this count as a reduction of density compared to the base material of the tube or hollow part. Would the impedance of the material be reduced?

Then applying that same logic of tubular/hollowed parts to porous/cellular parts with the base material where the reduction of density can be just as extreme as the tubular/hollowed parts. Would the tubular/hollowed parts have relatively same impedance as the porous/cellular parts?

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#### sandon

I am asking about all materials' impedance relationship to elastic waves. The purpose is for Hopkinson Bar Design.

#### Paul Colby

Gold Member
I see. So your question is how does the acoustic wave impedance differ for propagation along a hollow cylinder versus a solid bar. Interesting question. I have no idea. One could solve (or better look up) the boundary value problem solution for an infinite cylinder and answer the question.

#### Astronuc

Staff Emeritus
Acoustic impedance of material is defined by density multiply by the velocity of waves within the base material.

My question is if i have a tubular or hollow part with a base material, does this count as a reduction of density compared to the base material of the tube or hollow part. Would the impedance of the material be reduced?
In short, no. The density and speed of sound in a material are physics properties, independent of the geometry of the solid. The speed of sound in a material is related to the bulk modulus and density.

http://hyperphysics.phy-astr.gsu.edu/hbase/Sound/souspe2.html
https://www.nde-ed.org/EducationResources/CommunityCollege/Ultrasonics/Physics/acousticimpedance.htm

Porosity in a material is different, since the voids in the material result in reflection and refraction in the material, thus increase the attenuation.

#### sandon

In short, no. The density and speed of sound in a material are physics properties, independent of the geometry of the solid. The speed of sound in a material is related to the bulk modulus and density.

http://hyperphysics.phy-astr.gsu.edu/hbase/Sound/souspe2.html
https://www.nde-ed.org/EducationResources/CommunityCollege/Ultrasonics/Physics/acousticimpedance.htm

Porosity in a material is different, since the voids in the material result in reflection and refraction in the material, thus increase the attenuation.

"Acoustic impedance in Materials"

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