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# acoustic modal analysis_ Workbench

## acoustic modal analysis_ Workbench

(OP)
Hello!

I'm trying to do an acoustic modal analysis on a vocal tract mesh using Workbench 19. After importing the mesh from ICEM CFD (as .uns) a created a Named selection of the mouth and set a Boundary condition of 0 Pa accoustic pressure in there. I also created an Acoustic body so that the programme knows what body to use (i also set the speed of sound and mass density accordingly).

The goal is to compute the first 6 modes for the vowel /:a (their frequencies). After the calculation, I get 2.-6. mode calculated correctly, but the first mode is way off (see attached pic)(should be somewhere around 650). Any idea what I might be doing wrong?

### RE: acoustic modal analysis_ Workbench

Hi freiwyk,

Can you describe your boundary conditions a little more? How did you get a rigid body mode?

Suggestions:
- In your mode shapes, see if the 0 Pa acoustic pressure was respected. I suspect it is not, thus the presence of 0Hz rigid body mode.
- Since Ansys v18.2, "Modal Acoustics" is native in WB Analysis Systems now without need for ACT. You could give that a shot.
- Have you tried creating a simple room to check your process? (e.g. link)

Kind regards,
Jason

### RE: acoustic modal analysis_ Workbench

(OP)
Hello Jason,

The only things I set were Accoustic pressure in the mouth area to 0 Pa (see attached picture) and Acoustic body as my imported geometry (the entire body).

I'm not sure if the acoustic pressure condition was respected, but I guess not, because the pressure is constant throughout the whole tract in the first mode (see attached pic). The computation is being run on ANSYS 17.2., my mistake.

I think I might be missing a condition somewhere, but I can't seem to find it.

I'm new to both ANSYS and acoustics, so I apologize in advance for any misleading information I might have provided :)

Thanks,
freiwyk

### RE: acoustic modal analysis_ Workbench

Hi freiwyk,

Looks like the P=0 was not respected. Try V19.0 if you can. Otherwise, use a command snippet.

#### CODE --> apdl

cmsel, s, yourNamedSelectionMouthSet
D, all, pres, 0
allsel 

Kind regards,
Jason

### RE: acoustic modal analysis_ Workbench

(OP)
[SOLVED]

Hello Jason,

You were right, it was not respected. Although I still really can't understand the reason behind it, all I had to do was change the Acoustic pressure from 'Named Selection' to 'Geometry Selection' (but still using the exactly same nodes). After that, everything worked like a charm. Nevertheless, it took me quite some time to find :)

Thank you for your time and effort!

Regards,

Michael

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