Reply To: How to do Modal analysis of musical instruments by considering the coupling plates and air inside

[peteroznewman]

Dear Emir,

What worked for me on a different fluid, water, where the water was not flowing but just transmitting pressure, was to create a solid material that I called water. I solved a transient structural model of a drop test of a water tube and got excellent results. In a similar way, the air inside a musical instrument is not flowing either, but the equations that couple the fluid to the structure for acoustics is more complicated than what I needed. However, if you you go the Structures direction, the geometry and meshing will be appropriate for use in an Acoustic Modal analysis when you do obtain a license.

You will find that your model will quickly exceed the Student license limits, so you will need a full version anyway. One reference on modeling acoustics describes the need for a mesh with between six and ten linear elements per wavelength of the frequency of interest. Take an f = 10 kHz tone. Wavelength = c/f where the speed of sound c = 343 m/s, so the wavelength = 34 mm. If you need six elements per wavelength, then you need 6 mm elements. I created a sphere 0.5 m in diameter, and filled it with 6 mm tet elements. That mesh has 3.5 million nodes and 2.6 million elements. When you lower the frequency to a 1 kHz tone, 60 mm elements can fill that same sphere with only 4022 nodes and 2628 elements, but I believe you need a finer mesh to evaluate the higher modes above the first mode. Another reference mentions that the rule of six elements per wavelength can be halved for quadratic elements. However, the ANSYS Introduction to Acoustics training that comes with the Acoustics ACT says on page 60 to use 12 elements per wavelength for linear elements and six elements for quadratic.

Best regards, Peter