Learning Forum

[Gumang]

Hi Guys,

According to Mechanical User's Guide - A Modal Acoustic analysis models a structure and the surrounding the fluid medium to determine frequencies and standing wave patterns within a structure. In the Analysis Settings it says - The default is to extract the first 6 natural frequencies.

I have performed Modal Acoustic for a Plate enclosed with a Sphere. I gave the Sphere air properties. Solver Controls were Damped = No and Solver Type = Programm Controlled.

and expected it to give the first 6 natural frequencies of the plate.

[ErKo]

HinnYou are having a free plate.nnThe freq., of the pure plate modes (structure only) have 6 rigid body modes which are close to 0 Hz (so the plate moving like a rigid body - in the acoustic analysis we do not see to get that), but the 7th mode at ~129 Hz is not, and is probably the same as the 1st mode (~129 Hz) of the vibroacoustic model, so they are very likely the same structural plate modes.nnSo that is, and we need to check the modes in both models to see which are the same and so on, because in the vibroacoustic model we can also have acoustic and coupled modes on top of pure structural modes.nnAll the bestnnErikn

[Gumang]

Hey Erik (ekoston), nBut I actually get rigid body movement for certain frequencies in Modal Acoustics. For example: as shown in the Screenshot below at 152,04 Hz. nSo in order to find the natural frequencies of a structure surrounded by a fluid one has to look for pure structural modes and ignore the acoustic and coupled modes? nThank you very much for replying.nRegards nUmangnn

[Gumang]

^ (Forgot to tag you in my last comment ;P) n

[ErKo]

Hi Modal acoustics will calculate acoustic and / or structural modes if we have a structure present .nnSo we have a rectangular enclosure, then the first acoustic mode for (2mx1mx0.1m) say should f=343/(2*2)=85.75 HznnI would suggest to do some simple example without structure to study the pure acoustic modes.nnThen start add in a structure, but of course fix it as it would be in reality (there are no free floating structure, so we need some restraints), and look also on realistic enclosures (not a sphere).nnFinally for the theory and some possible examples see the help manual.nnAll the bestnnErikn

[Gumang]

Array Thank for the help n

[ErKo]

You are more than welcome - just to finish of this discussion, and perhaps this will help others - to the points that a vibroacoustic modal analysis predicts acoustic and / or structural modes if we have a structure present . In the right image below there is a room which is 3 m by 2.5 by 2 (the plate inside the room is 1 x 0.5 x 0.01), so the first acoustic mode along the longest direction, should be f=(343m/s)/(2*3m) = 57 Hz, and that is what we get (see highlight with red in the right image below). The plate mode is 52 Hz which is a free bending mode in that model (highlight in blue in the right image below), and is exactly like the pure structural modal analysis (52 Hz, left image highlighted in blue) and as expected. Of course since the structure is not fixed anywhere in the pure structural modal analysis in the left image below, it also has 6 rigid body modes before we get the 52 Hz plate bending mode.nnn