harmonic response analysis found only radially symmetric modes
i have performed a modal analysis of prestressed ideal circular membrane (homogenous), clamped on its edges.
Modal analysis found following normal modes (in Hertz): 55,8; 89,06; 119,46; 128,45; 148,55; 163,46; 176,89; 198; and so on
Next i performed a harmonic analysis, because i wanted to know the displacement at different frequencies.
The load was 20Pa pressure on whole area of membrane:
Suprisingly, amplitude is critical only for RADIALLY SYMMETRIC modes: 55,58Hz; 128,45; 198Hz
Why is that ?
Why only radially symmetric normal modes are critical?
By the way (just for explanation), the radially symetric mode from the following picture is 1st mode and 4th mode