Modal Analysis / Numerical Simulation
FE-Modell of the motor gearbox unit
Motor gearbox unit (Test bench)
A disturbing booming noise can be heard in the 250 Hz third-octave frequency band from a motor gearbox unit running at a speed of 1400 RPM. The gear mesh frequency of the gearbox at this speed is 443 Hz, which lies in the 400 Hz third-octave frequency band, and therefore cannot be responsible for the booming noise.
Animation of the mode shape at 236 Hz
A numerical modal analysis shows that the base structure of the unit has an Eigenmode at 236 Hz.
The 10th order of the motor at 1400 RPM causes an excitation at a frequency of 234 Hz. The base
structure is thus excited at a critical resonance frequency causing the booming noise.
The 12th order of the motor at 280 Hz coincides with another Eigenmode at 280 Hz, amplifying
the booming noise.
The 12th order of the motor at 280 Hz coincides with another Eigenmode at 280 Hz, amplifying the booming noise.
Animation of the mode shape at 280 Hz
The animation shows that both mode shapes are acoustically critical “pumping” motions of the base structure. The motion is similar to the motion of a loudspeaker membrane and causes the radiation of structure-borne noise. Early in the design phase, acoustical weak points of a machine can be
identified and optimized with numerical simulation using a 3D-CAD model even before a physical