  This study deals with spectral analysis of fluid-loaded vibrating structure.
It was recently observed in a numerical study on a high order perturbation
method under heavy fluid loading that a loaded vibrating plate results, not
only in the classical frequency shift of the in vacuo single resonance (in both
the real part because of the fluid added mass and the imaginary part because of
energy lost by radiation into the fluid), but also in an increase in the number
of the resonance frequencies : as a result of the loading, a single in vacuo
resonance frequency of the structure is transformed into a multiple resonance
frequency. Here we show that this phenomenon is said to be a refinement of the
Sanchez's classical result in their book (Vibration and coupling of continuous
systems, Springer-Verlag -1989-, paragraph 9.3) where it was established, using
asymptotic analysis, that in the case of a light loading conditions "the
scattering frequencies of a fluid loaded elastic structure (ie the resonance
frequencies) are nearly the real eigenfrequencies of the elastic body alone and
the complex scattering frequencies of the fluid with a rigid solid". A
theoretical explanation of the observed phenomenon of multiple resonance is
given using classical results of the distribution of zeros of entire functions.
It is established that every single in vacuo resonance frequency of a
rectangular plate is transformed into an infinite number of resonances when the
fluid-loading is accounted for.
