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Even the smallest
noise can make the crew of a submarine extremely vulnerable. Therefore,
the acoustic profile of modern submarines is taken into account right
from the design phase. Nevertheless, for various reasons (wear and tear,
ageing, poor assembly) an unacceptable level of sound may still arise.
That being the case, to be able to remedy this, there must be tools available
that enable these sounds to be attributed to one or more devices operating
in the vessel.
In this context, LABRADOR software (LArge Bande Recherche Analyse et Détermination des ORigines [Wide Band Search Analysis and Determination of Sources]) has been developed on Sun® and HP® workstations. This software is based on three main modules, which the user sees as three graphics windows, designed to be as user-friendly and therefore as simple to use as possible. The bulk of the software has been written in C, using IMSL C Numerical Library routines, while the user interface is based on X- Motif ® and Exponent Graphics®. Via an X-Motif-based user interface, the first window allows the user to define the problem that he or she wishes to resolve by specifying first the location of measurement sensors near potential noise sources and those measuring radiated noise and second, the signal-processing parameters that are going to be used for the analysis. In the second window,
known as standard analysis, the user can select from menus the display
of standard values for acoustic and vibration analysis (auto spectrum,
transfer and coherence). The calculation of these quantities relies heavily
on the routines contained in the C Numerical Library, in particular with
regard to Fourier transforms and all operations on complex vectors. The
display itself is produced using Exponent Graphics. In addition to very
careful presentation of the graphics, Exponent Graphics ensures interaction
between the user and the graphics, allowing customized presentation using
menus and buttons (colors, character sets, marker type, line thickness,
etc.) Exponent Graphics also allows the generation of output files in
HPGLTM and PostScriptTM, in particular guaranteeing
a faithful and careful reproduction of the window graphics, facilitating
incorporation into reports. In the third window
-- known as identification of sources -- and after the program has made
all the necessary calculations, users can select the display of different
graphics allowing them to diagnose the responsibility for the radiated
noise. Here also, the majority of operations rely heavily on the contents
of the C Numerical Library, in particular the formation of a spectral
matrix for each discrete frequency using the FFT routine, its breakdown
into distinct values and vectors and a set of product and addition calculations
on the complex vectors resulting from this breakdown. This set of complex
operations has the aim of constructing imaginary sensors independent of
one another (i.e., representing exclusively one source), based on real
sensors, of which each is a linear combination of all the sources in operation.
The most important
of the various graphical outputs in this attribution phase is the spectral
synthesis, allowing a simple glance to identify the device(s) responsible
for the radiated noise output. In effect, on this representation, each
of the devices is allocated a color; the area covered between the background
noise measured and the auto spectrum of radiated noise is colored proportionately
according to the influence of each of the devices with the color that
has been attributed to it. Another graphics
output enables a comparison to be made between the measurement of spectral
density of radiated noise for all the devices in operation and that calculated
by the software for the noise radiated by just one of these devices. This
function is particularly interesting, because for the majority of the
time, it is not possible for a device to function independently of the
others, and a signal-processing algorithm combined with linear algebra
enables simulation of this operating mode. A LABRADOR software function makes it possible to eliminate the contribution of one or more of the sources identified in the signals measured by each sensor. This enables a display of the spectral density of radiated noise when one or more incriminated devices are not in operation. The use of IMSL,
C Numerical and Exponent Graphics libraries has made it possible to reduce
the cost of development by exploiting tried and tested graph-plotting
calculation routines, with interactive possibilities and formatted print
outputs, while concentrating efforts on the specific processes. | ||||||
