Page 21 - Acoustics Today Summer 2011

SIGNAL PROCESSING IN PHYSICAL AND ENGINEERING ACOUSTICS
David H. Chambers
Lawrence Livermore National Laboratory Livermore, California, 94551
Brian E. Anderson
Acoustics Research Group, Department of Physics and Astronomy Brigham Young University
Provo, Utah 84602
Brian G. Ferguson, Kam W. Lo
Maritime Operations Division—Sydney, Defense Science and Technology Organization Pyrmont, 2009 New South Wales, Australia
Michael J. Roan
Department of Mechanical Engineering Virginia Polytechnic Institute and State University Blacksburg, Virginia 24061
Introduction
Signal processing is used extensive-
ly in physical and engineering
acoustics, with applications in
nondestructive evaluation, machine
and structural monitoring, tracking
and localization, and elsewhere. The
goal of signal processing is to extract
desired information from noisy and
uncertain measurements. In this
process we exploit both statistical
analysis and properties of acoustic
wave generation and propagation to
separate extraneous components of the
measurements from the signal of inter-
est. To illustrate signal processing in
physical and engineering acoustics, we present three exam- ples of signal processing that illustrate different methods and approaches to the problem of extracting desired infor- mation from measurements. The first example uses the symmetry of reciprocal wave propagation and timing of reflections to detect flaws (cracks) in plates. This is an illus- tration of a signal processing technique that exploits a prin- ciple associated with physical acoustics. The second exam- ple uses a sophisticated statistical approach to determine the condition of gears in a gearbox from accelerometer measurements. Machine condition monitoring is a large area of engineering acoustics—motivated by both cost and safety. The final example shows how combining informa- tion from different sound sources improves the ability to locate the origin of a bullet fired from a firearm.
Time reversal
Time reversal (TR)1 is a method of locating and character- izing sources and to intentionally focus energy at a selected
2,3
location in space.
The original time reversal experiments
were conducted by Parvulescu and Clay in the early 1960s to
demonstrate the reproducibility of signal transmissions in the
45 ocean. The technique has found applications in SONAR,
communications,6,7 medical ultrasound,8,9 nondestructive eval-
“Signal processing is used extensively in physical and engineering acoustics, with applications in nondestructive evaluation, machine and structural monitoring, tracking and localization, and elsewhere.”
uation,10,11 and seismic imaging12,13 (see selected references for recent work in these areas). The application of TR to nondestructive evaluation allows local- ization of cracks,10 which are nonlinear
11
To illustrate the application of TR to nondestructive evaluation, we will describe a basic TR experiment. During the forward propagation step, a source signal is broadcast from location A in a bounded sample. A reversible transducer at location B collects the directly propa-
gated signal from A and reflections of the source signal from the various possible reflected paths between A and B. The sig- nal recorded at B is then reversed in time and during the back- ward propagation step is broadcast from the reversible trans- ducer at B. This second broadcast signal traverses the propa- gation paths traversed during the forward propagation step. The energy broadcast along each respective path is timed such that they will simultaneously arrive at A.
Reverse time migration (RTM) is a variant of TR com- monly used in seismology applications to image scatterers of interest in the ground. Anderson et al. recently demonstrated that RTM may be fully implemented experimentally in 2-D laboratory samples to image scatterers on an inaccessible side of a laminated plate in places where these scatterers may par-
11
tially be due to delamination. To image scatterers using
RTM, the aforementioned TR experiment is carried out as normal, however during both the forward and backward propagation steps the vibration of the wave field at various points within a region of interest (ROI) must be sensed, (with a scanning laser vibrometer for example). RTM imaging cor- relates the arrivals of energy at specific times at a particular scatterer during the forward propagation with corresponding arrivals of energy at analogous times at the same scatterer during the broadcast and convergence of energy of the back-
scatterers, and linear passive scatterers. It will likely soon be shown that TR can be used to locate acoustic emission events, as work-in-progress continues.
Physical and Engineering Acoustics 17