As described in more detail hereinafter, the practice of this invention involves the application of interferometric holography as a means for measuring changes in pressure that occur along the boundary surfaces of a structure that contains or directs acoustic pressure variations. As is known in the interferometric holography art, when an optically reflective transducer diaphragm or other object that is subjected to time-varying displacement due to changes in fluid pressure or mechanical vibration is illuminated by a beam of coherent monochromatic light that is provided by a laser or other source and the light reflected from the diaphragm is combined with a portion of the original illumination, an optical interference pattern that is representative of the displacement of the illuminated object is produced.
For example, U. The light that is reflected from the object is directed to a photographic plate or other detector which also receives a reference beam that is derived from the laser illumination. The reference beam and the reflected object beam combine to form a diffraction pattern and, in particular, because of the movement of the object during the illumination interval, form an interference pattern that is representative of the displacement occurring during the period of illumination.
Another application of holographic interferometry is disclosed by Cindrich in U. The pressure sensor disclosed by Cindrich includes a cylindrical pressure chamber with one circular end wall thereof being formed by a diaphragm which deforms at the pressure within the chamber changes.
The outer surface of the diaphragm is illuminated with an image of the diaphragm at rest which is reconstructed from a hologram.
Thus, interference of the light from the reconstructed holographic image of the diaphragm and the light reflected from the actual diaphragm creates interference fringes whose number and arrangement are indicative of the pressure within the chamber. The principles of holographic interferometry have also been applied in systems for the optical reconstruction of an acoustically illuminated object, such systems commonly being referred to as acoustical holography.
In this respect, U. In particular, the Metherell patent discloses a system in which an object to be optically reconstructed is submersed in liquid and illuminated with ultrasonic energy so that wave energy that is reflected from the object impinges on a deformable, diffusely reflecting surface which forms one boundary wall of the tank that contains the illuminated object and the ultrasonic generator. The deformable surface is then illuminated by two relatively short pulses of coherent monochromatic light that are provided by a pulsed laser, with the to pulses of light energy being separated by one half the period of the ultrasonic signal utilized to acoustically illuminate the object.
Further, the distance traversed by the second optical pulse is increased by one quarter of an optical wavelength relative to the distance travelled by the first optical pulse.
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The light energy reflected from the deformable surface during each illumination pulse is caused to impinge on and doubly expose a photographic plate that also receives an optical reference beam that is derived from the laser illumination to thereby form an interferogram. Since the two photographic exposures are separated by a period of time equal to one half a cycle of the acoustic energy, the two exposures result in an interferogram wherein the variations in the optical density of the photographic plate represent twice the actual displacement of the various regions of the deformable surface.
Further, since the optical distance travelled by the second illuminating pulse is increased by one quarter of an optical wavelength, this pulse is effectively in phase quadrature with the first illuminating pulse and, as fully described in the Metherell patent, the optical density of the holographic interferometric record is linearly related to the displacement of the deformable surface. Once the interferogram has been formed in the manner disclosed by Metherell, the photographic plate is developed and illuminated with coherent monochromatic light to provide a second photographic record of the deformed surface.
Upon development, the second photographic record can be used in a conventional holographic wavefront construction system to generate a visual image of the original object. Although prior art such as the above-noted patents to Leith et al. In this respect, and with particular reference to determining the sound field within an aircraft fuselage or portions of an aircraft engine installation such as the air inlet or exhaust systems, the structure being considered is often subjected to mechanical vibration as well as acoustic energy and the disclosed methods of interferometric holography do not provide for distinguishing between the interference pattern that results from the vibration and the interference pattern that results from the acoustic field.
Further, present conventional pressure sensors or transducers that are utilized in forming holograms are not amenable to placement within structural enclosures such as the air inlets of gas turbine installations. Accordingly, it is an object of this invention to provide a method and apparatus for convenient determination of the spatial characteristics of complex sound fields within various acoustically excited structural enclosures wherein little or no modification of the structural enclosure is necessary. It is another object of this invention to provide a method and apparatus for determining the spatial characteristics of a complex sound field wherein holographic interferometry is employed to overcome the disadvantages encountered with previous measurement systems in which electroacoustic transducers are employed.
It is yet another object of this invention to provide a system for determining the spatial structure of sound fields wherein the structure enclosing or containing the sound field is also subjected to mechanical vibration. Still further, it is an object of this invention to provide means for rapidly determining the sound field within structures such as an air inlet of an aircraft engine installation wherein such determination can be made with the engine in operation.
These and other objects are achieved in accordance with this invention by positioning an array of small elastomeric sensors that are responsive to acoustic pressure along the boundary wall of structure whicn encloses or guides the sound field of interest and illuminating the array with a series of monochromiatic light pulses to form at least two holographic records. Each holographic record is formed on a photosensitive detector such as a conventional photographic plate by a pair of pulses of monochromatic coherent light.
When an aperiodic sound field is being analyzed, a series of holographic records are formed by a sequence of pulses of monochromic light that are spaced apart by a predetermined time interval. The holographic records associated with each pair of consecutive pulses are than utilized to form a number of interferograms. Regardless of the number of holograms utilized, each interferogram includes a pattern of optical interference fringes that are representative of the differential displacement of the elastomeric sensors occurring between two consecutive pulses of light energy, such displacement including displacement due to the acoustic field and displacement of both the boundary wall and the elastomeric sensors which results from movement or vibration of the structure.
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Each interferogram is then scanned with an optical reader to supply an electrical signal representative of the variations in photographic density and the spatial distribution thereof. To permit subsequent signal processing to distinguish between acoustically induced displacement of the elastomeric sensors and displacement due to vibration or movement of the structure, the optical scanning pattern traverses both the individual elastomeric sensors and regions of the structure lying between the sensors.
For example, in one disclosed embodiment of the invention wherein individual elastomeric sensors are arrayed in longitudinally extending spaced apart rows and vertically extending spaced apart columns, the optical scanning pattern comprises a series of longitudinal scans in which the optical reading device alternately passes over the center of each elastomeric sensor within a row of sensors and passes longitudinally along the portion of the structure that separates that row of sensors from the next most adjacent row of sensors.
A computer-based Fourier analysis is then performed on the set of electrical signals representative of each of the interferograms to determine the displacement spectrum of the elastomeric sensor array and the region of the structure upon which the array is mounted.
For example, with respect to the above-mentioned embodiment of the invention wherein the sensors are arranged in rows and columns, computer implemented fourier analysis is performed for each row of elastomeric sensors and for each of the longitudinally extending regions of the structure that separate adjacent rows of the sensor array. The portion of the spectrum which is due to the acoustic pressure is then obtained by subtracting the displacement spectrum of a longitudinally extending region of the structure from the spectrum obtained from an adjacent row of elastomeric sensors.
The amplitude and phase of the various spatial modes of the acoustic field are then determined by mathematically combining corresponding components of the acoustically induced displacement spectrum. In the disclosed embodiments of the invention, each halographic record is formed by conventional holographic apparatus of the type employed to record holograms for the optical reconstruction of an illuminated object and includes a pulsed laser and beam splitting arrangement for forming an object beam and reference beam wherein the portion of the object beam that is reflected from the sensor array and the reference beam are combined for impingement on a photographic plate.
Preferably, the laser is of the double pulse variety and is triggered by a control circuit that is responsive to the acoustic signal that is propagating through or in the structure of interest. A microphone or proximity pickup located in the structure that encloses the sound field or otherwise in acoustic communication with the sound field provides an electrical signal to te control circuit and the control circuit causes the laser to supply a pair of pulses having the above-mentioned temporal relationship.
In accordance with the invention, the elastomeric sensors are formed from a relatively thin sheet of elastomer such as foam rubber or one of the well-known polyvinyl materials. Preferably, the elastomer sensors include a gas impervious skin layer which is diffusely reflective to the impinging pulses of monochromatic light. Further, the surface of each of the elastomeric sensors is preferably contoured to control the distance between adjacent interference fringes in the interferograms to thereby produce an interference pattern that is amenable to the above-described optical scanning and computer analysis.
These and other objects and advantages of the present invention will become apparent to one skilled in the art after reading the following description, taken together with the accompanying drawing in which:.
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Upon understanding the invention, those skilled in the art will recognize that the present invention is not limited to the physical situation depicted in FIG. In this respect, the invention is especially advantageous in situations wherein the structure that contains or guides the acoustical energy is also subjected to mechanical vibration.
For example, with respect to the arrangement of FIG. In the arrangement of FIG.
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Each pulse of monochromatic coherent light that is emitted by the laser 12 is directed at a beam splitter 20, which passes a portion of the impinging light that is commonly called the object beam denoted by the numeral 22 in FIG. The object beam 22 impinges on the elastomeric sensor array 14, which is diffusely reflective, and thereby reflects a portion of the object beam A second beam splitter 28 is positioned within the reflected object beam 26 so as to also intercept the reference beam 24 and recombine a portion of both the reference beam 24 and reflected object beam 26 into a beam 30 which is directed to photosensitive detection equipment 32 symbolically depicted as a camera in FIG.
Those skilled in the art will recognize that the arrangement of laser 12, beam splitters 20 and 28 and camera 32 schematically depict a typical holographic recording system that is utilized for recording the optical interference pattern between light energy that is reflected from an object and a reference beam. As is known in the art, such a system records both the amplitude and phase of the reflected light so that, when the photographic record or hologram is developed and illuminated by coherent light, a three-dimensional image of the object is created.
In this respect, it should be recognized that various holographic recording systems are known to those skilled in the art and that many of these conventional systems are suitable for use in the practice of this invention. For example, such a holographic recording system often includes an arrangement of mirrors and divergent lenses to control and direct the object beam 22, the reference beam 24 and reflected object beam 26 and can also include a lens and photographic plate arrangement other than a conventional camera.
In accordance with the embodiment of the invention depicted in FIG. More specifically, when the arrangement of FIG. It will be recognized by those skilled in the art that various analog or digital circuit arrangements can be utilized within the trigger circuit 34 to provide the laser 12 with trigger pulses having the above-described temporal relationships. In this respect, the trigger circuit 34 is supplied an electrical signal by a microphone 36 or other conventional transducer such as an electromagnetic or electrostatic proximity pickup that is placed in the interior region of the structure containing the acoustic field of interest or physically attached to such structure so as to supply an electrical signal representative of the dominant acoustic signal guided by or contained in the structure e.
To supply the desired trigger pulses, the trigger circuit 34 can include, for example, a conventional comparator circuit or other type of circuit which supplies a signal pulse each time the signal supplied by the microphone 36 passes through a predetermined level e. Each of these conventional circuits supply a signal having a pulse repetition rate that is substantially equal to one-half the period of the dominant acoustical signal. By coupling this signal to a conventional digital frequency doubler, a signal having a pulse repetition rate equal to one-fourth the period of the dominant acoustic signal can be supplied and conventional counter circuits can be interconnected to receive the frequency multiplied signal and supply trigger pulses having the previously described temporal relationships.
Regardless of the exact configuration of the trigger circuit 34, each time the laser 12 supplies a pair of monochromatic pulses, a doubly exposed hologram is recorded by the photosensitive detector Since each exposure forms a holographic record that is, in effect, an instantaneous representation of the deflection of various regions within the illuminated area, the superimposed holograms form an interferogram wherein the photographic density and spacing of the interference fringes represent the differential deflection that occurred in the time interval between the two light pulses.
Thus, with reference to the arrangement of FIG. However, since operation of the gas turbine engine causes vibration of the boundary wall 18 and hence the elastomeric sensor array 14, typical interferograms formed in the practice of this invntion often include a more complex pattern of interference fringes that represent both the relatively high amplitude, low frequency, vibration of the inner wall 18 and the higher frequency, lower amplitude acoustic deflection of the elastomeric sensors As shall be described in more detail hereinafter, in accordance with this invention, computer implemented analysis of a signal representative of the composite interference pattern of each interferogram separates the acoustic displacement components from the combined acoustic and mechanical displacement information that is contained therein.
As is illustrated in FIG. Although the elastomeric sensors 16 can be positioned to form arrays of various geometry, it is usually advantageous to arrange the sensors 16 in a rectangular array in which the sensors form longitudinally extending spaced apart rows and vertically extending spaced apart columns. Regardless of the geometry of the array, each of the elastomeric sensors 16 is constructed of a conventional elastomer such as foam rubber or one of the well-known polyvinyl materials that include internal voids.
In this respect, the material utilized is selected to exhibit a mechanical compliance which results in relatively small deflection values when the sensors 16 are subjected to the acoustic field under study. Further, the size of each elastomeric sensor 16 is generally determined in view of the compliance value so that the interferogram that is formed by the two pulses of laser light includes a sufficient number of interference fringes in the region corresponding to each elastomeric sensor 6. In addition, it is often advantageous to establish the contoure of each elastomeric sensor 16 to control the spacing between the interference fringes of the interferograms.
More specifically, FIGS. In eacy of the arrays of FIGS. As can be seen by comparing FIGS. For example, with reference to FIG. Further, if desirable or necessary, the outer surface of the surface layer can be coated with a number of conventional materials to provide the desired amount of light reflection.
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Since each elastomeric sensor of FIG. That is, since the core material is of a relatively uniform density and pliancy, the central region of each elastomeric sensor yields more easily to acoustic pressure than do regions of the elastomeric sensor that are located outwardly of the central region. Since the interference fringes that are produced on the interferogram correspond to the locus of points on the surface of the sensor that are deflected by equal amounts, it can thus be seen that use of the sensor array depicted in FIG.
Like the sensor array depicted in FIG. However, neither the elastomeric sensors of the array depicted in FIG. In this regard, comparing the array of FIG. In this regard, the array of FIG.