Abstract:
An apparatus for sonic excitation of a surface includes a transparent pane. A sidewall structure cooperates with the pane to define an open cavity facing the surface. At least one speaker is positioned to introduce sound to the cavity. Sound cancellation may be provided for at least partially canceling transmission of the sound outside the cavity.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   Benefit is claimed of U.S. Patent Application Ser. No. 60/617,143, filed Oct. 7, 2004, and entitled “Sonic Exciter”, the disclosure of which is incorporated by reference herein as if set forth at length. 

   BACKGROUND OF THE INVENTION 
   The invention relates to art conservation. More particularly, the invention relates to sonic systems for determining the condition of frescos and similar murals. 
   A fresco mural (e.g., painting or mosaic) is a painting made on a masonry wall by brushing pigment-water mixtures into a fresh plaster layer, or by inlaying small pieces of colored glass, stones, or other materials into a fresh plaster layer. Fresco murals have endured thousands of years. 
   The interiors of the U.S. Capitol buildings have many fresco paintings, each over 100 years old, many in need of substrate repair. See, Barbara A. Wolanin, “Constantino Brumidi: Artist of the Capitol” (U.S. Government Printing Office, Washington, D.C., 1998). A similar situation exists in buildings of the Vatican and in other Italian locations and other locations near the Mediterranean Sea. 
   An exploratory technique has been used to evaluate fresco substrates by using a loudspeaker system to direct sound waves toward the fresco mural and a laser interferometer vibration sensor to measure the resulting motion of many locations of the mural. A sound pressure level of 90 to 100 dB is needed to obtain enough motion to measure, but makes the location near the painting too noisy for normal use. See, J. Vignola, J. Bucaro, J. Tressler, D. Ellingston, A. Kurdila, G. Adams, B. Marchetti, A. Agnani, E. Esposito, E. P. Thomasini, “Proper Orthogonal Decomposition Analysis of Scanning Laser Doppler Vibrometer Measurements of Plaster Status at the US Capitol”, 6th Int. Conf. on Vibration Measurements by Laser Techniques, Proc. SPIE Vol 5503. See, also, U.S. Pat. No. 6,728,661 of Cannelli et al., identifying use of a wideband acoustic detector. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention provides a sonic exciter which may be used in combination with one or more existing or yet-developed detectors (e.g., vibration sensing systems), components, and protocols. Contrasted with certain systems utilizing relatively remote and/or relatively exposed sound sources (potentially damaging to the hearing of people nearby), the present exciter may provide a relatively low level of sound within the room outside a cavity defined by the exciter. This may involve various measures for blocking leakage of sound and/or passive or active sound canceling techniques. 
   The exemplary sonic exciter provides a non-contact sound source with adjustable or selectable feet and a sound cancellation feature used to locate faults below the surface of a fresco mural. 
   The exemplary sonic exciter looks something like a heavy duty picture frame. The following text assumes that the large fresco painting to be studied is on a vertical wall of a room. The sonic exciter has a frame, typically, but not necessarily, rectangular, surrounding a work area, typically 30 inches by 60 inches in area. External supports are provided to hold the frame so that the soft feet on the painting side of the frame, typically of wood or other soft material unlikely to damage the painting in an accidental contact, are about 0.5 to 1.0 cm away from, but not touching, the fresco painting. The cross-sections of the arms of the frame typically are also rectangular, with loudspeakers mounted on each of the inner frame surfaces, directed inward, parallel to the wall, into the volume beside the work area. A window, through which laser beams can pass, is mounted to the room side of the frame. Around the outside of the frame, near the wood feet, are many adjustable sound canceling openings connecting the inside of the frame to the room. 
   The loudspeakers of the exemplary sonic exciter are driven by a sonic exciter driver, typically in the 50 to 1000 Hz frequency range. The sonic exciter driver includes a powerful audio amplifier with series resistor outputs for each of the loudspeakers of the sonic exciter. Since the loudspeakers have a very reactive load, and cannot radiate energy as they normally do, these resistors are provided to prevent the loudspeakers, and the amplifier, from failing due to overheating. These resistors provide an additional function of damping the major acoustic resonances of the volume beside the work area. Also provided are circuits to set the frequency, amplitude, and duration of the sonic pulses. The sonic pulse does not occur if such settings would produce a calculated excess of projected loudspeaker coil displacement, and/or a projected excess of the temperatures of the loudspeaker coil and/or of the amplifier. 
   For a flat wall, as described above, the wood feet are thin and flat, covering the entire fresco side of the frame. If, however, the fresco mural is on a curved wall, such as a cylindrical ceiling of a corridor, the flat feet are removed and replaced by thicker feet having curved surfaces toward the painting, shaped to be near but not touching, the painting surface, to minimize the amount of escaping sound. 
   With the sound cancellation off, and a sound pressure in the volume above the work area of 90 dB, the sound escaping from the gap/slot between the wood feet and the painting typically would cause a room sound pressure level of about 60 dB about 3 meters (11 feet) from the frame. Controls are provided on the frame to provide a canceling sound through the openings to reduce the room sound pressure level by 10 to 15 dB to about 45 to 50 dB, still very audible, but low enough so normal speech can be understood. 
   The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       FIG. 1  is a plan view of a first exciter. 
       FIG. 2  is a side view of the exciter of  FIG. 1 . 
       FIG. 3  is an end view of the exciter of  FIG. 1 . 
       FIG. 4  is a cross-sectional view of a side of the exciter of  FIG. 1 . 
       FIG. 5  is an end view of a side of the exciter of  FIG. 1 . 
       FIG. 6  is a view of the exciter of  FIG. 1  positioned for surveying a corridor. 
       FIG. 7  is a schematic view of a driver of the exciter of  FIG. 1 . 
       FIG. 8  is a view of an alternate exciter positioned for surveying a wall. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1-3  show one example of a sonic exciter  20 . The exemplary exciter  20  has four hollow arm-like housing components  22 ,  24 ,  26 , and  28 . The exemplary arms are generally straight and positioned end-to-end at right angles to form a generally rectangular frame or wall structure  29  surrounding a central chamber or cavity  30 . Each of the exemplary arms has a first end  32 , a second end  34 , an outboard side  36 , an inboard side  38 , a top  40 , and a bottom  42 . 
   In the exemplary non-square rectangular exciter, the longer cavity dimension is identified as a length L C  while the shorter cavity dimension is identified as a width W C . Overall nominal width, length, and height are shown as W, L, and H (ignoring any protruding adjustment knob  44  (discussed below) or other similar structure (e.g., switches, connectors, and the like). Exemplary L C  is 60 inches (1.5 m), W C  is 30 inches (0.76 m), L is 76 inches (1.9 m), W is 46 inches (1.2 m), and H is 8 inches (0.2 m). Exemplary ranges of lengths and widths are 0.1-2.5 m, depending upon intended use. Exemplary heights are 0.1-0.3 m. 
   As is discussed below, the structure of the arms may be formed from a relatively rigid and strong material. It may be desired to provide a protective material to prevent any structural material from damaging the mural in the event of intentional or accidental contact. Accordingly, the exemplary embodiment includes a foot or base material  46  secured to the assembled bottoms  42  to surround the cavity  30  (e.g., either as a single continuous piece or as separate pieces for each arm or side of the frame). Exemplary base material is lightweight and has a low adhesion to the mural paint (e.g., in case of an accidental contact). The base material may be removable and replaceable (e.g., to permit installation of base components shaped to accommodate desired shapes of mural surfaces). Exemplary attachment is by fasteners such as screws. Exemplary materials include soft woods and cellular plastics (e.g., cellular polyvinylchloride (PVC) such as is available under the trademark AVEK from AVEK Trimboards of Moosic, Pa. Such material may be particularly useful for applications where the exciter is intended as a non-contact device and there may be light accidental contact. A substantially more compliant material may be appropriate for contact use (e.g., a flexible polymeric foam such as polyurethane). Although the illustrated base material covers substantially an entirety of the undersides of the arm structures, other configurations are possible (e.g., along only a perimeter portion). 
     FIG. 4  shows further details of an exemplary arm with the exciter held adjacent a surface  50  of a mural with an underside  52  of the base  46  spaced apart from the mural by a gap  54 . Inboard and outboard edges  56  and  57  of the underside  52  may be radiused or otherwise convexly curved to reduce chances of damage to the mural by accidental contact. Exemplary gaps are 5-10 mm in height. The exemplary arm structure has a single layer inner/inboard wall  60  and a generally double layered outer/outboard wall having an inner layer  62  and an outer layer  64  with air passageway  66  therebetween. A loudspeaker  68  is shown positioned facing the cavity  30  with its flange mounted to the inner wall  60  and its magnet supported by an internal brace structure  70  of the arm. The inner wall may include vent openings in front of the speakers to permit sound passage into the cavity  30 . There may be multiple loudspeakers arrayed along the length of each arm. Especially with a metallic arm structure, the internal brace member  70  may help conduct heat from the speaker in addition to providing structural support. 
   The exemplary materials for the arm structure include metals (e.g., aluminum alloys), woods, and plastics (e.g., cellular polyvinyl chloride). The arm has a main interior volume  80 . An inlet  82  from the main volume  80  to the passageway  66  is formed by a valve assembly  84  cooperating with an upper wall  86 . An outlet  88  is formed below a bottom edge  90  of the outer wall layer  64  in close proximity to an exterior/outboard perimeter  92  of the gap  54 . 
   In operation and as discussed further below, the speakers  68  direct sound to the cavity  30 . Some of this sound enters the gap  54  at an inboard perimeter  94  thereof and exits at the outboard perimeter  92 . Sound is also produced in the volume  80  of each arm which forms a backspace of the associated speakers. Sound waves from the main volume  80  may pass through the passageway  66  and exit the outlet  88 . By appropriate dimensional selections and adjustment of the valve  84 , sound exiting the passageway  66  may at least partially cancel sound exiting the gap  54 . The level of sound experienced by operators and others in the room may thus be limited by use of this sound cancellation. The exemplary valve  84  includes an arcuate inlet sheet member  100  mounted to an outboard end portion  102  of the inner wall layer  62  by a hinge structure  104 . Rotation of the member  100  about the hinge axis may simultaneously vary the size of a collection area defined at the inlet  82  and the size of a throat area  106  at an intermediate location along the member  100  outboard of the hinge. Actuation may be achieved by a manually-controlled or automatically-controlled actuator. An exemplary manually-controlled actuator features an eccentric driven cam  110  contacting a concave surface of the member  100 . The cam  110  is mounted on an axle shaft  112  mounted in the arm for rotation about its central longitudinal axis. An end of the shaft may be connected to the associated knob  44 . The member  100  may be held against the cam by a biasing spring (not shown). An exemplary material for the member  100  is a high temperature plastic such as chlorinated polyvinyl chloride (CPVC). 
     FIG. 5  shows the knob  44  as bearing index indicia (e.g., numerical) which may register with a reference indicator  120  (e.g., an index post). The indicia may, for example, be selected to represent a characteristic gap height along that associated side. Accordingly, at least an initial adjustment may be made by measuring the gap height and adjusting the knob to the associated height for each arm. Further fine tuning could be performed via feedback. The knob may be provided with a detent mechanism, friction mechanism, and/or a locking mechanism. An exemplary friction locking mechanism comprises a friction clamp arm  130  having a friction under surface  132  and a handle  134  that may be used to tighten the friction clamp arm against the knob face to lock the knob after fine tuning. 
     FIG. 4  further shows a cover plate assembly  140  having a transparent plate or window pane  142 . Exemplary material for the pane  142  is glass, transparent polymer, or combinations thereof. An exemplary transparent polymer is polycarbonate which has advantages of light weight and shatter resistance. An exemplary thickness for a polycarbonate pane is 12-19 mm. The exemplary pane  42  may be mounted to the outboard wall  86  such as by clamps  144  and rubber or other resilient gaskets  146  and  148 , or within the cavity  30  by other means. The pane may have an anti-reflection coating on one or both faces. 
     FIG. 6  shows the exciter  20  positioned to examine a fresco  160  on the semi-cylindrical ceiling of a corridor  162 . An exemplary corridor is 12 feet (3.7 m) wide for a ceiling height of 18 feet (5.5 m). A scaffold-like or other support structure  170  is assembled within the corridor and may include a work platform  172 . The structure  170  may support an axle shaft  174  having an axis  510  at the center of curvature of the ceiling. 
   An exciter carrier  180  is pivotally mounted to the axle  174  for rotation about the axis  510 . The exemplary carrier  180  includes a centrally apertured frame  182  held on pivot arms  184 . The frame  182  carries mounting jacks  190  which have adjustable driving portions (e.g., handles  192  and jack screws) and driven portions (e.g., screw followers) (not shown). The driven portions are engaged to the exciter  20  to permit positional adjustments of the exciter  20  to minimize the gap  54 . In the exemplary implementation for curved surface, the base may have a curvature complementary to a local curvature of the ceiling. In the exemplary implementation, along the short sides of the assembled exciter, the base has a convex curvature R B  nearly identical to a concave curvature R C  of the ceiling. If the ceiling surface is singly concave along the long side, the base may have substantially no end-to-end curvature. For a doubly concave (e.g., domed) surface, the bases on all sides may be chosen for complementary curvature. 
     FIG. 6  further shows a laser generator and detector/sensor unit  200  carried at the end of an additional arm  202  of the carrier  180 . The laser may be scannable over essentially the entirety of the pane covering the cavity. In the exemplary embodiment, the perimeter of the scanning field is shown as  204 . Although the exemplary unit  200  may be positioned directly in front of the pane, to achieve a desired scanning range without interference from the axle it is off-center, with the beam and view reflected by a mirror  210 . Although shown as a flat mirror, a convex mirror may be used to further augment the effective field of view.  FIG. 6  also shows a driver unit  220  which may be connected to the exciter housing by means of cables  222 . In an automated system, the driver  220  and unit  200  may each be connected to a controller (not shown). The cables are coupled to the speakers to drive the speakers and may be configured to permit individual control of individual speakers or groups of speakers or may be configured to drive all the speakers in common. 
   In operation, after the scaffolding and exciter unit have been installed, the unit may be tuned in a first position. With the speakers being driven, the laser may scan across the area of the cavity. The detector receives laser light reflected from the surface of the mural. Depending upon subsurface condition, the mural surface will vibrate responsive to the sound. This vibration will affect the character of the reflected light. A preliminary laser scan may be made, including the coordinates of all of the intended test locations (points) within the field of view. This preliminary scan makes ensures that an adequate reflected signal is obtained at each point. A complex combination of factors including the paint topography and particular beam path to/from the point may prevent receipt of an adequate reflected signal. Even a slight repositioning of the exciter may cure this. For tuning, a low level sound may be generated, and each of the four sound cancellation knobs  44  may be adjusted for a minimum sound level (e.g., measured by ear or microphone adjacent the associated arm) The knobs may then be locked. Next, the higher volume desired sound excitation may be applied and the data taken at each of one-to all points in the scan and saved. If large surface velocities or displacements are present in an area, a fault below the surface is likely to exist in such area. Any of several known or yet-developed techniques may be used to determine the character, depth, and extent of any subsurface defect based upon the detected light. Depending on the particular protocol used, in a given position, there may be multiple scans (e.g., at different sound frequencies). Exemplary sound frequencies are in the range of 50-1,000 Hz, more narrowly 150-400 Hz (e.g., about 200 Hz). 
   After the scanning for any given position is completed, the exciter may be repositioned (e.g., by incrementally rotating the carrier  180  about the axis  510  or translating the carrier along the axis  510 ). In this way, a composite profile of the subsurface properties of the entire ceiling mural may be maintained. A mechanism may be provided for holding the carrier  180  in a given orientation relative to the axis  510 . In a simple embodiment, removable braces (not shown) may be formed for each of several orientations about the axis  510 . Alternatively, an angular detent mechanism or selective locking mechanism may be provided. Depending upon the nature of surface irregularities, consistency of shape, or other parameters, it may be necessary to retune the cavity after each repositioning or otherwise as appropriate. It may also be appropriate to retract the exciter away from the mural prior to moving to the next position (e.g., by use of the jacks  190 ). Other carriers may be provided for other environments (e.g., a purely translatory carrier for a flat wall or ceiling). 
     FIG. 7  shows a simplified driver schematic of an exemplary driver  220 . The driver includes a power amplifier  250  driving a plurality of power resistors  252  each connected in series with an associated one or more of the speakers  68  (e.g., through an associated twisted wire pair  254  assembled into the cable  222 ). Upstream of the amplifier  250  is a multiplier  260  receiving one input from an oscillator  262  and another input from a pulse generator  264 . A short duration input to an input terminal  266  will initiate one pulse of the pulse generator  264 . For example, a scanning control of the unit  200  may provide a next pulse signal. With non-scanning manual sensor positioning, the input may be from an operator-actuated button or other device. The exemplary driver  220  also includes an operator input program panel  270  and controller (e.g., computer or microcontroller)  272 . The operator may program desired inputs to the panel  270  (e.g., desired oscillator frequency, desired pulse duration, and desired magnitude (e.g., in dB) of the sound level inside the cavity). The controller  272  may be programmed with an out-of-range override in case the entered values would produce a pulse that might potentially damage the exciter, associated hardware, or mural. Feedback (e.g., a warning light) may also be provided. This controller may also comprehend temporary conditions (e.g., a programmed operation that would be safe with a cool system may become unsafe once the speakers have reached a particular temperature). In the case of a projected overheating, the controller may thus temporarily disable the pulse generator  264 . 
     FIG. 8  shows an alternate exciter unit  300  positioned along a vertical wall surface  302 . For ease of illustration, a support structure (if any) is not shown.  FIG. 8  further shows a non-scanning laser generator/sensor unit  304 . An exemplary unit  304  may be hand-held, tripod-mounted, or the like.  FIG. 8  further shows a driver/control unit  306 . For ease of illustration, power and control wiring to and between the various components or corresponding wireless communication pathways are not illustrated. As in the exemplary unit  20 , the exemplary unit  300  includes a frame  320  and a forward window pane  322  cooperating to define a rearwardly-open cavity/chamber. The exemplary frame sides may be simply box-structured in cross-section, with speakers (not shown) mounted along the inboard wall. The pane  322  may be mounted to the top of these box sections or spanning between the inboard walls. 
   In place of the passageways  66  of the exciter  20 , the exciter  300  has one or more tunable ports  330  in the outboard wall  332 . Each of the exemplary ports  330  are formed by an associated aperture  334  and a movable shutter  336 . The shutter  336  may be slidably mounted to the inboard surface of the associated outboard wall. The shutter  336  may be moved to determine the open cross-sectional area of the port  330 . In an exemplary implementation, there is one port  330  associated with and immediately outboard of each loudspeaker. The open portion of each aperture  334  is shown near the fresco-facing side of the unit (bottom of the box section) to be relatively close to the gap for sound cancellation. The exemplary shutter  336  may be moved by a pair of spring-loaded finger-buttons  338 . The buttons may be pressed toward each other (e.g., by operator thumbs and forefinger) to release from engagement with the lateral sides of the aperture  334 . When released, the shutter may be slid toward or away from the mural side of the aperture  334  to provide area control. When the buttons are released, they engage the lateral sides to secure the shutter (e.g., by friction or detect action). Iterative tuning may be as described above. 
   The exemplary unit  304  emits a continuous non-scanning laser beam  350 . The driver  306  may include a controller instrumentation panel  360  and other driver components (e.g., as discussed above). In operation, the operator manually directs the laser beam through the pane  322  to a particular location on the mural and then pushes the button  352 . The button push provides a signal to the input terminal  266 , causing the pulse generator  264  to cause the amplifier and loudspeakers to produce a sonic pulse. If the beam is aimed toward a substrate fault, the instrumentation panel  360  displays an indication of vibration at that location on the mural. More sophisticated displays may also be provided. 
     FIG. 8  also shows an alternate vibration detector unit  400  that may be used with an exciter unit as described above or otherwise. The exemplary unit  400  is a non-laser unit that may be representative of direct contact or other units. The exemplary unit  400  includes a geophone detector  402  having a housing  404  and a sensor foot  406 . An exemplary foot  406  has a thin rubber or other elastomeric/resilient contact face for directly contacting the mural surface. The foot may be spring biased by biasing springs (not shown) to contact the mural with appropriate force. Exemplary geophone technologies are implemented in oil ground surveys. A geophone operates responsive to the relative movement of a steel sleeve and a coil, spring supported relative to the sleeve and within a field of a permanent magnet. A common geophone construction involves a short metallic (e.g., steel) cylindrical sleeve body concentrically carrying the permanent magnet spaced by non-magnetic material inwardly of end plates of the sleeve. The coil is positioned surrounding the magnet in the annular space between the magnet and cylinder and may be mounted to an inner sleeve held at both ends by spring disks for spring-biased centering reciprocation relative to the magnet. Vibration-induced relative motion between the inner and outer sleeves generates a voltage, BLV, where B is the flux density through the coil, in webers, L is the length of the coil wire, in meters, and V is the relative velocity, in meters per second. In the present use, the foot  406  may be mounted directly to one of the end plates. 
   The housing  402  forms a carriage slidable along a guide arm  410 . The housing may be locked in position along the arm  410  by a set screw  412 , detect mechanism, or other means. At each end of the arm  410 , a thin leg wall (e.g., of sheet metal)  420  extends toward the mural. A thin foot wall  422  extends outward from the distal end of the leg wall  420  and is dimensioned to fit between the exciter foot/base and the mural (e.g., within the gap). A toe wall  424  extends back away from the mural and carries a set screw  426 . The set screws may be used to mount the unit  400  to the exciter (e.g., by engaging the adjacent outboard wall surface of the exciter base with the outboard surface of the foot wall  422  contacting the adjacent base/foot section of the exciter. 
   With the exemplary unit  400 , the exciter need not have a transparent pane. Advantageously, the pane (or an opaque replacement) may be formed as a door. The door may be openable to facilitate positioning of the geophone (e.g., by sliding the geophone along the arm  410  and/or moving the arm transversely (e.g., after loosening the screws  426 ). To facilitate such movement, the geophone may advantageously be provided with a retracting lever to retract the foot to avoid damaging the painting. 
   However, the mural sides of the sonic exciter feet and the foot walls  422  may be covered with resilient strips such as used for thermal insulation around doors and windows. In such a case, the sonic exciter may be pressed against the mural, and the counter-sound ports  330  closed, providing almost complete protection of the operator&#39;s ears from dangerous high intensity sound. 
   For initial surveys of very valuable murals to locate substrate failures, laser vibration sensors are preferred relative to contact sensors (e.g., press against the mural type vibration sensors, such as illustrated in the unit  400  of  FIG. 9 ), because laser scanning over the mural is fast and safe, and because the contact of the geophone foot may cause damage. For the repair of a substrate fault, however, the surface must be repaired anyway. A contact vibration sensor may provide a simple in process evaluation of the substrate re-attachment process, at a low equipment cost. 
   One or more embodiments of the present invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.