Abstract:
A method and apparatus for measuring the quality of live, standing timber employs deep-set spike probes angled to create a longitudinal compression wave. Alignment tools and isolation of the probes provide an improved signal, automated detection of probe separation and amplitude-independent pulse discrimination process ensure higher reliability and repeatability of the measurements, and wireless operation provides operational efficiencies.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims the benefit of U.S. provisional application No. 60/538,376 filed Jan. 22, 2004, hereby incorporated by reference. 
     
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT  
     BACKGROUND OF THE INVENTION  
       [0002]     This invention relates generally to an apparatus and method for acoustically testing wood properties and suitable for live, standing trees.  
         [0003]     Currently, it is difficult to nondestructively assess wood quality in standing trees. In order to evaluate wood quality, a forester must take samples of the wood and send them to a laboratory for analysis, or fell the tree first and make a visual inspection of the external characteristics of the logs cut from the tree. Often a complete assessment of wood quality cannot be made until the timber is in the milling process. By this time, considerable resources have been spent locating and transporting the timber. Discovering that the timber is not of usable quality is a waste of these limited resources. Furthermore, otherwise useful trees are unnecessarily removed from the forest.  
         [0004]     The measurement of sound speed through wood is a well-known method for evaluating logs and timber. Sound speed is generally related to the modulus of elasticity of the wood and may indicate additional properties related to wood strength and quality. Typically in such techniques, a resonance-based approach may be used or an acoustic signal is introduced into one end of a log or at the end or surface of exposed timber, and its time of transit to the other end or separate point is recorded. Sound speed is derived simply by dividing the transit time by the length of the sound path through the log or timber.  
         [0005]     PCT patent application WO 02/29398 describes a variation on this technique for use with standing trees. Following the method of this patent application, shallow spikes held in separation by a rigid bar are used to introduce sound waves into the cambium of the tree at a first location and extract the sound waves at a second location a predetermined distance away from the first location. The time between a tapping of the first spike and a receipt of the sound wave at the second spike displaced by the known length of the rigid bar provides a measure of sound speed that may be used to deduce modulus of elasticity. One drawback to this system is possible acoustic contamination from sound passing not through the wood, but directly between the probes through the connecting bar.  
         [0006]     New Zealand patent application No. 533153, filed May 26, 2004 by inventor Chin-Lin Huang, entitled: “System and Method for Measuring Stiffness in Standing Trees” and claiming priority from U.S. patent application Ser. No. 03600933 filed Jun. 20, 2003, describes a similar system, but in which the probes are separate, to be freely located on the sides of the tree held by spikes engaging the cambium wood. Each probe communicates by means of electrical cables to a measuring unit. Because the probes are connected only by flexible cables to the measuring unit, the sound detected by the receiving probe is not contaminated with sound through a connecting bar.  
         [0007]     The use of two separate probes, unconnected by a bar, reduces the weight and bulk of the system, simplifying its use by a single operator in the field. Yet because the probes are no longer held in rigid separation, their separation distance, which must be known for accurate sound speed measurement, will vary. This variation can be accommodated by the additional step of having the operator measure the separation between the probes, for example, using graduations on the connecting cables as a ruler as proposed in the 533153 patent, but this added measurement introduces possible operator error and decreases the convenience and speed with which the system may be used.  
         [0008]     When separate probes are used, they can be misaligned when installed on the tree, significantly decreasing the strength of the received acoustic signal, and hence accuracy of the measurement. The electrical connections between the probes and measuring unit, no longer routed along the separating bar, may become tangled and damaged.  
       BRIEF SUMMARY OF THE INVENTION  
       [0009]     The present invention provides the benefits of separate probes without many of the disadvantages of separate probes by employing a series of wireless links between the probes (and a measuring unit), each link tailored to a particular task. The result is a probe system that is easy to install and move and yet which provides high accuracy.  
         [0010]     In one embodiment, an ultrasonic pulse is used to automatically measure the separation between the probes. A laser checks alignment between the probes, while a high speed infrared pulse communicates the start time of the pulse from one probe to the other. Finally, a radio link is used to communicate data to a separate measurement unit.  
         [0011]     Each of these links connects the units without direct contact, eliminating any path of acoustic contamination between the probes and avoiding the need for cumbersome and damage-prone cables and electrical connectors.  
         [0012]     The present inventors have developed a number of other innovations that improve sound speed measurements applied to standing trees. First, spikes that may pierce the bark and cambium and extend into the sapwood are used to direct acoustic energy into a tree across a larger proportion of the cross-section of the tree trunk. Second, a sophisticated processing of the received acoustic pulse is used to provide an arrival time measurement that is less sensitive to signal amplitude variations than is a simple voltage threshold detection technique used in the prior art. In addition to sound speed measured in standing trees, several other predictor parameters have been incorporated into more sophisticated prediction models that are more reliable and accurate than is a simple sound-speed-only-based model. These additional parameters may include breast height diameter (DBH) of a tree, the age of a tree, tree species and wood temperature individually or in combination. DBH and wood temperature are the most significant parameters in addition to sound speed. Tree age and tree species are preferably added into the model if they are available at the time of assessing trees. In one embodiment, a tree-grading program then assigns quality levels to individual trees based on these measurements and derives plot, stand, or forest-level summary outputs.  
         [0013]     Specifically, then the present invention may provide a first and second acoustic probe attachable to a tree at different heights to engage the wood of the trunk and to provide an acoustic transmission signal and an acoustic reception signal indicating, respectively, a time of initiation of an acoustic wave into the wood by the first acoustic probe and a time of receipt of the acoustic wave through the wood at the second probe. The probes are adapted to transmit a longitudinal compression wave through the bulk wood (both sapwood and heartwood) in the tree from the first probe to the second probe and to detect the longitudinal compression wave passing through the tree at the second probe. Analysis circuitry communicating with the first and second acoustic probes may receive an acoustic transmission signal (start signal) and an acoustic reception signal (stop signal) to provide a speed of sound measurement of the acoustic wave through the wood. A wireless communication link may transmit at least one of the acoustic stop signal and acoustic start signal to the analysis circuitry, the wireless communication link providing a speed of transmission substantially greater than a speed of propagation of the acoustic wave.  
         [0014]     Thus, in at least one embodiment, the invention eliminates the possibility of conduction of the excitation signal through the support structure to the receiving transducer such as may introduce error into the timing measurements and eliminates the need for wires or other physical connections between the spikes, so improving greatly the ease with which the device may be used in the field.  
         [0015]     The wireless communication link may be an infrared diode transmitting the acoustic start signal from the first probe to the analysis circuitry.  
         [0016]     Thus it is an object of at least one embodiment of the invention to provide a wireless communication of the start pulse arrival time that can be transmitted reliably, at low cost, and with essentially no delay.  
         [0017]     The analysis circuitry may be attached to the second acoustic probe.  
         [0018]     Thus it is another object of at least one embodiment of the invention to ensure consistent and reliable communication between the transmitting probe and this circuitry. It is another object of at least one embodiment of the invention to eliminate the need for two wireless links in this critical measurement path.  
         [0019]     The invention may include a data logging unit storing speed of sound measurements and a second wireless communication link transmitting speed of sound measurement data from the analysis circuitry to the data logging unit.  
         [0020]     Thus it is another object of at least one embodiment of the invention to provide a convenient means of analyzing and transporting collected data free from the collection hardware.  
         [0021]     The invention may include a range-detecting sensor communicating between the first and second acoustic probes to provide a separation measurement signal indicating a separation between the first and second acoustic probes, and the analysis circuitry may use the separate measurement signal in determining the speed of sound measurement.  
         [0022]     Thus it is another object of one embodiment to reduce the error in length measurement between the spikes when a separating bar is not employed.  
         [0023]     The invention may further include an alignment sensor communicating between the first and second acoustic probes to indicate whether the spike of the first and second acoustic probes lies within a single common plane.  
         [0024]     Thus, it is another object of at least one embodiment of the invention to lessen signal loss and signal-to-noise induced error caused by off-axis or out-of-alignment probe insertion in a field environment.  
         [0025]     The second probe may include a transducer for generating an electrical signal measuring the acoustic wave received at the second probe, and the invention may provide a pulse discrimination circuitry receiving the electrical signal and generating a pulse stop signal using an amplitude independent detection process.  
         [0026]     Thus, it is another object of one embodiment to eliminate variations in sound speed measurement caused by amplitude variations that may occur when testing trees in a field situation.  
         [0027]     The first and second probes may include spikes adapted to be driven through bark and cambium of the tree to engage the sapwood beneath the cambium.  
         [0028]     Thus, it is an object of this embodiment to provide a method for introducing longitudinal compression waves deep into the tree without access to exposed wood or log ends.  
         [0029]     The pulse start signal is provided by an electrical switch actuatable by the striking of the first acoustic probe with a mallet.  
         [0030]     One object of at least one embodiment of the invention may therefore provide a simple means for detecting the start of the acoustic signal.  
         [0031]     One embodiment of the invention provides a method for predicting the modulus of elasticity (MOE) (a measure of wood stiffness) and other wood and fiber properties (such as strength, density, fiber length, microfibril angle, but not limited to these) through pre-established multivariate prediction models. The predictor parameters may include sound speed and DBH of a tree, as well as tree age and species if they are available.  
         [0032]     Another embodiment of the invention provides a procedure for generating statistic outputs for specific plot, stand, or forest evaluated. The statistic outputs include mean, standard deviation,  95  percent confidence level for various tree measures such as sound speed, DBH, and predicted wood and fiber properties.  
         [0033]     The invention may include a software program to assign quality levels to individual trees and specific plot, stand, or forest according to pre-determined grading criteria. The grading criteria may be adjusted by users based on their particular needs and the types of wood and fiber products that they wish to produce from the trees.  
         [0034]     These particular objects and advantages may apply to only some embodiments falling within the claims and thus do not define the scope of the invention. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0035]      FIG. 1  is a perspective view of a standing tree with spike probes of the present invention being inserted through its bark by an operator;  
         [0036]      FIG. 2  is a fragmentary elevational cross-section of the tree of  FIG. 1  showing positioning of the spike probes therein;  
         [0037]      FIG. 3  is a block diagram showing the components of the present invention in one embodiment such as may communicate with a personal digital assistant or the like;  
         [0038]      FIG. 4  is a front elevational view of the spike probes of  FIG. 1  showing alignment of the probes in a single plane by an alignment guide and housing reference surface;  
         [0039]      FIG. 5  is a set of three plots indicating from top to bottom, a received acoustic signal, a transmitted acoustic start signal, and an acoustic stop signal derived from the received acoustic signal using an amplitude-independent detection technique;  
         [0040]      FIG. 6  is a flowchart showing the steps of the method of the present invention;  
         [0041]      FIG. 7  is a side view of the probes similar to that of  FIG. 2  showing the use of a removable separation guide in an alternative embodiment of the invention; and  
         [0042]      FIG. 8  is a top plan view of a laser target and infrared transmitter used on the transmitting probe. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0043]     Referring to  FIG. 1  generally, the present invention may be used to measure the wood quality of a live, standing tree  10  by an individual forester  11  who may carry the components of the present invention easily from tree to tree.  
         [0044]     At each tree  10  to be measured, the forester  11  hammers two spikes into the tree using a hammer  13 . As shown in  FIG. 1 , the first spike associated with transmitting acoustic probe  12 , is angled upward into the tree  10  near the ground and a second spike, associated with receiving acoustic probe  14 , is positioned at about eye level and angled downward into the tree  10  toward the transmitting acoustic probe  12 . The distance  16  between the transmitting acoustic probe  12  and the receiving acoustic probe  14  is not critical and may vary between one foot and six feet with a distance of about four feet preferred because it is consistent with the average height that a forester can reach. Longer separations may also be possible. The transmitting acoustic probe  12  may alternatively be above the receiving acoustic probe  14  provided they are generally in a vertical line with respect to one another along the grain of the wood.  
         [0045]     After installation of the transmitting acoustic probe  12  and the receiving acoustic probe  14 , the forester  11  may make a measurement of the breast height diameter (DBH)  23  of the tree as may optionally be used in the calculation of wood quality to be described below. The forester  11  may carry a personal digital assistant (PDA)  22  providing for convenient interface with the electronics of the invention as will be described.  
         [0046]     Referring now to  FIG. 2 , the spike  26  of the transmitting acoustic probe  12  may be guided during insertion into the tree  10  by a housing  18  having an internal angled channel  19  providing the proper angulation of the transmitting acoustic probe  12  when a reference surface  21  of the housing  18  is approximately level. This angle θ is preferably about  45  degrees and results in a sharpened tip of the spike  26  of transmitting acoustic probe  12  being angled toward a tip of spike  28  of the receiving acoustic probe  14 . The spike  28  is held positioned by a second housing  20  of similar construction to housing  18  holding the spike  28  of the receiving acoustic probe  14  at a mirror image angle θ′.  
         [0047]     The angle of the spike  28  may also be guided by the use of a laser beam  56  (to be described below) emitted from a laser  76  at a 45 degree angle to spike  28  and perpendicular to reference surfaces  21 , and directed downward at a target  58  (shown in  FIG. 8 ) on the housing  18  of the receiving acoustic probe  14 . Both the laser  76  and the target  58  are at a same distance from the tips of spikes  26  and  28  on the respective housings  18  and  20 .  
         [0048]     Referring to  FIGS. 2 and 3 , the channel  19  is connected to the spike  26  through a spring coupling  31  to allow movement of the spike  26  without the need to accelerate the housing  18 , protecting the circuitry within the housing  18  (to be described) and providing a faster acceleration of the spike  26  upon impact by the hammer  13 .  
         [0049]     In contrast, the spike  28  of the probe  14  is removable and may be installed by striking a collar  33  on the outer end of the spike  28  with the hammer  13 . A handle  35  attached to the collar  33  and extending radially from the axis of the spike  28  may be used to hold and guide the spike  28  during this process. In this way, the circuitry (to be further described) in the housing  20  and other components of the acoustic probe  14  are wholly protected from the shock of installing the spike  28  into sapwood (as will also be described).  
         [0050]     When the spike  28  is installed, an upper shaft  29  is fit to the collar  33  by means of a mating pin and socket to attach the probe  14  to the spike  28  held by gravity and/or a loose press fit. Whereas such a separation in the length of the spike can be detrimental in the spike  26  of probe  12 , the present inventors have determined that this separation is readily accommodated in the spike  28  of the receiving probe  14 . Once the spike  28  is assembled to the upper shaft  29 , the handle  35  may be used to further adjust the alignment of the spike  28  as will be described.  
         [0051]     The reference surfaces  21  generally guide the installation of the probes  12  and probe  14  by the forester endeavoring to approximately place these reference surfaces  21  in a horizontal or other reference plane. Final alignment is accomplished with the laser  76 .  
         [0052]     The spikes  26  and  28  are preferably driven into the tree to a depth of from ten to twenty millimeters to allow a portion  42  of the spikes  26  and  28  to acoustically couple with sapwood  44  beneath both the bark  24  and cambium  25  being generally the older wood outside of a core layer of the tree as is generally understood in the art.  
         [0053]     The spikes  26  and  28  may be constructed of stainless steel, titanium, or other suitable materials to readily pierce the sapwood  44  and the bark  24  and cambium  25  and to provide a well characterized sound wave and to resist removal bending forces as will be discussed below. Generally, the portion  42  of the spikes  26  and  28  within the sapwood  44  tapers continuously from the tips outward as one passes through the bark  24  so as to provide compressive purchase against the wood fiber of the sapwood  44  that resists further insertion of the spikes  26  and  28  after a certain point and that eases removal of the transmitting acoustic probe  12  and receiving acoustic probe  14 .  
         [0054]     The spike  26  of transmitting acoustic probe  12  is surrounded at its distal end, by an elastomeric handle  60  having an outer grip portion  62  and a disk shield portion  64 . The disk shield portion  64  protects a user&#39;s hand holding the outer grip portion  62  from errant hammer blows and provides a grip for withdrawing or extracting the transmitting acoustic probe  12  and receiving acoustic probe  14  after completion of the test.  
         [0055]     The distal ends of the spike  26  holds a replaceable impact cap  66  constructed, for example, of a metal so as to resist damage by the hammer  13  and to control the characteristics of the acoustic signal that will be generated during the testing process by striking the transmitting acoustic probe  12  with the hammer  13  or other impact means. Impact cap  66  for transmitting acoustic probe  12  may include a mechanical contact switch  36  providing normally open contacts biased by a contained spring (not shown) that will contact during a hammer strike to produce an acoustic start signal as will be described. The switch  36  communicates with an infrared transmitter  37  exposed at the upper reference surface  21  of the housing  18  to communicate a pulse start signal to be described below. The housing  18  further holds batteries (not shown) for powering the infrared transmitter  37 .  
         [0056]     Referring still to  FIGS. 2 and 3 , a piezoelectric accelerometer  82  is attached to the upper shaft  29  of receiving acoustic probe  14  to sense axial vibrations conducted along the length of the spikes  26  and  28 . The lower reference surface  21  of housing  20  exposes an ultrasonic range finder  50  that may transmit an ultrasonic beam  52  to a target, being the upper reference surface  21  of the probe  12 , to thus determine the separation between housing  20  and  18 . By the known geometry of the probes  12  and  14  and their respective spikes  26  and  28  with respect to the reference surfaces  21 , the separation between the tips of spikes  26  and  28  can be determined. The lower reference surface also holds an infrared receiver  77  to detect the pulse start signal from the infrared transmitter  37 .  
         [0057]     Housing  20  also holds processing circuitry and a wireless communication device  92  on a contained circuit board  74  holding a microcontroller  80 . The microcontroller  80  or the circuit board may incorporate a multiplexing A/D converter (not shown) which may receive the output of the accelerometer  82  and an ambient air temperature probe  86 . Wood temperature may be deduced from ambient air temperature, or a separate temperature probe (not shown) can be incorporated into one or both spikes  26  and  28  to communicate with the A/D converter  84 , or an average ambient temperature may be assumed from typical seasonal temperature data. Alternatively, a separate instrument may be used to measure wood temperature for selected trees during the measuring process. The microcontroller  80  may receive a digital output of the A to D converter as well as digital output from the range finder  50  and the infrared receiver  77  for the acoustic start signal from the impact cap  66  of the transmitting acoustic probe  12  as described above. The microcontroller  80  may also communicate with a wireless transmitter providing data exchange with a PDA  22  to read out or store data or provide programming input to the apparatus. The wireless transmitters and receivers of this type are well known in the art and include short radio links per the Bluetooth or IEEE wireless standards or alternatives known in the art.  
         [0058]     Microcontroller  80  also communicates with a internal memory which contains an operating program, as will be described below, for processing the data collected by the apparatus and in handling other tasks that would be understood to those ordinarily skilled in the art. The circuit board  74  also provides an attachment to a battery pack (not shown) holding replaceable or chargeable batteries to provide electrical power to the described components. The microcontroller  80  may also communicate with a numeric display  81  exposed on a front vertical face of the housing  20 . The microcontroller  80  may also communicate with an audio transducer providing audible signals to the forester  11 .  
         [0059]     Referring now momentarily to  FIGS. 2 and 4 , the laser beam  56  allows for housing  18  and  20  to be aligned to ensure that acoustic energy in the form of longitudinal compression waves  70  are well coupled through the tree  10  between the transmitting acoustic probe  12  and receiving acoustic probe  14 , and that automated length measurement between the spikes  26  and  28  is as accurate as possible. This alignment provides that angle α defining angular deviation between a plane perpendicular to the bark  24  and bisecting the spike  26  of the transmitting acoustic probe  12  and a plane perpendicular to the bark  24  and bisecting the spike  28  of the receiving acoustic probe  14  be close to zero. This alignment also provides that a separation of these planes Δx be close to zero. This alignment contributes to precision of automated length measurement and also to the strength of the signal passing between transmitting acoustic probe  12  and receiving acoustic probe  12  reducing signal-to-noise ratio of that signal such as may result in erroneous or irreproducible results.  
         [0060]     Referring now to  FIG. 6 , in a first step in the use of the present invention, the forester  11  enters the species and age estimate of the tree as indicated by process block  100  through the PDA  22 .  
         [0061]     At process block  102 , the DBH  23  described above with respect to  FIG. 1  is optionally measured (for example, with a tape measure or alternative measuring device) and entered through the PDA  22 .  
         [0062]     Per process block  104 , the forester  11  then may drive the spikes  26  and  28  of the transmitting acoustic probe  12  and receiving acoustic probe  14  into the tree  10  as shown in  FIG. 1 , and after assembling the spike  28  to upper shaft  29 , align the housings  18  and  20  per the discussion associated with  FIG. 4 . The transmitting acoustic probe  12  and receiving acoustic probe  14  are inserted into the tree  10  to a maximum depth such that tips of the spikes  26  and  28  are coupled to the sapwood.  
         [0063]     After completion of driving the spikes  28  and  26  into position, the microcontroller  80  is armed either through a signal from the PDA  22  or a pushbutton (not shown) on one of the housings  18  and  20 . At this time, the microcontroller, executing a stored program in memory  90 , measures the separation distance of the housings  18  and  20 , determines the separation of the receiving acoustic probe  14  and transmitter acoustic probe  12 , and stores this number and displays it on the display  81  for confirmation.  
         [0064]     At process block  106  of  FIG. 6 , and referring also to  FIG. 3 , the forester  11  may strike impact cap  66  of transmitting acoustic probe  12  with the hammer  13  to generate longitudinal compression waves  70  passing generally along the length of the tree  10  as shown in  FIG. 2 . The angle θ of the transmitting acoustic probe  12  ensures that energy is directed into a compression wave along the trunk of the tree  10 . The angle θ′ of receiving acoustic probe  14  in turn provides that the longitudinal compression waves  70  produce an axial force along the spike  28  that may be sensed by a piezoelectric accelerometer  82  as has been described.  
         [0065]     When the hammer  13  strikes the impact cap  66 , the internal switch provides the acoustic start signal received by infrared receiver  77 , causing the initiation of a timer internal to the microcontroller  80 .  
         [0066]     Referring now to  FIG. 5 , at a given time after a closure of the switch in impact cap  66  as indicated by an upward transmission of an acoustic start signal  112 , the receiving acoustic probe  14  will detect a transmitted compression wave signal  114  through accelerometer  82 . This digitized signal may be filtered to reduce high frequency noise unlikely to be a compression wave through wood, and then is analyzed to detect a slope exceeding a predetermined slope  118 , the particular value being dependent on the amplification of the signal and the time base of the measurement. The filtering and analyses may be performed through software executed by the microcontroller  80  or by a digital signal processor (DSP) or by discrete components implementing analog filters and slew rate filters as would be understood in the art from this description. If the signal equals or exceeds the slope  118  and is at least of a predetermined threshold  120 , then the acoustic pulse is considered properly received and a stop pulse is generated, and a time fixed on the clock as derived from the retrospective analysis of the rising edge of a logical acoustic stop signal  122 . The slope sensitive detection system reduces sensitivity of the detection time on the amplitude of the signal such as may vary widely according to how hard the forester  11  strikes the transmitting acoustic probe  12  with the hammer  13 , the coupling of the transmitting spike  26  and spike  28  to the sapwood  44  and other factors unrelated to the quality of the tree. Other amplitude-independent detection systems can also be envisioned including those that detect particular waveform shapes or frequency content.  
         [0067]     At decision block  108 , the receive signal is analyzed as described above and if it is acceptable, this is indicated to the forester  11  by a single beep from the audible transducer  79  and at process block  110 , and if the rolling three-hit average is acceptable, the forester removes the transmitting acoustic probe  12  and receiving acoustic probe  14 . If the signal is not acceptable in meeting the above slope/amplitude criteria, the forester  11  is instructed via three beeps from the audible transducer  79  to strike the transmitting acoustic probe  12  again at process block  106  until a suitable signal is obtained.  
         [0068]     The elapsed time between the start of the acoustic start signal  112  and rising edge of the acoustic stop signal  122 , t 0 , is considered the time required for a pulse to proceed from transmitting acoustic probe  12  to receiving acoustic probe  14 . The acoustic wave speed through the tree C tree  is determined by the microcontroller  80  using the following equation:  
               C   tree     =       L   0       t   0               (   1   )             
 
         [0069]     where L 0  represents the distance determined by the ultrasonic range finder  50  corrected for the displacement of the transmitting acoustic probe  12  and receiving acoustic probe  14  with respect to the housings  18  and  20  as described above.  
         [0070]     In a preferred embodiment, to may be modified by subtracting the time of flight in the spikes  26  and  28  between their tips and their respective sensors of switch in impact cap  66  and accelerometer  82  as follows: 
 
 t   0   =t   m −2 Δt   (2) 
 
         [0071]     where Δt is the time of flight through the steel or other material of the spikes  26  and  28  and t m  is the measured time of flight by the apparatus. The sound speed may be corrected by the measured temperature of the wood.  
         [0072]     If the sound speed for a given measurement is within a predefined velocity range, the system provides feedback to the operator by audible or by some other means to eliminate the requirement for the operator to view the PDA screen.  
         [0073]     At process block  110 , the measure of the quality of the wood is then determined using the sound speed and a number of predictor parameters as arguments to an empirically or theoretically derived function. In the preferred embodiment, the predictor parameters include sound speed as determined above, DBH and the age of the tree, however, tree species, green moisture content, and basic density may also be used. These parameters may be measured for selected trees of a stand or deduced from tree species or other measurements.  
         [0074]     In the present preferred embodiment, the function may be implemented in one of the following forms: 
 
 Y=αX   1   b   X   2   c   X   3   d   (3) 
 
         [0075]     or 
 
 Y=β   0 +β 1   X   1 +β 2   X   2 +β 3   X   3 +β 11   X   1   2 +β 22   X   2   2 +β 33   X   3   2 +β 12   X   1   X   2 +β 23   X   2   X   3 +β 13   X   1   X   3   (4) 
 
         [0076]     Y is the wood or fiber property to be predicted;  
         [0077]     a, b, c, or the β values are experimentally derived constants specific to the equipment and species of the tree; and  
         [0078]     X 1 , X 2 , and X 3  are the predictor parameters of C tree , DBH and estimated age of the tree.  
         [0079]     This data Y may be stored in the PDA  22  and associated with the particular tree which may be marked or identified by location through the use of a GPS device or the like. The information of various trees may be collected to create a “stand summary” providing for stand average values and value ranges for a particular stand of trees. While the forester is collecting data on a plot/stand, when data collected achieve a predefined stand point average, the system provides feedback to the operator by audible or by some other means to eliminate the requirement for the operator to continue any further data collection for that plot/stand.  
         [0080]     Referring now to  FIG. 7 , some features of the present invention, for example the pulse detection technique, may be used in an alternative embodiment having a removable separator rod  130  of fixed length which may be fit against each of the housings  18  and  20  on pegs  132  to assist in attaching the transmitting acoustic probe  12 , and receiving acoustic probe  14  to the tree  10 , and then is removed before measurement.  
         [0081]     Although in an ultrasonic range finder  50  is preferred, other distance measuring techniques may be used including those using laser or optical range finders or radio-based range finders may also provide measurement of the separation of the transmitting acoustic probe  12  and the receiving acoustic probe  14 . Removable mechanical caliper systems may also be used where the separation is automatically measured by encoders, linear transformers, or the like. Different methods than a hammer strike may be used for generating the excitation signal including an ultrasonic piezoelectric transducer attached to the transmitting acoustic probe  12 .  
         [0082]     It is specifically intended that the present invention not be limited to the embodiments and illustrations contained herein, but include modified forms of those embodiments including portions of the embodiments and combinations of elements of different embodiments as come within the scope of the following claims.