Patent Publication Number: US-2006000281-A1

Title: Method and apparatus for assessing or predicting characteristics of wood or other wooden materials

Description:
FIELD  
      The invention comprises an improved method and apparatus for acoustically assessing or predicting one or more characteristics of a tree stem, log or wood piece, or of a wood composite material.  
     BACKGROUND  
      Acoustic technology is increasingly being used in the forestry and processing industries as a means of predicting the inherent characteristics of wood and wood composite materials. It is a requirement of the building industry that the strength of a timber piece be sufficient for its purpose hence measurement of a log&#39;s modulus of elasticity utilising longitudinal acoustic waves as a probing means provides a convenient measure for the forestry industry as such measure is largely independent of the cross sectional area of the timber piece. Typically the sample, be it a tree stem, log, or other wood piece or piece of a wood composite material is hit by a hammer which induces a stress wave within the sample. This stress wave traverses the sample length with a velocity indicative of one or more inherent characteristics of the sample.  
      One type of instrument measures the time taken for a single traverse of the sample length and, knowing the sample length, the acoustic velocity is calculated. This method necessitates transducing both ends of the sample, or alternatively one end of the sample and the hammer. Most instruments use accelerometers to transduce the disturbance although in some instances displacement transducers are used. Commonly the stress wave is induced directly with a mechanical or pneumatic hammer, however the stress wave may be also be induced by an electronic hammer e.g. Silvatest. Usually in these instances the hammer comprises an electronic method of exciting a piezoelectric transmitter or transducer. The controlling electronic signal may be used to indicate the excitation of the sample. The crux is that the stress wave transit time measure is the time measured between excitation of the stress wave and its detection at the receiving transducer. A limitation to the usefulness of transit timer instruments is that the measure is prone to corruption by noise, due at least in part to the need for wide bandwidths to correctly identify starting and stopping points.  
      Another type of instrument records the reverberation of the stress wave within the sample, for a duration equivalent to many transit periods. A single receiver transducer only is required. The hit may occur at the same end as the receiving transducer. The spectral composition of the reverberation is determined typically by Fourier analysis and, knowing the sample length, the velocity calculated. Since many transits of the sample are recorded the calculated velocity is an average for the recording duration, preferably dominated by the plane wave reverberation. The hit must contain frequencies which match and excite the resonance&#39;s of the sample hence ideally an impulse is required generating the stress wave which has fast transitions and short period. To accurately determine a sample&#39;s velocity it is a requirement that the combination of hit amplitude and material absorption be such that the resonance is recorded for many reverberations. The sample&#39;s acoustic absorption dampens the stress wave and imparts an effective window function on the spectral signature, as the absorption increases the resonance peaks broaden (and shift) resulting in reduced accuracy. Generally resonance is less susceptible to random noise; interference on the other hand appears in the output as a sample resonance. Resonance techniques have an inherent ambiguity. A consequence of either the hit spectrum or sample support loading is that the fundamental or other overtones may not be excited or correctly identified. Then the velocity may be incorrectly identified by an integer or integer fraction for example 3, 2, ½, ⅓. Similarly the range of possible velocities may be constrained to less than a factor of two.  
     SUMMARY OF INVENTION  
      In one aspect the present invention may be said to consist in a method of determining a characteristic of a wood specimen to assist in optimising use of the specimen including: exciting the specimen with a frequency varying excitation to impart an acoustic wave into the specimen, sensing a response indicative of the acoustic wave behaviour within the specimen, determining a response characteristic of the specimen using the sensed response, and determining the characteristic using the response characteristic.  
      In another aspect the invention may be said to consist in apparatus for determining a characteristic of a wood specimen to assist in optimising use of the specimen including: a transmitting transducer coupled to the specimen for generating a frequency varying excitation to impart an acoustic wave in the specimen, a first receiving transducer adapted to sense a response indicative of the behaviour of an imparted acoustic wave, and signal processing means adapted for determining a response characteristic from the sensed response, wherein the processing means is further adapted for determining the characteristic using the response characteristic.  
      In another aspect the present invention may be said to consist in apparatus for determining a characteristic of a sample log, stem, wood piece or a wooden composite to assist in optimising use of the sample including: a transducer for generating a frequency varying excitation coupled to the sample at a first position to impart an acoustic wave into the sample, a waveform generator to generate an excitation signal to drive the transducer, a first sensor in proximity to the transducer to sense the frequency varying excitation, a second sensor positioned to sense the response of the imparted acoustic wave within the sample, and a signal processor for determining a response characteristic of the sample from the sensed response, the sensed frequency varying excitation and the excitation signal, wherein the signal processing means further determines the characteristic from the response characteristic.  
      In another aspect the present invention may be said to consist in apparatus for determining a characteristic of a sample log, stem, wood piece or a wooden composite to assist in optimising use of the sample including: a transducer for generating a frequency varying excitation coupled to the sample at a first position to impart an acoustic wave into the sample, a waveform generator to generate an excitation signal to drive the transducer, a sensor coupled to sense a response indicative of the imparted acoustic wave, and a signal processor for determining a response characteristic of the sample from the sensed response, wherein the signal processing means further determines the characteristic from the response characteristic.  
      In another aspect the present invention may be said to consist in apparatus for determining a characteristic of a log or stem to assist in optimising use, the apparatus adapted for use with harvesting equipment and including: one or more drive rollers adapted to move the log or stem longitudinally a waveform generator to generate an excitation signal which stimulates the drive rollers to oscillate and excite the log or stem, a first sensor adapted to sense the excitation signal, a second sensor adapted to sense the response of the log or stem during oscillation, and a signal processor for determining a response characteristic of the sample from the sensed response and the sensed excitation signal, wherein the signal processing means further determines the characteristic from the response characteristic.  
      A method for assessing or predicting one or more characteristics of a tree stem, log or wood piece, or of a wood composite material (herein: specimen) comprising exposing the sample to a continuous excitation energy which varies at least in frequency over a defined time period, simultaneously detecting the resultant acoustic wave energy in the sample over the same time period via a receiver contacting or in proximity to the sample, and determining the sample characteristic(s) using the detected signal.  
      In another aspect the present invention may be said to consist apparatus for assessing or predicting one or more characteristics of a tree stem, log or other wood piece, or of a wood composite material, comprising transducer means arranged to expose the sample to excitation energy which varies at least in frequency over a defined time period, receiver means arranged to simultaneously detect the excitation energy in the sample over the same time period, and means arranged to determine the sample characteristic(s) from the detected signal. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
      The invention is further described with reference to the accompanying figures by way of example and without intending to be limiting, wherein:  
       FIG. 1  is a block diagram of an apparatus for determining a characteristic of a wood specimen,  
       FIGS. 2   a - 2   e  show various waveform representations used to generate an excitation signal for the apparatus,  
       FIGS. 2   f - 2   i  show waveforms present at various locations in the apparatus,  
       FIG. 2   j  shows a known resonant behaviour for analysing a characteristic response of the specimen,  
       FIGS. 3   a ,  3   b  are block diagrams show a preferred embodiment of the apparatus,  
       FIG. 4  is a flow chart showing the functionality of the apparatus,  
       FIG. 5  is block a diagram showing an alternative embodiment of the apparatus,  
       FIG. 6  is a flow chart showing the functionality of the alternative apparatus,  
       FIGS. 7   a ,  7   b  show output waveforms resulting from a multitone excitation signal,  
       FIG. 8   a  is a block diagram of an alternative waveform generator for generating a multitone excitation signal,  
       FIG. 8   b  is a flow chart of the functionality of the alternative waveform generator,  
       FIG. 8   c  shows a response resulting from processing of the output waveform shown in  7   a,    
       FIG. 9   a  is a flow chart of the functionality of an alternative embodiment of the apparatus shown in  FIG. 6  which implements segmented FFT analysis,  
       FIG. 9   b  is a graph showing the segmentation scheme,  
       FIG. 10  shows an implementation of the apparatus in harvesting equipment for logs and stems,  
       FIG. 11  shows a hydraulic system for driving the apparatus shown in  FIG. 10 ,  
       FIG. 12  shows an implementation of the apparatus for sooks, ASTM samples and bolts,  
       FIGS. 13 and 14  show experimental results from the implementation shown in  FIG. 12 ,  
       FIG. 15  shows an implementation of the apparatus in a portable form for testing logs. 
    
    
     DETAILED DESCRIPTION OF PREFERRED FORMS  
       FIG. 1  is a schematic diagram providing a general overview of an apparatus  100  according to the invention for determining a characteristic of a sample or specimen of wood material, such as a log, stem, wood piece, wooden composite or the like. The characteristic may be for example the stiffness of the specimen, determined by the Modulus of Elasticity (MoE). Alternatively it may be another characteristic such as the velocity of an acoustic wave within the specimen, which in turn can be used to establish a characteristic such as MoE. In broad terms the apparatus  100  may assess or predict one or more characteristics of a tree stem, log or wood piece, or of a wood composite material (herein: sample or specimen) by exposing the sample to a continuous excitation energy which varies at least in frequency over a defined time period, simultaneously detecting the resultant acoustic wave energy in the sample over the same time period via a receiver contacting or in proximity to the sample, and determining the sample characteristic(s) using the detected signal. For slow sweep rates, sweep periods typically near or greater than a second, a low drive power level may be desirable. The consequential acoustic stimulation of the sample is small and linear so that harmonic and intermodulation distortions are minimised. All frequencies are generated uniquely and independently stimulating all resonance frequencies individually. Without limitation, the signal may increase in frequency from 1 Hz up to 200 kHz or above, and for a log may typically be in the range 100 Hz to 20 kHz or 500 Hz to 10 kHz for example. Typically, the sweep time period will be less than 20 seconds, preferably less than 10 seconds, most preferably about 5 seconds or less and greater than 0.1 second, and about 2-3 seconds for example. Processing algorithms interpret the spectral characteristics for the samples acoustic wave propagation effects including absorption and velocity properties and the combination of these propagation effects whilst accounting for other effects.  
      More particularly the apparatus  100  imparts a varying frequency acoustic wave into the specimen and then detects and analyses the response in the specimen  104  to that acoustic wave to assist in determining the desired characteristic. A controller  101 , such as a microcontroller, microprocessor or the like controls operation of a waveform generator  102 , DSP  107  and other components of the apparatus as required. The waveform generator  102  generates an excitation signal which is passed to an excitation apparatus  103  including various filters and amplifiers as required and a drive transducer, such as a loudspeaker, roller, piezoelectric element or the like. The transducer is coupled at a first position to the specimen  104  either directly or via a suitable coupling medium to vibrate the specimen  104  in accordance with the excitation signal thereby producing an excitation, for example an acoustic wave, which is imparted into the specimen by way of the coupling. Similarly a receiving sensor  105  is coupled at another position where the response in the specimen  104  to the acoustic wave is detected, and the resulting signal filtered and amplified as required.  
      This signal is then processed using analogue  106  and/or digital signal processing  107  components to determine the desired characteristic, or a suitable intermediary characteristic  108  or parameter which can be used to ultimately determine the desired characteristic. This characteristic may be determined from a response characteristic  109  which indicates directly or indirectly the acoustic response of the specimen. The response characteristic is derived from a sensed response of the specimen, preferably the receiver sensor  105  signal, which is indicative of the acoustic wave behaviour within the specimen. Preferably, although not essential, the characteristic response can be an acoustic transfer function of the specimen which is derived from the receiver  105  signal and one or more signals indicative of the excitation, for example signals relating to the excitation signal and imparted acoustic wave. To do so the apparatus may further include a sensor in the excitation apparatus  103  in proximity to the driving transducer to detect the acoustic wave output from the transducer. The excitation and acoustic wave signals can be sent to the analogue signal processing components  106  to be processed in the analogue domain  106  as shown, or alternatively could be processed in the digital domain.  
      It will be appreciated however that a response characteristic could be determined in various other ways. For example the response characteristic may be the receiving sensor signal itself or a derivative thereof. Further in certain circumstances, such as when the excitation transducer  103  is hard coupled to the specimen, the signal from the sensor in the excitation apparatus  103 , in addition to providing information indicative of the output of the transducer  103 , will also provide a response signal indicative of the acoustic wave behaviour and therefore can be used to derive a response characteristic itself. Similarly an excitation signal used to drive the excitation apparatus may respond to the transducer and therefore exhibit characteristics which indicate the nature of the acoustic wave behaviour in the specimen. To achieve this circuitry in the waveform generator  102  can be implemented to sense the excitation signal. One or more of these signals may be used alone or in combination to determine a response characteristic of the specimen.  
       FIGS. 2   a - 2   i  show, by way of example, waveforms generated at various points in the apparatus during operation. The frequency varying excitation signal is generated from a sample store in the waveform generator  102 .  FIGS. 2   a - 2   e  show plots of example stored samples which can be used to generate the frequency varying signal. The signal may linearly increase or decrease in frequency as shown in  FIGS. 2   a  and  2   b  respectively, or may increase or decrease at a linearly increasing or linearly descreasing rate as shown in  FIGS. 2   c  and  2   d . Alternatively the excitation signal may be a combination of two or more signals, each of which may increase or decrease, for example as shown in  FIG. 2   e . It will be appreciated that many other suitable varying frequency excitation signals could be envisaged by somebody skilled in the art and those depicted are not exhaustive. Preferably the frequency is varied in a continuous sweep over a time period at a suitable rate such that a steady state response is reached within the specimen which can be analysed. The frequency varying excitation signal which is used to drive the transducer, causes the transducer to output a frequency varying excitation, for example an acoustic wave. The acoustic wave is then imparted into the specimen although it will be appreciated that this wave may differ from the excitation due to coupling effects and the like. Similarly the acoustic wave in the specimen may differ in nature from the excitation signal due to various responses of components in the apparatus and the response of the specimen.  
       FIG. 2   f  shows an excitation signal generated from a sample store such as that shown in  FIG. 2   a .  FIG. 2   g  shows the detected acoustic wave which is output from the transducer and used to excite the specimen  104 . The frequency has been scaled down by a factor of 100 to 5 to 30 Hz in  FIGS. 2   f  and  2   g  to clearly indicate the phase continuous nature of the sweep. The attenuation visible in  FIG. 2   g  provides an example indication of a possible response characteristic of the driving transducer.  FIG. 2   h  displays the response wave which is set up in the specimen as a result of the acoustic wave stimulus. The resonant frequencies  209 - 213  of the specimen are apparent in the plot and can be used to assist in determining the desired characteristic of the specimen.  FIG. 2   i  shows a plot of an admittance transfer function in the frequency domain of the specimen which also displays resonance information. It will be appreciated that the excitation signal frequency varies with time and therefore the time domain plots have at least some shape correlation to the equivalent frequency domain plots. Preferably the excitation signal is frequency varying and substantially continuous over a predetermined period over which the analysis is conducted. Further, preferably the indicative response of the acoustic wave behaviour in the specimen is sensed substantially simultaneously to the excitation and over the same predetermined period.  
       FIGS. 3   a  and  3   b  show a preferred form implementation of the apparatus  100  adapted to carry out a three second single tone sweep of a sample over up to around ten octaves. In the embodiment shown the specimen is a log although it will be appreciated that the apparatus can be adapted for other specimen types. As the apparatus described can be adapted for use with a large range of different types of specimens details set forth here may deviate from implementations for other specimen types. For example the sweep frequencies and durations will differ depending on the nature of the specimen being tested. Any such variations will be known to those skilled in the art. Part of the functionality of the apparatus is implemented using various DSPs and other components configured to perform the required signal processing in the digital and/or analogue domain as required. The components along with the function they are configured to carry out will therefore be described with reference to  FIGS. 3   a  and  3   b  in combination with the flow chart depicted in  FIG. 4 .  FIG. 4  indicates which components shown in  FIG. 3  carry out the steps. It will be appreciated that the digital signal processing portions of the apparatus  319 - 325  may be implemented in any manner known to those skilled in the art, for example on one or more DSPs.  
      The accompanying diagrams of  FIG. 2   f - 2   i  are indicative of the waveforms that may be observed at the points indicated in  FIG. 3  for a sweep from 500 Hz to 3000 Hz in 3 seconds.  
      The tone or wave generator  102  of the apparatus  100  includes a counter  301 , waveform memory store  302  DAC  303  and antialias filters  304 . The samples describing the linear increasing frequency sweep are stored in time sequence in the waveform store  302  before the sweep occurs.  FIG. 3   b  shows further detail of the wave form store along with and example plot of samples stored within, which in this case represents a linearly increasing frequency waveform.  FIG. 2   f  is a representation of the sweep or excitation signal which may result. The frequency has been scaled down by a factor of 100 to 5 to 30 Hz in  FIG. 2   f  to clearly indicate the phase continuous nature of the sweep as mentioned previously.  
      The graph in  FIG. 3   b  indicates the tone frequency within the waveform store in this case increasing from F 1  to F 2  in T seconds. A single tone sweep with a constant sweep rate [HZ/sec] as shown implies an unequal acoustic wave energy accumulation period across the sweep. At the fundamental for the sample, say 100 reverberations occur in the receiver bandwidth period. Then at the tenth harmonic 1000 reverberations will occur in the same period. This need not be the case; a linear increasing sweep rate (for increasing frequency) implies equal accumulation. In the system shown in  FIG. 3   a  the counter  301  includes sweep time and phase counters. The memory  302  stores one full or multiple period of the sampled I and Q  326 ,  327  waveform components to be generated, driver and receiver sweep gain control samples plus an instantaneous phase increment value. The result of the phase counter  301  is used to address the waveform memory  302  samples, and is hence the instantaneous phase. The result of the sweep time counter  301  is also used to address the sweep gain samples in the memory  302  and instantaneous phase increment value. At every cycle of the system, about 2 MHz, the waveform memory I and Q samples are combined in a summer  328  and used to update the DAC  303  and analogue output antialias filters  304  to generate the excitation signal  205 . The sweep gains are then adjusted, and the instantaneous phase increment value is added by the phase counter  301  to its current value. For a constant phase increment value a single tone will result; for a constant rate of increase of the phase increment value a linear increasing frequency ramp will result as shown in  FIG. 2   a . A linearly increasing rate in the phase increment value achieves a linear sweep rate as shown in  FIG. 2   c  and hence equal energy accumulation.  
      The excitation signal  205  is then passed to a drive gain amplifier  305 , power amplifier  306  and the output signal passed to the driver  307  such as a loudspeaker or other suitable transducer. The driver  307  is coupled the specimen  104 , preferably at one end, via a coupling frame and coupling media  309  which may be air or another suitable acoustic coupling media. Preferably a driver sensor  308  is mounted on the coupling frame either in proximity to or directly on the driver to detect the actual acoustic wave output from the driver  307  which in turn is imparted into the specimen. This sensed wave is indicative of the excitation, ie the output of the driver  307 , as well as in certain circumstances providing an indication of the acoustic wave within the specimen. The actual drive amplitude and spectral characteristic may be able to be transduced by the drive accelerometer  308  mounted on the coupling frame. The drive forcing function can be inferred from the acceleration waveform. In this circumstance the drive level can be adjusted instantaneously or sweep-to-sweep by the driver gain stage  305  to achieve the desired excitation drive amplitude from the power amplifier  306  and driver  307  whilst limiting peak resonance amplitudes. This method compensates for any broad spectral characteristics, for example a roll off of the driver with frequency, and best preserves the dynamic range of the overall system. Residual drive spectral characteristics are recorded in the drive waveform.  
      A receiver sensor  311  such as an accelerometer is also mounted on the sample using a suitable coupling media  310  so that sample vibrations are fully coupled to the receiver. For sinusoidal waveforms sample velocity and displacements can be calculated from the receiver output. The output of the receiver sensor  311  is indicative of the acoustic wave behaviour in the specimen. A representation  206  of a possible excitation recorded at the driver sensor X  308  is indicated by  FIG. 2   g  and a possible sample response  207  recorded by the receiver sensor R  311  is indicated in  FIG. 2   h . A plot  208  of the resulting magnitude of the transfer function, in this case admittance, is indicated by  FIG. 2   i . It should be noted that the graph indicates the function in frequency space. Translating between time and frequency is a consequence of the sweep frequency definition in time as mentioned previously. The receiver signal is amplified  312  and band pass limited  313  to the sweep bandwidth, amplified  314  and applied to multipliers  317  enabling the complex receive signal to be synchronously detected. More particularly the receive signal  207  is multiplied by the I and Q components of the excitation signal  205  respectively using analogue multipliers. The resultant signals are filtered using antialias filters  318  to extract the receiver signal  207 .  
      The multiplied and filtered receive 207 waveform is then sampled by an ADC  319  and stored in a sample store  320 . A range of signal processing is then carried out by a DSP  321  including bandpass filtering and normalisation of the stored waveforms, low pass filtering and conversion of the results between vector and polar coordinates. The multipliers provide a down converter function enabling a wideband excitation with low sample rate ADC converters  319  and to limit the sample store  320  size and subsequent data processing. It has been found that for very small samples excitation frequencies up to 200 kHz can be desirable. It has also been found that a receiver bandwidth, set by the low pass filter corner frequency implemented in the DSP  321 , of about 100 Hz provides adequate electronic resolution to enable-measurement of typical wood samples of length less than around 0.3 metre i.e. the low Q of the wood samples implies a reverberation bandwidth significantly greater than that of the receiver bandwidth. This receiver bandwidth is implemented in firstly the analogue antialias filters  318  and then subsequently in the DSP  321 . Narrower receiver bandwidths, which may be desirable for longer sample lengths, are then conveniently achieved in the digital domain; a bandwidth of typically less than 10 Hz would be used for logs of several metres length.  
      The driver sensor signal  206  is similarly amplified  315  and band pass limited  316  to the sweep bandwidth, multiplied  317 , filtered  318  and detected in identical fashion to that of the receiver waveform  207 . The multiplied and filtered sensor  207  waveform is then sampled by an ADC  319  and stored in a sample store  320 . A range of signal processing is then carried out by a DSP  321  including bandpass filtering and normalisation of the stored waveforms, low pass filtering and conversion of the results between vector and polar coordinates. The drive bandwidth is achieved identically to that of the receiver. The channels are identical except in the sensing function and that the receiver channel incorporates an additional gain function  314  identical to that incorporated in the driver output i.e. the receiver stage gain can be adjusted instantaneously or sweep-to-sweep to achieve the desired detection amplitude. The receiver signal  207  and the actual drive signal  206  are detected synchronously by the excitation signal  205  ie the measurements occur simultaneously with the excitation, and further the excitation extends for the entire measurement period. A plot of the magnitude term alone is the usual representation of the processed X and R waveform functions i.e. the magnitude of R determined in this way represents the magnitude of the received sensor signal sine wave  207  and similarly the magnitude of X determined in this way represents the magnitude of the sensed acoustic wave signal  206 . Timing is achieved through the use of controller  330 . For suitably sized samples i.e. logs or stems with a diameter say greater than 50 mm, the measurement can be single ended i.e. the driver and receiver may be located at the same end.  
      Once the samples have been processed a response characteristic is produced using a transfer function calculator  322 . In the preferred embodiment an admittance transfer function  208  is produced although it will be appreciated an impedance transfer function could be obtained. To find the transfer function the sample or specimen can be modelled as a number of mechanical resonant filters stimulated by the drive forcing function in a manner known to those skilled in the art. The complex mechanical impedance Zm, defined as the ratio of driving force F to the resultant acoustic wave velocity v at the particular driving frequency is 
 
 Zm=F/v   (1) 
 
 which has a small real amplitude when the drive frequency is coincident to a mechanical resonant filter frequency, and consequently the instantaneous power transferred from the drive to the sample is high—large vibration amplitudes result. Similarly when the drive frequency is not at a mechanical filter resonant frequency the mechanical impedance is high [includes reactive terms], the instantaneous power transferred between the drive and sample is low and low vibration amplitudes result. This system measures waveforms closely approximating the forcing function and resultant vibration velocity. By determining the receiver and drive waveform ratio the sample admittance Y may be approximated at the particular drive frequency since 
 
 Y= 1 /Z=v/F   (2) 
 
 For a sweep, determining the admittance throughout the frequency sweep determines the sample admittance transfer function spectrum  208 . Admittance is a less commonly used concept however in this instance more intuitive. At resonance admittance peaks, high velocity amplitudes result for a constant driving force, the velocity being in phase with the applied force. Further by measuring the in phase and quadrature components of these complex waveforms the real and reactive components are identified, and a precise measure of each resonant frequency determined. For such sweep rates the spectral characteristic is “time invariant” since the stimulating drive is effectively constant i.e. the receiver waveform at all frequencies is a consequence of many transits of the acoustic wave and consequently measures the plane wave response. Slow sweep rates do not impart an envelope function on the resonance characteristic in wood samples and therefore the admittance can directly provide the spectral transfer function of the sample i.e. it does not require subsequent transformation or spectral modification. The relative resonance peak amplitudes and resonance peak shapes [Q] reflect the acoustic absorption effects within the sample. In some instances the samples spectral characteristic is approximated, to a first order, by examining the receiver waveform amplitude  207 . For loosely coupled constant amplitude spectrally flat drive the receiver response amplitude will exhibit peaks at the sample&#39;s resonances eg  209 - 213  in  FIG. 2   h . In a preferred embodiment however the actual admittance transfer function  208  is obtained. This done by finding the ratio between the receive waveform  207  and the sensor waveform  206  which are synchronously detected. This ratio relates to an approximation of the admittance function in defined in equation 2. The admittance measure can be thought of, in a limited way, as an extension of this first order system whereby it is enhanced to accounting for the driver  307  spectral characteristic i.e. the receiver amplitude response may be normalised by dividing it by the driver  307  amplitude response. In doing so residual spectral characteristics in the driver response  307  are removed. 
 
      Unlike the transit time and resonance measures this method ensures that all frequencies are stimulated and that noise is less of a concern. Hence it is not uncommon for logs of length l metre or greater to clearly distinguish overtones up to and beyond the tenth harmonic. The low inherent noise is a consequence of the sweep drive method. The acoustic wave energy effectively integrates within the log within the receiver bandwidth period. For example for a log of length 2 metres a sweep may start at say 500 Hz and stop at 10 kHz (10th overtone) and with a single tone occur over say a 3 second period. Then for a receiver bandwidth of 100 Hz the energy accumulation period is 32 milliseconds, for all frequencies individually in the sweep. Identical, precise stimulus even to high input energy levels can very easily be attained. This has to be compared to the energy input period of the hammer methods. One would expect the hammer hit induced stress wave for such a log to occur with a period consistent with the swept bandwidth i.e. 2 milliseconds to 0.1 milliseconds, for all frequencies simultaneously. The greater the energy accumulation period the greater certainty in the result. The sampling rate and the sweep definitions determine the actual spectral resolution. In the system described 8192 samples are collected in a 3 second sweep i.e. approximately 3 Hz sample resolution in the 2 metre log example.  
      The resonance peak shape and amplitude of the receiver signal  207  and admittance transfer function  208  reflect the acoustic phenomena within the sample. One of the resonances, preferably the fundamental can then be used to find the velocity of an acoustic wave in the specimen  104 , and then the velocity used to find the MoE. To extract the resonances the digital signal processing portion of the apparatus further includes a peak shape analyser  323  which determines peaks which correspond to a desired shape, overtone analyser  324  which determines related harmonics and characteristic calculator  325  which determines acoustic velocity and MoE. The resonance extraction process will described in relation to the admittance transfer function however it could be applied to the receiver signal  207  if required. Algorithms implemented in these components identify the peak shape and overtones sequences in the sample admittance transfer function spectrum. These alogorithms find a resonance from the transfer function  208  firstly identifying peaks in the response characteristic which exceed a magnitude threshold, have a shape which substantially correspond to a general resonance model within a predetermined level of fit, and have a Q which falls within a predetermined range. these peaks are then analysed to identify those groups of peaks which have centre frequencies which substantially correspond with known resonant behaviour of the sample and then to identify the group of peaks which best correspond with the known resonant behaviour. Once the group of peaks are identified, which are assumed to correspond to resonances, one or more of the peaks are used to calculate a resonant frequency, preferably the fundamental. Preferably the fundamental is found by analysing most or all the peaks in the identified group.  
      More particularly, peaks are individually analysed by correlation or other means to ascertain the best shape factor and degree of fit with known predetermined peak shapes. Appropriate predetermined shapes would be those derived from the general resonance Q equations known to those skilled in the art, which have a form  
       Q   =     fr   fb         
 
 such as that shown in  FIG. 2   j , where fr is the resonance frequency and fb is the resonance bandwidth. Resonance peaks that are either insufficient in relative magnitude or exhibit a poor degree of fit or have a shape factor which is unacceptable, for instance an apparent Q say less than 5 or greater than 500, are rejected.  FIG. 2   j  is the first three peaks of the 2.2 m log of  FIG. 6 . The measured data is shown as the dotted line. Overlying the three peaks is a solid line being the expected form of the peaks from the general resonance equations with the same Q value applying at each peak in this instance. The fit is clearly acceptable. It is to be expected that successive overtones of a sample response will have similar shape factors for similar magnitudes and that the shape factor will trend with the overtone number consistent with absorption phenomena that generally increases as the square of the frequency for viscous damping for instance. It is by these means that interference signals are firstly rejected. The resonance frequency of the overtones can also be expected to trend with the overtone number and not necessarily be harmonic. Sweeping a sample over a ten harmonic range implies a possible absorption range of two orders of magnitude. Consequently, especially for small samples say with fundamental frequencies exceeding 10 kHz, dispersion may be evident in the overtone resonance frequency sequence dependent on the magnitude of the absorption. This effect is distinguishable from sample loading effects since dispersion effects due to damping correlate with the peak shape factor and also by the nature of the relationship with frequency. In all circumstances the actual fundamental is determined by iteration. Each resonance peak is individually tested with each other peak for consistency with the relationships defining the fundamental and overtone frequencies for the above effects. 
 
      By way of example consider a harmonic sequence of overtones with an additional interference signal at one half the fundamental. By simply matching the interference signal without regard to the expected relationship, in this instance fn=n*f 0  a supposed perfect match is achieved since the harmonic sequence appears as the even harmonics of the interference, an incorrect velocity could be attained. It will be appreciated by those skilled in the art that other relationships between the fundamental and harmonics may be displayed by specimens. For example where a specimen has a large width or diameter in relation to its length, the harmonic relationship may not be an integer multiple. By testing each peak with each other peak using the expected relationships the interference and harmonic sequences can be differentiated, an error occurs for missing or misidentified peaks. The sequence that maximises the number of admittance peaks accounted for and minimises the number of errors is accepted as the harmonic sequence. For example, the sequence or group of peaks which have the greatest combined amplitude are selected as the resonant peaks of the specimen. The fundamental frequency can then be determined, or alternatively one of the other harmonics. Preferably however the fundamental is determined using most or all of the detected harmonics. Incorporating the overtones enhances the precision to which the fundamental is determined since having identified the sequence the fundamental may be calculated as the average of the calculated value determined for each overtone existing in the sequence. From the fundamental f and knowing the sample length l the acoustic velocity v is calculated in the characteristic calculator  325  according to: 
 
 V= 2 fl  
 
 From the acoustic velocity the modulus of elasticity MOE may be determined using the standard formulation, for a known density p 
 
 MOE=pv   2  
 
 or other suitable means. 
 
      In some instances it is preferable but not essential to implement a modified detector scheme as shown in  FIG. 5 . The components along with the function they are configured to carry out will therefore be described with reference to  FIGS. 5   a  and  3   b  in combination with the flow chart depicted in  FIG. 6  which indicates the components shown in  FIG. 3  that carry out the steps. In this implementation both the drive  206  and receiver  207  waveforms are simultaneously digitised directly by the ADC  319  and the individual components detected digitally. This is possible since the waveform generator  301 - 304  and analogue converters are synchronised enabling sample by sample evaluation of the drive  206  and receiver  207  outputs.  FIG. 6  indicates the time sequence of operations for this alternative embodiment using a single tone sweep. In this instance the system comprises two processes, a stimulus and data capture process and a subsequent analysis process. The samples describing the sweep are stored in a time sequence in the in phase (I) and quadrature (Q) buffers  326 ,  327  shown in  FIG. 3   b  before the sweep occurs. The graph in  FIG. 3   b  indicates the tone frequency within the waveform store in this case increasing from F 1  to F 2  in T seconds. During the sweep the waveform samples are added and then output to the DAC  303  in the time sequence and filtered  304 . Drive amplitude control  305 ,  306  may be applied as previously described to generate the stimulus waveform. The driver sensor (X)  206  and receiver sensor (R)  207  waveforms are filtered  315 ,  316  and  312 - 314  respectively, gain amplitude controlled  314  as previously described, converted by the ADC  319  and recorded in the sample store  320  in time sequence. On the completion of the sweep the waveform and sample store are subsequently used by the processing and analysis routines implemented in the digital signal processing components  321 - 325 . The X and R waveforms  206 ,  207  are band pass filtered and normalised in amplitude by routines implemented in the DSP  321  before presentation to the tone detection stage. In this implementation the tone detection is achieved by a digital multiply and low pass filter method which are also implemented in the DSP  321 . The waveform store  326  I data is multiplied with the sample store  320  X data in the time sequence and the result stored in an intermediate product buffer in the DSP  321 . This is repeated for the waveform store  327  Q data on the sample store  320  X data, and also both I and Q data on the sample store  320  R data. The intermediate product buffers comprise frequencies (A+B) and (A−B) for each frequency in the sensor waveform since the product of two sine waves is given by: 
 
sin A *sin B =sin A ( A+B )+sin( A−B ) 
 
 The unwanted frequencies are rejected by filtering the intermediate buffer product with a low pass filter leaving (A−B) implemented in the DSP  321  which in this instance for low sweep rates is close to dc. The bandwidth of the filter is set as suggested earlier, about or less than 100 Hz for sample lengths less than 0.3 m and less than 10 Hz for samples several metres long. The resulting complex X and R waveforms then describe a bandwidth limited version of the sample store  320 , the bandwidth being the low pass filter bandwidth with the center frequency of the filter at any instantaneous point in time being the original waveform store frequency i.e. the stimulus  205 . The waveforms are converted by a routine in the DSP  321  to a polar form for convenience, a plot of the magnitude term alone being the usual representation of the processed X and R waveform functions i.e. the magnitude of R determined in this way represents the magnitude of the received sensor signal sine wave  207  and similarly the magnitude of X determined in this way represents the magnitude of the sensed acoustic wave signal  206 . The admittance transfer function for the sample is calculated as previously described, for slow sweep rates this measurement outcome directly provides the spectral transfer function of the sample. The admittance transfer function peaks reflect the resonance and absorption phenomena occurring within the sample. The resonance peaks are detected using the peak analyser  323  from other effects based on the apparent Q and degree of fit to the Q curve as discussed previously. The centre frequency of peaks meeting the criteria is then used by the overtone analyser  324  to determine a fundamental resonance frequency from which the acoustic velocity is calculated. 
 
      The alternative embodiment of the apparatus shown in  FIGS. 5 and 6  can be modified to impart multitone or arbitrary compression drive of the sample. For example, a multitone acoustic wave can be generated from stored samples such as that shown in  FIG. 2   e  in which two or more varying frequency acoustic waves are substantially simultaneously imparted into the specimen  104 . This facilitates a major reduction in the sweep period, preserving the receiver bandwidth, energy integration effects and spectral resolution. This is especially desirable in the testing of sooks for instance, the short 100×50 mm or similar lengths used in finger jointing where throughput is high. In the above 3 second single tone sweeps over 10 overtones have been given as an example. Since the system is linear with small amplitudes tones can be superimposed. For example, the sweep could consist of a tonal sequence comprising a fundamental f o  acoustic wave superimposed with both 2.2*f o  and 4.84*f o  acoustic waves for a linear sweep rate. Then the total sweep period equivalent to the single 3 second single tone linear sweep f o  to 10*f o  is 0.4 seconds. The tonal sequence is recorded in the waveform memory and operates as previously described. This sequence was chosen to avoid the simultaneous occurrence of harmonic overtones thereby minimising harmonic distortion effects and the possibility of harmonically related large combined vibration amplitudes. Other sequences could be used in preference. If a harmonic sequence is used then the equivalent total sweep period can be reduced to 0.3 seconds. It can be advantageous to utilise say six tones or more. As the number of tones approaches or exceeds the number of octaves spanned the sequence is more easily chosen to be nonharmonic.  
      When viewed in the time domain the resultant waveform  700 , for example as shown in  FIG. 7   a , looks increasingly noise like. This is not in fact the case, the waveforms must be interpreted by a synchronous detection system as described previously, for example in relation to  FIGS. 5 and 6 . In particular, the synchronous detection extracts resonance peaks from the output waveform  700  relating to the resonances setup in the specimen in response to each of the component acoustic waves which were imparted. That the time domain waveforms look this way is of great significance. A stress wave hit waveform imparts energy to the sample at the hit only, which by necessity must be a very small fraction of the measurement period. For a swept drive system energy transfer occurs whenever the samples admittance allows, at each overtone. For a complementary multitone sweep, ie an increasing and decreasing frequency for each tone, nonharmonic multitone stimulus waveform energy couples to the sample substantially constantly throughout the measurement period.  FIGS. 7   a  and  7   b  show plots  700 ,  701  for a log of length 2.2 metres for a swept frequency range 500 Hz to 7000 Hz for 0.5 seconds with a complementary 12 tone multitone sequence exciting the sample.  FIG. 7   a  shows the receiver sensor time trace which unless observed by a synchronous detection system is difficult to interpret. However the magnitude plot  701  indicated is similar to that of the peaks in  FIG. 2   h  and  FIG. 2   j , the points of maximum energy transfer and signal. The plot  701  in  FIG. 7   b  is the admittance transfer function spectrum. On average the tones are only 270 Hz apart spanning 541 Hz, with a resonance bandwidth of about 50 Hz. As can be observed in the time trace energy coupling occurs at all times during the entire sweep period.  
       FIGS. 8   a  and  8   b  show block diagram and a corresponding flow chart of an embodiment which expands the single tone system of  FIG. 5  to facilitate multitone drive. The waveform store  302  is extended to include I and Q component sample stores A 1l , A 1Q , A 1′I , A 1′Q    800 - 803  to A n1 , A nQ, A   n′1 , A n′Q    804 - 807  for each additional tone added.  FIG. 8   a  shows two plots  812 ,  813  showing a matching increasing A x    808 ,  809  and decreasing A x ′  810 ,  811  frequency respectively for each of the tones of the multitone. The graphs  812 ,  813  indicate the tone frequency at the point in time within the waveform store  302 . Note that in this example tone  1  spans from frequency  1  to  2  and that tone n spans from frequency n to n+1. The waveform store samples  800 - 807  are summed  814  as before to form the stimulus waveform. Each of the tones of the multitone may be detected in similar fashion to that of the single tone sweep as indicated in  FIG. 5   b , individually the I and Q of each of the tones of the waveform store are multiplied and low pass filtered by the X and R waveforms. The complementary increasing and decreasing complex resultant X and R waveforms are combined during the conversion for each tone. An expanded buffer is formed by sequentially recombining the overlapped time multitone into a nonoverlapped concatenated frequency axis as shown in the graph in  FIG. 8   c.    
      Whilst the multiplier/low pass filter method of tone detection provides excellent detection and rejection performance and preserves the complex X and R waveforms it does require significant computational effort since each tone must be processed individually for the duration of the sweep period.  FIG. 9   a  shows a portion of an alternative process undertaken by the DSP  321  where detection of tones occurs in parallel in the frequency domain, the remaining steps not shown being the same as described previously.  FIG. 9   b  show a corresponding representation of the segmentation scheme. In normal use this can achieve a good compromise between spectral performance and computational speed, however phase information would usually be lost. Referring to  FIG. 9   a  the X and R waveforms are detected and band pass filtered and normalised as before. A segmented FFT algorithm is then implemented in the DSP  321  wherein the sample store buffer period  901  (visible in  FIG. 8   b ), is divided into a number of equal time width samples, for example 32. Each of these is windowed  900  and Fourier analyzed  901  as a part of the segmented transform. Usually for a segmented transform the spectrum outcome is then recombined by summation  903  of each of the individual transforms since the outcome power spectral density is the combination irrespective of the time sequence sample. If however the resulting frequency space for each sample is truncated in such a way that it only contains data corresponding to frequencies spanned by the waveform store in the same time period then a quantised tracking filter function is achieved. Multitone signals may be processed in parallel in the frequency domain by appropriate truncation of the sample transform using a bandpass truncation algorithm  902 .  
      The resulting waveform is then processed in the usual way by calculating the transfer function, determining the peaks, detecting the harmonics and calculating the characteristic. As is sometimes done in segmented Fourier analysis, an additional set of samples offset by one half of the sample width is also analyzed, ie windowed, FFT, frequency truncated and summed into the resulting outcome. In this way the effect of the window function is diminished. For a six tone sweep over 5 octaves using a overlapped segmented transform as above the resulting bandwidth is equivalent to that stated above i.e. about or less than 100 Hz for length 0.3 m or less than 10 Hz for samples of several metres length.  
      Implementations of the apparatus will now be described with reference to  FIGS. 10-15 .  FIG. 10  indicates a means  1004  by which tree stems  1000  may be measured during harvesting. It is common practice for trees to be felled, delimbed and cut into logs by a log harvester which typcially cromprises a digger tractor unit with log harvester head  1004  attached to its boom. The head  1004  utilises one or more hydraulic motor driven rollers  1001  (of which one is visible) to propel the stem through the head and to remove branches and the like from the stem by way of a delimbing apparatus  1002 . A stem motion roller  1003  can detect movement of the stem to establish various parameters such as stem length. To implement the present invention in such an apparatus the rollers  1001  can be adapted to impart an acoustic wave into the stem  1000  by oscillating the stem longitudinally using. Usually stems are longer than 25 metres long hence the swept frequency range is preferably from say 20 Hz to 400 Hz (6 th  overtone) for typical acoustic velocities. This is within the useful frequency range of servovalves which control the hydraulic rollers  1001 .  FIG. 11  includes a simplified hydraulic circuit which could be operated in a manner to drive one of the hydraulic roller such that it oscillates the stem  1000 . The stem propelling rollers  1001  spaced around the stem are driven by bi-directional hydraulic motors. A system pump  1103  channels hydraulic fluid from a sump  1104  to a high pressure reservoir  1102 . The high pressure hydraulic fluid is used to rotate the roller  1001  in a controlled manner by way of a pressure proportioning servovalve  1105 . The left port of the motor is maintained at an intermediate pressure, about one half of the high pressure feed from the high pressure reservoir  1102 , by a pressure regulating system including an intermediate pressure regulator  1100  and intermediate reservoir  1101  which can both source and sink flow. The stem  1000  may be propelled through the head by a control system which sets the pressure proportioning servovalve  1105  so that the right port of the hydraulic motor is say at high pressure to drive the stem in one direction or to sump  1104  pressure to drive the stem  1001  in the other direction. Alternatively an acoustic wave of known frequency can be imparted within the stem by modulating the pressure portioning servovalve  1105  with the desired frequency and amplitude. Stem motion is detected by the idler rollers  1103  and independent sensors (not shown) which may be in the rollers  1103 , or coupled in another manner to the stem  1000 . The swept drive method facilitates a means by which the existing harvesting system can be utilised to implement the invention by imparting acoustic energy into the stem by the circumferential propelling rollers over a period of time, for example three seconds. The narrow bandwidth detection system described previously can be used to eliminate nonlinear responses, extraneous vibrations or other interferences.  
       FIG. 12  indicates an implementation used for the testing of bolts, sooks, ASTM samples and the like. These samples are usually small and therefore direct mechanical coupling of a driver and in some instances a receiver may influence the acoustic properties. A transducer  1204  such as a piezoelectric or conventional loudspeaker is air coupled to the specimen  1201  and is driven by an excitation signal which is passed through a power amplifier  1203 . An accelerometer  1206  is attached to the transducer  1204  to detect the acoustic wave output by the transducer. An optical vibrometer  1205  measures response to the acoustic wave which is imparted into the specimen  1201 . A system controller  1202  contains the functionality required to operate the system as described previously. The swept system can provide reliable data with low level acoustic excitation, typical of air coupled systems. This is a consequence of the energy accumulation within the sample in a given bandwidth period coupled with narrow band detection. The use of multitone stimulus enhances the signal level by effectively extending the accumulation period in proportion to the number of tones used. For instance it has been found that for bolt testing a small loudspeaker may be placed several hundred millimetres from the bolt end and reliable measurements achieved for single tone sweeps of about a second and substantially less than a second for multitone sweeps. That the sample need not be held or positioned dramatically reduces the mechanical complexity in a production environment. The method equally applies to any larger sample and may be single ended i.e. the driver and receiver located at one end of the sample. It is found that for bolts, samples 100 mm diameter and of length 300 mm or greater direct coupling of the receiver does not significantly influence the result, the sensor can be selected for dimensions and mass very small compared to the sample thereby minimising sensor cost.  
       FIG. 13  indicates the outcome of a trial of 110 ASTM size samples of length 300 mm and of cross section 20 mm×20 mm. Swept drive apparatus was used to determine an acoustic velocity from which a ‘dynamic’ acoustic modulus (MOE) can be calculated. The graph plots the acoustic modulus with that measured by static deflection of the sample when loaded by a mass. Clearly the data indicates a good agreement, the correlation coefficient R2 being 0.98.  FIG. 14  indicates the outcome of a trial of 31 bolts, sections of length 200 mm to 600 mm of a 3 to 10 year old tree stem, typically the diameter range is 60 mm to 200 mm. The ‘dynamic’ acoustic modulus was determined using swept drive equipment. The bolts were then fitted with displacement sensors and an axial compression applied to determine the static modulus. Given the irregular shape of the samples, being simply debarked tree stem, a good agreement was found, correlation coefficient R2 of 0.91.  
       FIG. 15  shows one embodiment of a transmitting transducer and acoustic wave sensor arrangement which can be used for in relation to analysing specimens such as logs or stems. A coupling frame  1506  supports a magnet and coil assembly  1503  which drives a former  1504  to provide the compression force. Alignment dampers  1500  support the former  1504 . A coupling face plate  1507  of the frame  1506  is rigidly coupled to an end of the log  1500 . An accelerometer is placed at the face plate  1506  to sense the acoustic wave which is generated. For large samples, such as logs about or greater than 2 metres long, loading by the driver motional elements is unlikely to be significant i.e. the natural resonance frequencies are unlikely to be significantly disturbed by the driver mass. In this circumstance the driver motional elements may be rigidly coupled to the sample. A single monitor accelerometer only may be required. The driver stiffness, a combination of the driver source and coil impedance, transformation factor, relating electrical to mechanical quantities, and motional element compliance is chosen along with the excitation levels to match the energy transfer requirements. The accelerometer waveform by itself conveys the resonance signature within the limitations of the drive spectral characteristics. Both conventional dynamic or piezoelectric speakers can be used. Piezoelectric devices prove to be particularly useful since the motional component mass may be as low as 1 gram and the sample and accelerometer may be directly coupled to the piezoelectric element face which is usually flat. A magnetic actuation is used since this provides a wide frequency range controlled actuation with a large possible range in quiescent point, necessary for handheld operation. A pair of alignment dampers maintains the former alignment, the coupling face may be brought into contact with the log directly or via a compliant medium. The unit is sealed and mechanically isolated by the incorporation of the sealing foam.  
      The loudspeaker and its driving method are chosen to achieve a constant or known amplitude characteristic over the sweep. If wide frequency range conventional dynamic loudspeakers, or some other types, form the basis of the excitation the drive sensing function may be, in an indirect way, achieved by monitoring the driving waveforms. For a loudspeaker excited with a voltage waveform the resultant complex current in the voicecoil is a consequence of the electrical and motional impedances. Typically an exciter system is designed for low losses hence the motional impedance is dominated by the reactive component. The electrical reactive component will be necessarily small, compared to the resistive component, to allow wide bandwidths to be achieved. Thus detection and analysis of the complex current, as could be achieved with the apparatus shown in  FIG. 3 , could be used as the drive monitor. Except at resonance the cone is largely motionless and consequently the motional impedance large. At resonance the motional impedance is significantly smaller. As above for large samples the drive monitor waveform may display the sample resonances; a receiver accelerometer may not be essential.  
      The foregoing describes the invention including preferred forms by way of example. Alterations and modifications as will be obvious to those skilled in the art are intended to be incorporated within the scope hereof.