Plant, Cell and Environment (1989) 12, 371-382

Characterization and propagation of acoustic emissionsignals in \A/oody plants: to\A/ards an improved acousticemission counter

M . T, TYREE & J. S. SPERRY Department of Botany, University of Vet tnont, Burlington, VT 05405, U.S.A.

Received 15 July 1988; received in revised form 21 September 1988; accepted for publication12 December 1988

Abstract. The physics of ultrasonic acoustic emissions( A E s ) was investigated for AE transmission throughw o o d and transducers. The physical propertiesmeasured were velocity, attenuation and frequeneycomposition ol" AEs produced by two sources:cavitation events in xylem and peticil lead breaks.T h e authors also measured the relative sensitivity ofvar ious combinations of ultrasound transducets andampliflers to aid in the selection of a measuringsystem optitnized for cavitation detection iti woodyplan ts . Some of the authois' conclusions are; (1)Softwoods {Thuja, Pinus) attenuate AEs more rapidlyt h a n hardwoods (maple, birch), (2) The velocity ofA E s in wood exceeds that tneasured by others inw a t e r so the main medium of AE transmission tnustbe the cellulose, (3) The strongest frequencies of AEsare in the range of 100-300 kHz, (4) Cavitation-induced AEs tend to shift to higher frequency asw o o d dehydration progtesses. (5) One cannotdetermine the locus of origin of AEs from itsfrequency cotnposition, (6) The frequencycomposition of the acoustic emissions probablycanno t be determined at all with the sensors usedbecause of their tendency to 'ring'. The data collectedin this paper were used to aid in the design of animproved AE counter having a seven-fold increase insignal to noise ratio compared to counters previouslyused in our laboratory. The improved counter, tnodel4615 Drought Stress Monitot, is now cotntnetciallyavailable frotn Physical Acoustics Corp,, Princeton,N J , U,S,A,

Key-words: acoustic emission; xylem cavitation; .icer saeehaiimi:Thuja oecidentalis: Pitius strobus.

Introduction

Since 1983, mote thati a dozen papers have appealedin the literature in whieh eavitation events in thexylem of crops and woody plants have been detectedby the amplification of ultrasonic acoustic emissions(AEs); for example, Tyree & Dixon (1986), Tyrcc etal. (1986), Pena & Gtace (1986) and Salleo &LoGuUo (1986), The ultt"asonic transducers used inthese studies were generally tnodel 8312 (Bruel &

Correspondence: Dr Mel Tyrcc, LX-partmcnt of Botany,University of Vermont, Burlington, "VT05405, U,S,A,

Kjaer, Dentnark); for exatnple, Tyree & Dixon (1983)and Sanford & Grace (1985), The ultrasonicatnplifiers used were either comtnercial (Bruel &Kjacr tnodel 2368) or custom made owing to the highcost of purchase. Counting citcuits have generallybeen custotn-tnade also because of the high cost ofpurchase of commercial units that were oftendesigned with more sophisticated applications inmind (Tyree, Dixon & Thompson, 1984), The presentauthors are not aware of any studies which detertninewhether the ttansducers currently used are optimallysensitive for the detection of AEs in plant systems orwhich evaluate the design of a relatively inexpensivettatisducer-atnplilier-filter-counter combination. In1986, the authors approached a tnajor matiufacturcrof ultrasonic equiptnent. Physical Acoustics Corp.(PAC), to seek funds to research and develop a low-cost cotnmetcial AE counter and transducercotnbination having characteristics significantlyitnproved with tespect to what is currently used in theplant sciences. Funding for this research waseventually obtained ftotn the United StatesDeparttnent of Energy as part of their Stnall BusinessInnovation Research (DOE-SBIR) ptogtam. In thisreport, the authors present work conducted in theirlaboratory as part of the research and developtnentprogram.

The purpose of this study was to investigate thephysics of AE signal propagatioti in the stems ofsotne woody plants in order to aid in the selection ofthe transducer with the best signal to noise ratio forthe detection of cavitation-induced AEs, The physiealproperties investigated wete: (1) the veloeity of AEsin woody stetns gcnctated frotn pencil lead breaks;(2) the ftequency composition of AEs in woody stemsotiginating ftotn cavitation events during thedehydration of excized stem tissue; (3) theattenuation of AEs in woody stems tneasured on AEsboth frotn pencil lead breaks and from cavitationevents; and (4) the measuretnent of the t"elativesensitivity of various AE transducers. Thisinfbnnation was used to design a commercialacoustic emission counter that is optimized for thedetection of ultrasonic acoustic emissions in woodyplants. This new instrutnent, the model 4615 DroughtStress Monitor, is now available from PhysicalAcoustics Corp,, PO Box 3135, Ptinceton, NJ 08540,U,S,A,

371

372 M. T. TYREE & J. S. SPERRY

- 6 0

a:<m

tn•o

- 6 5

- 7 0

- 7 5

i -80

- 8 5

- 9 0 _L

RI5S/N26I3

_L

(RI5S/N 2618

J_50 IOO 150 2 0 0 250 300 350 4 0 0

Frequency (kHz)

Figure 1, Erequency response of the two R15 t tansduccrs used in this study (tnanufacturcr 's data) .

Materials and methods

All measurements were performed on woodysamples. Most measurements were performed onliving woody stem segments collected from thefollowing trees: maple (Acer .saeeharutn). Thujaoeeidentalis and pine (Pinus strobu.s). In all cases, thebark was removed and transducers were coupled tothe wood medium with silicone grease in order toenhance sound transfer and to reduce localizeddehydration. The transducers were held in place byspring-loaded clamps with a constant force of 30Newtons, In some cases, tneasurements wereperformed on rectangular blocks cut from the stetnsegments. Some measurements were also performedon a birch dowel (probably Betula populifolia) ofcommercial lumber origin. The dowel was air dried,90 ctn long and 1.33 cm in diameter.

Velocity mea.surements

Velocity measuretnents of AEs were perfortned onthe birch dowel and on stems of maple and pine. Twotransducers (PAC model RI5) were selected withnearly tnatched frequency responses (Fig, 1), Thetransducers were clamped on the same side of thestem segment or dowel at fixed spacing (usually10 cm). Velocities were tneasured on AEs created bybreaking a mechanical pencil lead (0.5 mm HB) afixed distance to the right or the left of the pair oftransducers. The leads were extended 3 tnm beyond

the tip of the mechanical pencil and broken on thesurface of the wood while tnaintaining a contactangle of about 40 . Cavitation-induced AEs weresuppressed in living wood segtnents by keeping oneend of the stem segtnent in contact with water.Velocities were again measutcd in tnaple and pinesamples after the stem segments were air dried.

A highly reproducible AE signal resulted from thelead break, the wavefortns picked up by bothtransducers were very similar in shape (Fig. 2), TheAE signals wete atnplified by a preamplifier (PACmodel 1220A) and a post-atnplificr (PAC tnodel3104). The preamplifier and post atnplifiers werefitted with sixth-order band pass filters withfrequeticy cut ofi"s of 100 and 300 kHz. The AEsignals were captured on an 8-bit wavelbrtn digitizer(Tektronix 7D20). A tniciocomputer (CompuPro 8-16) was in comtnunication with the digitizer andsome digital waveforms were copied and displayed ona dot tnatrix printer.

The titne of arrival of the leading edge of thewaveforrns were measured directly on theoscilloscope display of the digitizer and the velocitycotnputed from the distance between the twotransducers and the dilTerencc in arrival times of theleading edges of the wavefortns. The overall gain ofthe amplifiers were adjusted as needed to allow forsignal attenuation as the waveform travelled downthe wood sample. In the birch and tnaple samples,attenuation was small so the gain setting for the twotransducers were set equal. In the pine samples.

ACOUSTIC EMISSIONS IN WOOD 373

F i g u r e 2 . T y p i c a l A H s igna l o b s e r v e d for a penc i l l ead b r e a k o n a h i t c h d o w e l . T h e A E s o u r c e w a s 2 5 c m frotn t h e R I 5t r a n s d u c e r .

attenuation was large so the gain of the transducerfarthest from the AE source was set about 20 dBhigher.

Acoustic emission Jrecjuency cotnpositiotx

Ultrasonic AEs created by eavitation events indehydrating maple and Thttfa satnples were measuredwith a broad band transducer (Bruel & Kjaer tnodel83 1 2). The transducer was seleeted at the authors'request by Bruel & Kjaer to have the flattest iesponseof sensitivity vet"sus frequency of all transducersproduced in 1984. The calibration curve isreproduced in Fig. 3. The objective of thesemeasuretnents was to detertnine which frequencies ofcavitation-induced AEs were of latgest atnplitude.This pertnitted us to select narrow band transducershaving a tnaximutn sensitivity centred around thestrongest ft"equency component of the cavitation-induced AEs. Although these tt"ansducet"s were notbroad band, they do have a better signal to noiseratio.

Cavitation-induced AEs picked up by the tnodel831 2 transducers were atnplified by the intetnal 40dBpre-atnplifier contained in the ttansducer and

85 r-

;•£ 75/l

-^ 70

CE6 5 _L

0 200 400 600Frequency (kHz)

800 1000

Figure 3. Fiequency response of the B&K 8312 broad bandtransducer used in this study (manutactiner"s data).

atnplified further by a Bruel & Kjaer tnodel 2638atnplilier with an eight-order 50-kHz high-pass filter.The AEs were captut"ed on the Tektronix 7D20waveform digitizer and the wavefortns up-loaded intoa tnict"ocotnputer. The sampling rate of the digitizerwas 10 MHz (100 ns per voltage teading) andantialiasing filters wete used. The raw wavefortns(which usually represented mixtures of severalftequencies) were analysed by standard discrete fastFourier ttansfottnation (FFT) techniques to produceplots of amplitude versus ft"equency. The plots werethen tnodified to account for the non-flat response ofthe model 8312 transducer. Frequency components inthe FFT, which cortesponded to frequencies wherethe actual sensitivity of the ttansducer was below theaverage sensitivity (horizontal line in Fig. 3), wereincteased in amplitude by the nutnber of dB that thecalibration curve was below the average sensitivity;similarly lYequency components that were above theaverage sensitivity of the transducer wet"e decreasedin atnplitude. Because of the limited dynatnic rangeof the 8-bit wavefortn digitizer, the overall gain of thetneasuring system was adjusted so that only a few percent of the AEs went beyond the full scale range ofthe digitizer (set to +5.12 V full scale). At this gain,the majority of the wavefortns wet"e below 1 voltmaxitnutn atnplitude. Ultt"asonic AEs below 1 voltamplitude or beyond the full scale were rejected inorder to itnpt"ove the accuracy of the discrete FFTanalysis. All plots of amplitude versus ftequency wetenortnalized so that the tnaximum amplitude had arelative value of one.

The above procedures were repeated using anotherbtoad-band transducer (Panatnetrics model VI05).This transducer diffeted frotn the Bruel & Kjaermodel 8312 in the method used to achieve broadband characteristics. The model 8312 used a verystnall transducer crystal having many closely spacedhartnonics and the non-linear response of the crystalwas partly cotnpensated by a cotnplementary non-linear preatnplifier built into the transducer housing.The model VI05 uses a large heavily dampedtransducer crystal designed to have a very fiat

374 M. T, TYREE & J, S, SPERRY

frequeney response (plot of sensitivity versusfrequency), and there was no built-in preamplifler.The model 8312, while broadly sensitive, tends toring harmonically at characteristic frequenciesfollowing a 'hit' by an acoustic event in the same waythat a bell struck by a hammer will ring on for a longtime compared to tbe duration of the initiating event.The model VI05 is much less sensitive than the model8312 but it does not have strong harmonics and ringsfor a much shorter time.

Acoustic emission signal attenuation

Ultrasonic AE signal attenuations were measured onmaple and pine samples using low-frequency andhigh-frequency transducers. The low-frequencymeasurements were made with PAC model R15transducers having a nominal harmonic frequency of150 kHz. The high-frequency measurements weremade with PAC tnodel R80 transducers having anotninal harmonic frequency of 800 kHz. The pre-ampliflers and post-ampliflers were PAC models1220A and 3104, respectively. The band pass flltersused were 100-300 kHz and 600-1200 kHz for thelow- and high-frequency measurements, respectively.

The placement of the tnatchcd pairs of transducerswere as shown in Fig, 4a for maple and birch(satnples having low attenuation) and as shown inFig, 4b for Thuja and pine (samples having highattenuation). Since the transducers were separated aflxed distance apart and usually unequal distancesfrotn the source of the AEs, the amplitude of the AEsreceived by the transducers differed because of signalattenuation. Signal attenuation. A, is defined in thisstudy as the ratio (in dB) of tnaximum AE amplitudedetected by the two transducers, TRl and TR2, i.e.

= (20dB)log[TRl/TR2] (1)

The amplitudes TRl and TR2 are deflned here as themaximum minus minimum voltage amplitude of thewave form (Fig, 4c),

Signal attenuations were measured for AEs ofknown locus of origin in the samples (i,e. a pencillead break at a known place on the sample) and forAEs of unknown loeus of origin (i.e, a cavitationevent frotn an unknown spot in the sample).

Signal attenuations give some crude measure of theeffective 'listening' distance over which AE can bedetected using any given transducer. In order torelate signal attenuations between wood samples, thesignal attenuation. A, was divided by the distanee incentimetres that the transducers were separated. Thisquantity is called here the speciflc attenuation A, (dB/ctn). The value of A, was easily calculated for pencillead breaks because the source of the AE was at aknown locus. For birch, tnaple. Thuja and pinesamples, the transducers were arranged as shown inFig, 4a and pencil lead breaks were to the right or leftof the transducer pair.

The computation of attenuations for tbe

F*"igure 4. Experimental arrangement for attenuationmeasurements; 'd" indicates the distance ol separation betweentransducers, (A) Showing placement of two RI5 transducers (TIand T2) on a birch dowel or maple stem, (B) Showing placement oftransducers on a pine or 'thuja block, (C) An AE curve showingthe meaning ol amplitude.

cavitation-induced AEs was more cotnplex since thelocus of origin of the AEs was unknown. Theamplitude of AEs frotn cavitation events was one ortwo otders of tnagnitude smaller than AEs frotnpencil lead breaks. In otder to get signiflcantnutnbers of coincident events in both transducers, thearrangetnent in Fig, 4b had to be used for Thuja andpine satnples. Several thousand AEs were countedsitnultaneously in two transducers separated by aknown distance, d, A histogratn was constructed ofthe number of AEs having a signal attenuationwithin discrete ranges of 1 dB each. For maplesatnples (Fig, 4a), the histogram had two distinctpeaks separated by XdB; A, was equated to X/2d.For pine and Thuja satnples (Fig. 4b) the histogtamhad one peak of width Y dB measured at the halfheight of the peak; A, was equated to Y/2d. See theappendix for a discussion of how A was calculatedfor AEs frotn unknown loci, and for a discussion ofthe physical factors likely to mitigate against theaccurate determination of A ,

Relative transducer sensitivity

Signal attenuations were tnore in pine and Thuja thanin maple and birch. The effective listening distancefor cavitation-induced AEs in pine is of the order of 1or 2cm, This fact was used to devise a scteetiingtechnique to select transducers having the highestsignal to noise ratio.

Pine stem segments typically 20 ctn long werecollected, rehydrated in water and stripped of bark,A reference transducer (PAC model RI5) wasclamped near the centre of the segtnent and thecomparison transducer was clamped as elose to thereference transducer as possible. The gain of theatnplifiers attached to each transducer was adjusteduntil the peak to peak noise was 0,4 V (maximumpositive spike 0,2 V); the sample was kept wet so thatno cavitation events occurred during the gainadjustment. The pine segtnent was fully air

ACOUSTIC EMISSIONS IN WOOD 375

dehydrated while counting all AE events havingpositive going spikes in excess of 0,25 V, Thetransducer with the largest signal to noise ratio wasdeemed to be the one recording the largest nutnber ofA E events. The relative sensitivity, S was computedfrom the following definition:

S, =

[cumulative AEs of thecomparison transducer][cumulative AEs of thereference transducer]

(2)

The comparison transducers used were: ModelsN30W, R6, R15 and R80 (all from PhysicalAcoustics Corp,, Princeton, NJ, U,S,A,), modelMAC175L (from Acoustic Emission Technology,Sacramento, CA, U,S,A,), model 8312 (frotn Brueland Kjaer, Marlborough, MA, U,S.A.), a 'pinducer'(from Valpey-Fisher, Hopkinton, MA, U,S,A,) andthe 1151 transducer in conjunction with the Dt oughtStress Monitor built as part of the DOE-SBIRresearch. All AE events were counted using custombuilt AE counters of the type described by Tyree &Dixon (1983) and Tyree et al. (1984), The pinducerused was one of a lot of 12 and was selected becauseit had a sensitivity to pencil lead breaks nearest themedian of the 12 pinducers.

Results

Velocities

Velocity measurements for AEs travelling axially areshown in Table 1, All velocities range frotn 2,4 to6.3 km/s. These flgures are all in accord with thosereported elsewhere for longitudinal soundpropagation in wood (McDonald, 1978; Bucur,1983),

Frequency composition of AEs

The wavefonns of AEs arising from cavitation eventstended to have two or three dominant frequeneycomponents superimposed. Occasionally, cavitation-induced AEs were predotninantly one frequency. TheF F T of each AE was cotnputed and the atnplitudesof each frequency cotnponent of the FFT wasnormalized to one by dividing each amplitude by thevalue of the largest. The nortnalized FFTs of 200AEs were then averaged and renormalized to one. In

Table 1. Velocity of AE propagation in the longitudinal direction.The source of AEs was a pencil lead break on the cylindrical

surface of a dowel (liircli) or stem surtace (maple and pine)

Sample

BirchMaplePine

Velocity

Stem wet

4,20 + 0,022,4 ±0,05

(km/s±S,E,M,)

Stem air dry

6,3 + 0,14,48 + 0,032,78 + 0,01

Fig, 5 are plots showing the average frequencycotnposition of 200 AEs collected early and late inthe dehydration of tnaple and Thi4ja shoots collectedon the Bruel & Kjaer model 8312 transducer. Similardata collected on the Panametrics model VI05 areshown in Fig. 6.

Means of 200 AEs were plotted in Figs 5 and 6 tobetter show long-term trends. Individual AE eventstended to be quite variable in frequency composition,i.e. some AEs had predominately low frequenciesothers predotninately high frequencies others werefairly broad-band. There was a tendency for moreAEs of high frequency and/or less AEs at lowfrequency as wood samples dehydrated. During adehydration experiment, 30-60 sets of 200 AEs werecollected and there tended to be a smooth transitionfrom the early to late frequency spectrums as shownin Fig. 6. The trend was less distinct in Fig. 5. TheFFTs shown ate fully representative of hundreds ofdata sets. The 95% confluence limits on all means ofrelative amplitude at any given frequency tended tobe +0,03 relative units or less. So tneans that differby more than about 0.06 relative units aresigniflcantly difl'erent at better than the 0.99confldence level.

Cotnparing Figs 5 and 6 reveals two features: (1)The apparent frequency cotnposition of AE signalsobtained by the two transducers differ signiflcantly,(2) Regardless of what transducer is used it appearsthat the dominant frequency tends to drift a littletowards the higher frequencies late in thedehydration period ( > 90% cavitated) than early inthe dehydration period ( < 10% cavitated). Anincreased atnplitudc of any given frequency betweenthe average FFTs of early and late dehydrationperiods indicates that there were tnore AEs havingthat dotninant frequency. Generally, frequencies inthe range of 100-300 kHz were dominant or co-dominant in amplitude. The frequency compositionmeasured in Thuja was independent of the size of thewood sample dehydrated for samples ranging fromI cm diameter by 10 cm long to samples 0.05 cmdiameter and 0.3 cm long.

Attenuation of AEs

The apparent attenuation of AEs generated by pencillead bleaks was measured on a birch dowel asillustrated in Fig. 7, Transducers TI and T2 wereplaced 5 cm to the right and left of the centre of a90 cm birch dowel 1.33 cm in diatneter. Pencil leadswere broken at different distances to the right or leftor at the centre of the dowel. The attenuation wasmore for AE sources near the pair of transducersthan for sourees farther away. The amplitude ratiowas one (attenuation = OdB), when the AE sourcewas at the midpoint between TI and T2, The trendindicated by the line in Fig, 7 is approximatelysymmetrical. Deviations from symmetry wereprobably because of asytntnetrical fibre orientation in

376 M. T. TYREE & J. S. SPERRY

Q.ED

1,0

0.8

0.6

0.4 k

0.2

0.0

1

1

1 Thuja

II t late

200 400 800 900 1000 1200

a>

200 400 600 800Frequency (KHz)

1000 1200

Figure 5. Plots of amplitude versus frequency of cavitation-induced AEs during the dehydration of 77(»/« and maple stems. Allplots were derived from a discrete fast Fourier transformation analysis of AEs collected on a Bruel & Kjaer model 8312transducer. Each plot is the average of 200 AEs. 'Early' and 'late' refer to when during the dehydration the AEs weremeasured; early means that < 10% of all AEs had oecurred and late means that > 90% of all AEs had occurred.

Q.

oc

i.O

0.8

0.6

0.4

0.2

nn

1 1{•A Pine

r A^ 1 1

1

lateearly

1.0

0.8

0.6

0.4

0.2

nn

[.••

1 1 1Thuja— late

V — early

1 1 l" ' •

1

-~>

• "-t — . .

C

1.0

08

0.6

0.4

0.2

0.0

i

J

2 0 0

j

ff

1

4 0 0

1

1

6O0

18 0 0

Maple— late

early

1

1000 I2(

1

) 0

1.0

0.8

0.6

0.4

0.0200 400 600 800 1000 1200 0

400 600 8 0 0 1000"1 r"~ I 1—AE source pencil-lead break— on transducer

on wood on transducer

1200

200 400 600 800 1000 1200Frequency ( k H z )

Hgurc 6. Similar to Fig. 5, but AEs were collected with a Panatnetrics model V105 ttansckiccr. Fourier ttansformations arc alsoshown for pencil lead breaks direetly on the surfaee of the VI05 transducer or on the surface a piece of pine wood mounted onthe V105 transducer. The Fourier transformations for pencil lead breaks are for single but representative AE signals.

ACOUSTIC EMISSIONS IN WOOD 377

ODXI

- 5 0 - 4 0 -30 -20 - 1 0 0 10 20 30

AE source: distance from centre (cm)

50

Figure 7. Attenuation ineasuremciUs in a birch dowel measured for pencil lead breaks at various distances from the centre ofthe dowel, TI and T2 show the placement of the two R15 transducers.

the dowel. The velocity of sound differs according tothe direction of sound propagation with respeet tothe fibre orientation, i,e, for longitudinal, radial andtangential directions of propagation in wood samples(McDonald, 1978; Bucur, 1983), The fibreorientation did not follow ptecisely the long axis ofthe dowel and the line of eut of the dowel appearedto have been in the vicinity of a side branch (knot). Inother experiments, the asytnmetrical properties of thedowel were confirtned by placitig the transducersfarther apart. When the transducers were spaced50 cm apart (25 cm eaeh side of the tnidpoint), theamplitude ratio was no longer 1 when the AE sourcewas located at the tnidpoint between the ttansdueers.

The pritnary value of this experiment is toillustrate, in the simple geometry of the birch dowel,how AEs tend to bounce ofT cylindrical surfaces toeither reinfotce or cancel out to produce differentapparent attenuation factors by the time the AEsreach tbe transducers. If thete were only one tnode ofwave propagation and only one pathway from the

soutcc of the AE to the two transducers, then tbeapparent attenuation would be independent of thedistance of tbe source to the ttansducers. Theseresults suggest more complex patterns of waveinterfetence. When AEs originate 5 ctn frotn thenearest transducer then the sound wave travelling theshortest route to the transducers tend to cancel outthe waves that travel a longer route and refleeting offthe opposite surfaee of the dowel. These effects (asexpected) are less significant tbe further tbe souree ofthe AE is frotn the nearest transducer. Qualitativelysitnilar things ought to happen in stems having moreeotnplieated geometries.

Attenuation properties ought to be a function ofwood hardness and satnple geotnetry. Althoughtnaple wood dowels wete not available to thetn forsitnilar cxperitnetits, the authors would expectqualitatively sitnilar results ibr all hardwood satnples.Unpublished results on maple stems which tapperedin diatneter from about 2 to 1 ctn diameter over adistance of 1 tn confirtn this presutnption.

Table 2. Specific attenuation values. A,, measured for AEs tVom pencil-leadbreaks and for AEs from cavitation events. Sec Appendix I for detailsregarding A, measured tor cavitation events. All measurements apply to thefrequency range of 100-300kHz except where indicated. Numbers in bracketsgive the number of AE sources (pencil-load break sources) or the number of

stem samples dehydrated (cavitation event sources)

Sample

BirchMapleTliuidTliitia (600 1200 k l l / )P ine

Ranges of A,

AE source; pencil break

0,2-0,9(100)0,,VI,4(40)5,0-14(20)

4-12(2)

dB/cm

Cavitation event

1,1-1,9(3)8-12(3)

15-18(3)8-20(3)

378 M. T. TYREE & ,1. S. SPERRY

A summary of specific attenuations. A,, appears inTable 2. The range of specific attenuations measuredfor cavitation-induced AEs tended to be consistentlyhigher than for AEs originating from pencil leadbreaks. However, the trends were not strong enoughto meet statistical test of significance. Note that inThuja samples the specific attenuation is more forhigh-frequency AEs than for low-frequency AEs,During the measurement of attenuation forcavitation-induced AEs in pine and Thuja, more than90% of the AEs detected by one transducer were alsodetected in the other. This suggests that most of theAEs were within listening distance of bothtransducers. In the maple samples, only about 30% ofthe AEs detected in one transducer were alsodetectable in the other. This is about what shouldhave been expected for the experimentalconfiguration (Fig. 4a); most of the stem tissue wasbeyond the listening distance of both transducers.

Relative sensitivities of tran.sdueers

A summary of relative sensitivities of transducers isshown in Table 3. The R15 transducer, used as areference, was more sensitive than most othertransducers. The MAC175L was approximately thesame. However, the MAC175L transducers had apotentially serious problem. Spurious AEs could begenerated on the AET transducers by gently movingthe signal cable. The coupling where the cableentered the body of the transducer appeared to be thesource of the problem. The AET transducers werefound to be unsuitable for field measurementsbecause wind-induced motion of stems and causedspurious AEs because of the poorly designed cablejunction.

The best transducer appeared to be that designedfor use on the drought stress monitor (DSM) built as

part of the DOE-SBIR project. The 1151 transduceras tested with the DSM amplifiers and countingcircuit. The authors cannot say at this point if the1151 transducer or the amplifiers (or both) were thesource of the superior sensitivity. The previousstandard transducer used for cavitation studies wasthe Bruel & Kjaer model 8312 (Tyree & Dixon, 1983;Sandford & Grace, 1985; Tyree et al,, 1986). It can beseen that the new system built by Physical Acousticsis about seven times better in terms of signal to noiseratio.

Discussion

It is difiicult to say exactly what the mode of originand the mode of transmission of the cavitation-induced AEs is in wood samples. From Table 1, it canbe seen that the velocity of longitudinal tnotion of theAEs is about the same in fully hydrated and air-driedwood samples. The velocity of ultrasound in water(1.53 km/s) is much less than that reported in Table I,The volumetric water content of fully hydrated woodis 0.5-0.8 by volume. So the higher velocitiesmeasured strongly suggests that the medium throttghwhich the sound propagates is the cellulose materialof the cell walls of wood. Thus, the properties of thewood cellulose in combination with the properties ofthe transducer together determine the frequencycomposition of the AEs detected.

Ultitnately, Table 3 contains the information tnostpertinent to the selection of transducer types. Theother information provides some insight into why theresults should be as they are in Table 3.

In all experiments in which S, was tneasured, thegains were adjusted until the typical maximumamplitude of the background noise was about 2dBbelow the voltage that triggered the detection of anAE event on the event counters. With these settings.

Table 3, Relative sensitivity, S,, of transducers. The reference transducer in allcases was a model R15. The comparison transducer is listed in the left handcolumn. A value of S, more than one indicates that the comparison trans-ducer had a better signal-to-noise ratio for the detection of cavitation events.A ratio less than one indicates that the comparison transducer was inferior.All errors reported are standard errors of the mean. Numbers in brackets arcthe number of measurements used to compute Ihe means. Band pass frltcrsarc indicated by low and high frequency cut off values. High pass filters arc

indicated as such

Comparisontransducer

R15R6MAC175LN.30WB&K 8312R80Pinducer1151-t-DSM

Filter(kHz)

100-120050-200

125-250200-400

100 hi pass600-1200

IOO hi pass100-300

S, [Re.; R15 with 100-300 kHz filter]

0.75-1-0.06 (6) A,E*0.50-1-0.07 (6) A,E0.91+0.08 (6) B,E0.48 + 0.07 (5) A,E

0.185 + 0.020(7) C,E0.09 + 0.01 (4) A,E0.10 + 0.02 (4) A,E1.37 + 0.09 (6) D,F

•Letters refer to amplifier systems used with the transducers. Pre-amplifiers;A=1220A, B= C = B&K 8312 internal, D = I151 internal Pcst-ampliliers:E = custom built based on LH0032 Op-Amp, F=DSM internal.

ACOUSTIC EMISSIONS IN WOOD 379

the background count rate was in the range of 0.1-0.5 events per minute. The S values in Table 3effectively reflect the frequency dependenee of theattenuation properties of the pine samples plus thesignal-to-noise ratio of the tneasuring system, i.e.transducer -\- filter + preatnplifier -f post-atnplifier.In many eases, the preamplifier and post-atnplifiersare the same so that differences reflect the signal tonoise ratio of the transdueer + filter combinations. Itcan be seen that the 1151 transdueer and atnplifiers ofthe drought stress monitor (DSM) developed for theSBIR research contract is superior to all othersystems studied. Sinee the 1151 and DSM haveunique transdueers and amplifiers, it is not possibleto say if the itnprovetnent lies in the transducer or theamplifier designs.

The R15 with a 100-300 kHz band pass filter issuperior to the same transducer with a 100-1200 kHzband pass filter. This is because it was necessary toturn the overall gain down a few dB lower toeliminate the higher frequency noise that the widerband pass filter allowed to pass.

The superior perfortnance of the best threetransducers (1151, R15, and MACI75L) can beattributed to the fact that the typical cavitation-induced AEs have a strong frequency cotnponent inthe 100-300 kHz range (see Fig. 5). Althoughfrequencies about as strotig or stronger sometitnesexist in the 400-1200 kHz range, the petfortnanee oftransducers in the higher frequency bands are inferiorfor three probable reasons: (1) transdueer atid/oramplifier noise is worse at the higher ftequencyranges, (2) the inherent sensitivity of the transducersis less at the higher frequency ranges, or (3) there ismore rapid signal attenuation in wood samples in thehigher frequeneies (Table 2). The cost of thetransducers precluded the possibility of testing arange of transducers of the satne tnodel to see if theseresults are representative of all transducers of thesame model. However, all tnanufaeturers haveinformed us that the transducers supplied were'representative' of their 'typical' product.

Of considerable significance is the question ofwhether one ean determine the locus of origin of acavitation-induced AE frotn the amplitude orfrequency composition of the AE signal as receivedby the transducer. Although sotne people (Milburn,1973) have suggested that latge xyletn cotiduits storemore elastic energy when under sap tension thansmall conduits, it is clear that atnplitude would not bea good indicator of the size of the conduit which isthe source of the AE. The AE signals attenuaterapidly (Table 2, Fig. 6) and constructively ordestructively interfere as they bounce off surfaces itithe stetn while ttavelling en route frotn the source tothe transducer.

Others have suggested that large conduits mayproduee AEs of eharacteristieally longer wavelengththan would small conduits, for example, Sanford &Grace (1985). One of the present authors (MTT)

c _ _

V

c

Figure 8. Possible modes of vibration of a vessel (V) wall attachedto paratracheal eells (C). The walls are shown in solid line. Thedashed line represent possible wall displacements during a mode ofvibration. (A) Shows hypothetical vibration along the full length ofthe vessel requiring distortion of cross walls (bold vertical walls).(B) Shows hypothetical vibration about the nodes formed by erosswalls.

must assume responsibility for having either instilledor at least reinforced this idea in the mind of DrGrace when he worked in MTT's lab in 1983 andviewed some of their early AE signal analyses. Theauthors no longer subseribe to this dubioushypothesis. They do not know the mode of AE signalpropagation in the wood nor the mode of vibt ationsthat conduits tnight experience. Even if it is assumedthat conduit walls might vibrate in and out radiallyfollowing a cavitation event, the frequency ofvibration will depend on all of the following factors:(1) the length between vibrational nodes, (2) theelastieity of the wall, (3) the mass of the wall, (4) thenature of the attachment of the walls to surroundingcells. Two different hypothetieal modes of vibrationbetween different fixed nodes are illustrated in Fig. 8.In the first (A), the whole length of the wall of thevessel is shown vibrating in and out. In the seeond,the nodes of vibration are confined to the location ofcross walls of the para-vaseular cells (C). In theauthors' view, the second mode of vibration (B) istnore likely than the first (A) since (A) would requireconsiderable stretching and cotnpression of the crosswalls of cells (C).

There is also positive evidenee for the relativeitnportance of wall elastieity and/or tnass indetertnining the characteristic frequency of AEs. Inconifers, earlywood tracheids cavitate beforelatewood tracheids as shown by dye perfusions andother evidence (Tyree et ai., 1984; Tyree & Dixon,1986). This study shows that the AEs have higherfrequency cotnponents near the end of thedehydtation when latewood tracheids cavitate thanat the beginning when earlywood tracheids cavitate.But thete is no diffetenee in the length of earlywoodand latewood tracheids in Tiiuja and pine(utipublished results of ttacheid lengths tneasured ontnacerated earlywood and latewood tissue).

Finally, there is evidence that the frequencycomposition of AE signals can be signifieantlychanged when they travel through the wood orthrough the transdueer. A pencil-lead break producesa step-wise decompression of about 0.5//s duration(Dr Richard Weaver, personal eomtnunication). Butby the titne this short duration AE is received by thetransducer, the authors have found it to last 50-

380 M, T, TYREE & J, S, SPERRY

400 (s on the Bruel & Kjaer model 8312 transducerand 10-20/is on the Panametrics model VI05transducer even when the peneil lead is brokendirectly on the surface of the transducers (data notshown). This suggests that the frequencies observedare largely the artifact of the transducers themselves.Compare the large difference between the Fouriertransformations of AEs from Thuja and maplecollected on the Bruel & Kjaer transducer (Fig. 5)and the Panametrics transducer (Fig. 6). Bycomparing the Fourier transformations resultingfrom a pencil-lead break directly on the VI05transducer to the Fourier transfortnation for apencil-lead break on a pine wood sample resting onthe VI05 transducer, it can also be seen that thewood also alters frequencies received. Certainfrequencies are selectively removed and others appearto be enhanced. The possibility that eaiiywoodtissues remove more lower frequency signals when airfilled late in the dehydration than when water filledearly in tbe dehydration cannot be discounted. Theauthors feel it is premature to conclude anythingabout the loeus of origin of AEs based solely onfrequency cotnpositions of raw Fouriertransformations.

As a consequence of this work and of thecollaborative efforts of Physical Acoustics Corp,, anAE counter has been designed for commercialdistribution, the model 4615 Drought Stress Monitor(DSM), Some useful features of the device are: (1) itis small, light weight and battery powered, (2) it has aseven times better signal-to-noise ratio than systemspreviously used in the authors' laboratory, (3) it ismicroprocessor controlled and programmable. (4) itwill log AE data in memory and dump it to a hostcomputer for further analysis. One of the authors(MTT) has written software to allow an IBM-PC orclone to communicate with, program, dump and plotdata from the 4615 DSM and is available at no cost.

Acknowledgments

This work was made possible by a subcontract fromPhysical Acoustics Corp, who also provided the loanof some equipment used and provided mueh usefuladvice. Some transducers, the waveform digitizer andthe computer used in this study were purchased withan equipment grant from the Natural Sciences andEngineering Researeh Council of Canada (NSERC).Some of the Fourier transformations reproducedhere were obtained in 1985 while MTT was at theUniversity of Toronto and funded by NSERC grantnumber A6919. We also thank Dr Richard Weaver,University of Illinois, who is a noted expert on thetheory of ultrasound propagation in metals, for manylong and valuable discussions, Dr Weaversummarized for us many signifieant concepts derivedfrom diverse papers too technical and mathcrnaticalfor us to understand, Mel Tyree must assume fullresponsibility for any distortions of theory or fact

that may result from our simplified discussion of theconcepts explained to us by Dr Weaver,

References

Bucur, V, (1983) An ultrasonic method for measuring the clasticconstants of wood increment cores bored from living trees,Vltrasotiics, 21, 116-126,

McDonald, K,A, (1978) Lumber defect detection by ultrasonics.Research Paper FPL 311, Forest Products Laboratory, ForestService, USDA, Madison, Wis,

Milburn. J,A, (1973) Cavitation in Ricitnts by acoustic detection;Induction in excised leaves by various factors, Planta. 110, 253-265,

Pena, ,1, & Grace, ,), (1986) Water relations and ultrasoundemissions of Pitius sytvestris L, before, during and after a periodof water stress. New Phytologist, 103, 515-524,

Sandford, A,P, & Grace, J, (1985) The measurement andinterpretation of ultrasound from woody stems. Journal ofExperitnental Botatiy. 36, 298-311,

Salleo, S, & LoGullo, M,A, (1986) Xylem cavitation in nodes andinternodes of whole Chorisia insigtiis II,B, et K, plants subjectedto water stress; relations between conduit size and cavitation,Atmats of Botany, 58, 431^41,

Tyree, M,T, & Dixon, M,A, (1983) Cavitation events in 77»(/«oeeidentalis L,',' Ultrasonic acoustic emissions trom the sapwoodcan be measured, I'lant Physiology. 72, 1094-1099,

Tyree, M,T, & Dixon, M,A, (1986) Water stress induced cavitationand embolism in some woody plants, Physiologia Plantarutn, 66,397-405,

Tyree, M,T,, Dixon, M,A, & Thompson, R,G, (1984) Ultrasonicacoustic emissions from the sapwood of Thuja oeeidentalismeasured inside a pressure bomb, Ptatit Physiology. 74, 1046-1049,

Tyree, M,T,, Fiscus, E,L,, Wullschleger, S,D, & Dixon, M,A,(1986) Detection of xylem cavitation in corn under fieldconditions, Ptant Physiology, 82, 597-599,

Appendix

Attenuation of ultra.sonie acoustic emissions in wood

In this analysis, the authors start out with severalsimplifying assumptions that are needed to arrive at away of calculating a specific attenuation factor. A,,,These assumptions serve to produce a relativelysimple set of equations. Later, they will discuss howthe real situation will tnodify the expected theoreticalbehaviour. The simplifying assumptions are:

(1) The AE amplitude attenuates exponentially withdistance travelled down the stem, (Thisassumption is eorrect for plane waves movingthrough an infinite mediutn; waves attenuatebecause of visco-elastic energy loss,)

(2) Attenuation is approximately independent of therange of frequencies contained in AE waves,

(3) The distance the AE waves travel is much longerthan the diameter or thickness of the woodsample,

(4) The AE transducers makes a point contact withthe wood sample, i,e, the size of the transdueer issmall compared to the separation between thetwo transdueers.

ACOUSTIC EMISSIONS IN WOOD 381

Pencil-lead break as a source of known locus(Pig. 9a)Consider transducers Tl and T2 separated by adistatice d and AE signal sources at Bl or B2 at anydistance beyond but not between the transducers. Ifthe AE signal level at the source is L , it will decline,by assutnption, exponentially with distanee awayfrom the source:

L = L., 10 "^ (Al)

where L is the signal level at distance x, cm, from thesource and K is the an attenuation constant. For asource at Bl the signal level at TRl and TR2 are:

T-r> 1 I in1 K. 1 — LQ "-'andTR2 = L,, 10 -K(D +

(A2a)

(A2b)

where D is the distance frotn Bl to Tl, d is thedistance between Tl and T2, and TRl and TR2 arethe AE atnplitudes at Tl and T2, respectively, as inEqn (1). Dividing Eqn (A2a) by Eqn (A2b), takingthe log of both sides of the result, and multiplying theresult by 20 (to get dB units) yields the attenuationfactor A in Eqn (1):

A = 20 log[TRl/TR2] = 20Kd. (A3)

From this, it is clear that A is related to the productof K and d and it also follows that the specificattenuation defined in the 'Materials and tnethods'section is given by

= A/d = 20K. (A4)

From Eqns (A3) and (A4) it can be seen that thedistance of the soutce of the AE frotn the transducerpair is unitnportant in the calculation of A and A .For a source located at B2, the sign but not themagnitude of A and A will change.

The relationships developed here are only crudeapproximations of the teal physical situation inwhich attetiuation factors are tneasured. The use ofan 8-bit wavefortn digitizer poses serious litiiitationsbecause of its narrow dynamic range (48 dB). Toassure an accurate value of TR1/TR2 it is necessaryto keep the ratio in the range of 20 dB and theminitnum amplitude over 20-30 dB so that themaxitnutn atnplitude is also within range of thedigitizer. For the maple and birch satnples this tneantthat Tl and T2 could be separated by only about 5-10 cm which is not tnuch mote than the diatneter ofthe R15 transducers (1.8 cm). Since thete is sotneuncertainty regarding where along the transducer theAE enters the transducer the evaluation of d isuncertain. There are a number of other confoundingfactors including: (I) The AEs ate not all of the samefrequency and frequently have several ftequencycomponents. Since visco-elastic attenuation is afunction of frequency the waveform and amplitude isdistorted as the wave travels (iti water the attenuationconstant, K, is invetsely proportional to the square

Bl Tl T2 B2^ •\j \ ^

AE af X

T l • -T2

o

c

r\/

D

M1

+ - 0Attenuation, (dB)

Figure 9. Theory of attenuation tneasutetnents. (A) experitnentalset up where transducers Tl and T2 arc separated a distance, d cm.apart near the eentre of a stetn segtnent. Bl and B2 show placeswhere leads tnight be broken. (B) experitnental .set up wherettansdueets Tl atid T2 ate at the ends of a stetn segment d ctn long.(C) Theoretical curves of nutnber of AEs versus attenuation forcase B. (D) Theoretical curves of nutnber of AIEs versusattenuation for case A. See Appendix I for details.

root of the frequency). (2) There are two tnodes ofwave propagation in solids: one is a cotnpressionwave and the other is a surface wave. These wavestravel at dilTetent velocities, the surface wave beingabout twice the velocity of the cotnpression wave.These waves travelling at different velocities willinterfete with each other sotnetitnes nullifying eachother and sotnetitnes teinforcing each other. (3) Asthe waves propagate through the tnedium, somecotnponents of the waves will reflect off the surfacesof the wood and internally where large densityehanges are encountered. The reflected waves, havingtravelled a longer distance will also interfere with thewaves that have followed the tnost direct path frotnthe source to the transducers. The ultimate result ofall wave interference is that the waves will not declineexponentially with distance. At some locations, thewave atnplitude will be tnueh larger than expectedand at other points will be much stnaller in amplitudethan predicted by the simple theory above. (4) Thetransducers thetnselves will tnodify the waveforms asthe waves bounce around inside the body of thetransducer and because harmonic frequenciescharacteristic of the transducers will make sometransdueers ring on long after the AE has ceased inthe wood sample.

Attentiation of AEs from an unknown loeus

Pine and Thuja satnples had a large specificattenuation. In order to tneasure attenuation of AEsfrom eavitation events, stnall blocks of length, d,were elatnped between two matched transducers.This situation in illustrated in Fig. 9b. Consider anAE at sotne arbitrary distatice x ftom Tl. Using anargutnent sitnilar to that above it can be shown thatthe measured attenuation will be given by

A = 20 log[TRl/TR2] = -K(2x-d) . (A5)

382 M. T. TYREE & J. S. SPERRY

<O

(DX}

E

•o

(D

a.

- 2 0 -15 - 1 0

Amplitude ratio TRI/TR2 (dB)

Figure 10, Actual attenuation curves for AEs from cavitation events in maple and Thuja slcms. The Ttitija curve is for theexperimental set up in Fig. 7B using R80 transducers (600-1200 kHz filter). The maple curve is for the experimental set up inFig. 7A using R15 transducers (IOO-3OOkHz filter). In the maple experiment, the transducers were .separated by 4cm. In theThuja experiment, the transducers were separated by 0.4 cm.

Note that when the AE originates in the centre(x = d/2) that A = OdB, Also the maxitnum andminimum values of A occur when x = 0 and x = d,respectively. Although the authors do not knowwhere any one AE occurs, it is very likely that theoccurrence of AEs will be more or less evenlydistributed overall possible x values during the courseof a dehydration. The maximum and minimumvalues of A can be determined from a frequencyhistogram of the number AEs versus the attenuationvalue from which values of K and A, can beevaluated. The expected histogram is illustrated inFig, 9c, thin sohd line.

On maple samples the transducer could be spaced5-10 cm apart in the middle of stem segments 50 cmlong. This case is illustrated in Fig. 9a, The onlydifference here is that AE outside of the spacebetween the two transducers will yield the maxitnumor minimum values of A, The expected frequencyhistogratns are given in Fig. 9d, thin solid line.

When account is taken of the reality of thesituation (large transducers, internal reflection ofAEs, interference and the frequency dependence of K)then the expected histograms would probably look likethe thick lines in Fig, 9e and d. Two examples ofactual experimental data are shown in Fig, 10.