Acoustic white noise generator

A narrow banded acoustic transducer is driven by a VCFO whose output is varied by input signals of variable amplitude. One can simulate acoustic emissions for purposes of calibrating transducers which monitor a structural part for such emissions. Different signals for the control of the VCFO are generated to provide for specific test pulses.

BACKGROUND OF THE INVENTION 
The present invention relates to the generation of acoustic waves and more 
particularly, the invention relates to the reproducible production of high 
frequency noise bursts. 
The generation of acoustic signals has become increasingly important for 
the detection of flaws and defects in structural parts. Ultrasonic signals 
and particularly pulses are caused to traverse the material, and the 
interaction thereof with the acoustic waves is used to obtain information 
on the uniformity of texture (or lack of it) of the material. It is 
apparent that the resolution of this detection method depends on the wave 
length of the acoustic signal. Thus, it is desirable to operate with as 
high a frequency as possible to permit the detection of minute cracks and 
fissures. 
Another field of applying acoustics to flaw detection relates to acoustic 
emissions. These are spontaneous, high frequency acoustic waves emitted 
upon relief of localized tension and stress in the structural part. The 
emission of these waves can be used as a criterion to indicate the 
internal development of a crack, fissure or the like. It is inevitable 
that during the continuous monitoring of acoustic emissions in a structure 
noise is also picked up from non-relevant sources. This noise can be 
similar in many ways to the acoustic emissions generated by a growing 
crack, and it is a particularly vexing problem to identify an acoustic 
emission and separate it from the noise. One characteristic which is used 
to accomplish this is their frequency spectrum. Identification and 
interpretation of acoustic emissions from their spectrum relate directly 
to the calibration of the monitoring pickup transducers and of the various 
wave propagation paths in the structure under test. It was found that the 
existing devices for producing bursts of acoustic energy for purposes of 
calibration are either too narrow banded or afford little control over 
their frequency spectrum. 
DESCRIPTION OF THE INVENTION 
It is an object of the present invention to provide a new and improved 
source for high frequency, ultrasonic signals. 
It is another object of the present invention to provide for a new and 
improved method of acoustic signal generation and transmission in 
structural parts to be tested in the general sense with regard to flaws. 
It is a specific object of the present invention to provide a new and 
improved acoustic wave generator for producing reproducible bursts of 
acoustic energy to be used, e.g. for calibrating transducers which are 
provided for monitoring a structural part for acoustic emissions. 
In accordance with the preferred embodiment of the present invention, it is 
suggested to use a solid state electrically controlled transducer 
(ferroelectric or piezoelectric) which provides acoustic signals in 
response to an electrical signal being provided by a voltage-controlled 
frequency oscillator which, in turn, is controlled by a signal and wave 
form generator being preferably of the variety which provides particularly 
contoured voltage signals on a repetitive or, at least, repeatable basis. 
The VCFO-transducer combination is capable of providing a well-defined 
spectrum which is narrow-banded in each instance, but the narrow emission 
band is shifted over a wide range of frequencies for corresponding 
variable inputs to the VCFO, and for each burst or chirp being produced. 
The narrow bandedness of such a burst or chirp results from the transducer 
emission spectrum provided for a particular input voltage for the VCFO; 
the broadbandedness results from the range of voltages covered by the 
input signal as generated and applied to the VCFO. 
An additional parameter in the general sense is the time sequence of the 
voltage variations for the input of the VCFO. The same amplitude range may 
well be covered by differently contoured voltage pulses, but the acoustic 
signal generated and/or picked up at a remote location will be quite 
different in these instances. It was found particularly that by suitably 
contouring the wave shape of a voltage pulse applied to the VCFO input one 
can simulate readily a variety of acoustic emissions. It was also found 
that by suitably contouring the wave shape of the VCFO input one can 
correct or modify the particular response characteristics of a pickup 
transducer because these characteristics may exhibit certain 
irregularities. It is important that for each field of application, the 
frequency spectrum of the generated acoustic signals is well controlled 
which is particularly important for the megahertz range of acoustic 
frequencies because relatively little direct interference from outside 
sources can be expected at such high frequencies so that broad band 
control per se may provide valuable information, at least in some 
instances.

Proceeding now to the detailed description of the drawings, FIG. 1 shows an 
electro-acoustic transducer 10, which generates acoustic waves in direct 
response to electrical input signals. The transducer 10 includes a housing 
11 in which the outer conducter 12 of a coaxial connection 13 ends and is 
terminated. The bottom of the transducer housing 11 includes a metallic, 
ceramic, or plastic wafer 14 covering a ferroelectric or piezoelectric 
disk 15. The disk 15 may, for example, be of lead metaniobate or lead 
zirconate-titanate or PZT for short. A specific material which is quite 
useful here is traded under the designation PZT-5A. The disk 15 has its 
top and bottom covered with silver coatings serving as electrodes. One of 
these electrodes connects to the housing 11 assuming its potential (as 
reference) while the top electrode 16 is connected to the inner conductor 
of the coaxial cable. The disk 15 is acoustically loaded by a body 17 made 
of rubber in which metal particles have been dispersed. 
The transducer as depicted in FIG. 1 has a frequency response shown as a 
solid line curve in FIG. 2, the center peak frequency being, for example, 
at 1 MHz or thereabouts, with litle or no response for frequencies higher 
than about twice that value. This specific response has validity for 
steady signals at any of the frequencies within the plotted range. 
However, the same or about the same curve is valid for sweep or sweep-like 
signals in which the frequency varies rather slowly, for example, at a 
rate of one gigahertz per second or less. Thus, the plotted frequency 
response as to each frequency has validity for stationary inputs or quasi 
stationary inputs as regards to such frequency. 
The situation is different for fast changes in frequency. The dotted curve 
in FIG. 2 is representative of an amplitude vs. frequency response in 
which the frequency changes much faster. Fast, in this context, is to mean 
a noticable change in frequency from cycle to cycle or, to put it 
differently, the rate of change in frequency becomes comparable to the 
frequency itself. An inherent delay in the transducer operation causes, in 
fact, the response to flatten. 
The transducer 10 is coupled to a solid object 50 such as a structural part 
and transmits thereto acoustic signals. These acoustic signals propagate 
through the object serving as transmission medium and are picked up at a 
location (or several spaced-apart locations) being remote from the point 
of transmission. A transducer 60 is coupled to the object 50 for this 
purpose and picks up any acoustic signals arriving at the (physical) 
interface of transducer 60 and object 50. The acoustic signal as picked up 
by the transducer 60 is converted therein into an electrical signal which 
is passed through an output circuit for example, for further study. 
Details of the pickup are conventional and do not constitute per se a part 
of the invention. 
The object 50 as illustrated may represent a variety of structures. 
Moreover, the present invention can be explained fully with respect to 
different modes of operation which may be practiced on the same object. By 
way of example, the object 50 may be a stainless steel pressure vessel for 
use in a chemical process plant. The vessel may be 8 ft. in diameter, 10 
ft. tall, and may have a wall which is about one half inch thick. 
Alternatively, the vessel may have a diameter of 10 ft., a height of 111 
ft., and a wall being about 5 inches thick and made of, for example, a low 
alloy steel covered with a 4 inch thick layer of thermal insulation. 
Such a structural part can be expected to be tested acoustically in a 
two-fold manner. The first test is carried out prior to use and involves 
the determination of the attenuation and dispersion characteristics of 
typical propagation paths in the structure. In such a case, acoustic 
signals are generated by the transducer 10, and the signal picked up by 
transducer 60 is indicative of these characteristics. Subsequently, the 
structure 50 is continuously monitored during proof load and service load 
conditions and the material may undergo local stress relief with 
concomitant emission of bursts of acoustic waves. Some of these acoustic 
emissions may be quite harmless; others may indicate the beginning of the 
formation of a crack. Most prominently, however, the emission of such 
acoustic bursts by the material from the same location may well indicate 
the growth of a crack which has to be detected. Particularly for the 
latter case the supervising transducer requires calibration in order to 
distinguish such acoustic emissions from other noise. In this case then 
the transducer 10 can also serve as a source for acoustic calibration 
signals. Thus, the transducer 60 may be interpreted as being one of those 
transducers whose response in this particular environment and for this 
particular purpose, namely detection of acoustic emissions, requires 
calibration. 
Generally speaking, however, transducer 60 will be of that kind that is 
needed or wanted for a specific purpose. This aspect should be born in 
mind because the transducer 10 will be operated to provide signals adapted 
to the specific use that is desired, and different specific uses require 
only a change in the signal that is applied to transducer 10. The pickup 
transducers may differ as to structure in each instance of use. 
Proceeding now with the description of the system, the coaxial connection 
13 is connected to output terminals of a voltage controlled frequency 
oscillator 20 or VCFO for short. The oscillator 20 is of conventional 
design and produces a narrow band of output frequencies for a particular 
input signal, usually being d.c. The frequency band may, for example be 
about 30 KHz wide, but the generator covers a much wider range. For 
example, the generator 20 may produce no noticeable output for 0 input 
voltage and a maximum frequency of, for example, 10 MHz for an input of 
+V. 
The VCFO 20 may have an output circuit which includes a particular, low 
pass filter 21 which cuts off frequencies above a particular desired 
range, for example, 2 MHz. Thus, no signal above that frequency is 
transmitted to the acoustic signal transmitting transducer 10. As will be 
understood shortly, the use of this lowpass filter 21 is a suitable 
expedient for the convenient formation of pulses and bursts. 
It can readily be assumed that the relationship between the input voltage V 
for the VCFO 20 and its output frequency f is a linear one, i.e., dV/df = 
constant. However, this is not a necessary requirement as any 
non-linearity can be compensated in the particular stage which generates 
the input wave form for the VCFO and which will be described next. 
The input signal for the VCFO is generated by a wave form generator 25. The 
wave form generator can be constructed in a variety of ways, and are 
commercially available, including generators with digital memory chips for 
producing particular wave and pulse forms. However, the following analog 
representation for obtaining a specific variety of wave forms will suffice 
for explanatory purposes. By way of example, generator 25 includes a 
capacitor 30, a charge control circuit 31, and a discharge control circuit 
32. The charge control circuit includes a plurality of electronic switches 
such as FETs 331, 332, by means of which different impedances, e.g. 
resistors, such as 35 and 37, can be connected between the capacitor 30 
and a voltage source, for example, +V. 
The set of impedances includes also an inductance 39, but its connection to 
+V is an indirect one as will be explained below. One particular circuit 
connection that can be completed by a FET, 333, does not include a 
resistor at all to symbolize a low impedance, rapid charge circuit for the 
capacitor 30 when the respective FET is gated on. 
The discharge circuit 32 includes also a plurality of FETs, 341, 342, and 
343, to place discharge resistors such as 36 and 38 or no resistor between 
ground potential and the capacitor 30. 
The number of resistors in the charge and discharge circuits shown here by 
way of example, is not limited to the particular number illustrated, but 
the number of resistors is simply representative of a variety of different 
wave forms to be generated. 
The switches 331 etc. and 341 etc. are under control of a wave form 
selector 40 which may include a dial or the like for purposes of selecting 
charge and discharge impedances in order to produce a desired wave form. 
The FETs 331, 332, 341 and 342, providing switching signal for non-zero 
charge or discharge resistances, are gated on by the respective selection 
switch for the duration of the selection. The low (zero) impedance charge 
and discharge portions in circuit 31 and 32 will be controlled in that the 
respective selector switches connect a clock 41 to the gate of the 
respective FET, 333 or 343, to obtain a brief turn-on pulse for producing 
a rapid charge or discharge of the capacitor 30 as desired. 
Usually, the selection will be made so that for a selected non-zero 
impedance in the charge circuit the capacitor 30 will be discharged 
rapidly with the clock via 343 while on the other hand, a rapid charge of 
the capacitor by clock control and via 333 is followed by a gradual 
discharge through a selected non-zero impedance in the discharge circuit 
32. The clock 41 determines the repetition rate of the production of wave 
forms and pulses that make up the effective wave form signals for driving 
the VCFO 20. Typically, the clock may produce pulses at a rate of 200 Hz 
but of a very short duration. 
One of the impedances in the charge circuit 31 is the coil 39 which 
completes an oscillating circuit when connected to capacitor 30. The coil 
when so connected receives a voltage pulse at the rate of the clock, but 
at a selectively adjustable delay to each normal clock pulse. This will 
provide an oscillatory voltage across the capacitor until such time the 
respective next clock discharges the capacitor to ground. For example, 
circuit 43 may provide an adjustable delay for the clock pulse to set a 
flip-flop 44 and the next clock resets it. That flip-flop provides a 
voltage to the coil 39 for the duration of the set state. The reset state 
provides ground to the coil so that upon occurrence of each clock, 
capacitor and coil are both grounded across all terminals. This way, one 
obtains a wave form of an oscillatory nature followed by a pause. 
The capacitor 30 has its non-ground electrode connected to a summing poing 
26 whose output is the output proper of the wave form generator 25 and is 
applied to the control voltage input of VCFO 20. In addition to the 
capacitor signals, summing point 26 receives an adjustable bias from a 
circuit 27, the bias being variable between -V and +V. It can thus be seen 
that the various signals generated across the capacitor 30 are combined 
with the adjusted bias to select the particular signal configuration and 
wave form that is being applied to the VCFO 20. 
FIG. 1 shows also an additional bias for summing point 26. A FET 28 does 
not clamp the output of summing point 26 to ground or a negative potential 
for the duration of the astable state of a monostable device 29, being 
operated by the clock, i.e., in the beginning of each wave form pulse. 
After the monostable device 29 is run, the FET 28 is rendered conductive 
and holds the signal input for the VCFO 20 to ground. This additional bias 
may be used in lieu of or in addition to filter 21 for the generation of 
the trailing edge of the pulses applied to VCFO 20. 
The FIGS. 3 depict a plurality of wave forms generated by the wave form 
generator 25 which provides the input for the VCFO 20 via the summing 
point 26 in conjunction with biasing circuit 27. In each instance of FIG. 
3, the wave form proper generated and developed at the output of summing 
point 26 is drawn in a dashed line. The range of voltages covered thereby 
is, however, in most instances, extended beyond the particular range 
operative for producing frequencies that actually drive the transducer 10. 
The lower boundary for this whole range is given by zero volts for the 
input of VCFO 20 so that all negative voltages applied thereto remain 
ineffective. On the other hand, the low-pass filter 21 cuts off 
frequencies above the 2 MHz range limit so that any voltage applied to 
VCFO 20 and having a value x or larger will produce an output frequency 
higher than 2 MHz. Therefore, such a voltage &gt; x will actually remain 
ineffective, as far as the output of circuits 20 and 21 is concerned. 
Each of the FIG. 3 includes a solidly drawn curve which will be called a 
virtual wave form signal. This virtual wave form represents the equivalent 
voltage value and contour which would produce the same VCFO output as the 
VCFO plus filter combination produces by operation of the actual (dashed) 
input for the VCFO. Generally speaking, the virtual wave form differs from 
the actual or real wave form as produced by circuit 25 in that the virtual 
wave signal is zero for all negative values of the actual wave form and 
for values above the particular value x resulting in VCFO frequencies 
above the cutoff range of filter 21. 
Referring first to FIG. 3a, the figure shows the wave form generated when 
the bias circuit 27 is adjusted to zero volts, and when the wave form 
generator 25 is in a select state in which a relatively high impedance is 
effective in the charge circuit 31 (FET 331 being on), while the clock 41 
turns on briefly FET 343 to reset and discharge the capacitor 30 
periodically. The charge control circuit is permanently on for the 
duration of the generation of this particular wave form, for the 
periodicity of the generated wave form results from the clock controlled 
discharge. 
Whenever the sweep signal traverses, the level denoted x the resulting high 
frequency output of VCFO 20 is suppressed by low-pass filter 21 so that 
the output signal of the latter drops to zero. Consequently, the signal 
applied to and driving transducer 10 is composed of individual bursts or 
chirps which begin with the lowest possible frequency VCFO 20 can actually 
produce (such devices usual produce a noticable output only above a few 
KHz). The frequency increases to 2 MHz whereupon the transducer driver 
signal actually drops to zero until the next clock pulse resets the sweep 
circuit for the next burst to begin. 
FIG. 4a illustrates the frequency spectrum of a burst when received by 
transducer 60 in response to a burst produced by transducer 10 when 
receiving a wave form as per FIG. 3a. 
FIG. 3b denotes a situation in which the charge circuit for capacitor 30 is 
clock pulse controlled (FET is on with the clock) and applies a voltage 
pulse +V to the capacitor 30 as a high charge pulse of short duration. The 
discharge circuit 32 may be adjusted for high impedance discharge via 
resistor 36 and FET 341. Again, the solidly drawn curve is the effective, 
virtual ramp signal. Thus, each sweep begins at a level well above the 
level x so that for a relatively long period of time, the VCFO output has 
frequency above the 2 MHz range, and these oscillations are cut off by the 
filter 21 so that stimulating signals are not applied to the acoustic wave 
transducer 10. 
As soon as the output of summing point 26 traverses the level x, a 2 MHz 
signal corresponding to an upward jump in the virtual wave form curve is 
produced, and as the ramp signal slopes down further, the frequency of the 
transducer driver signal decreases accordingly until traversing the near 
zero level. The bias in circuit 27 may be adjusted so that the zero level 
will be traversed prior to the next clock which produces a new charge 
pulse for capacitor 30 in this case. Thus, transducer 10 provides 
oscillation bursts at a chirp rate of 200 Hz, and each burst or chirp 
begins at 2 MHz and drops to zero frequency followed by a pause until 
another burst is produced. 
It can readily be seen that, for example, in the cases of FIGS. 3a and 3b 
and, as will be shown in other cases, the clock pulse operation could be 
replaced by an amplitude response in that upon obtaining a particular 
charge state (or discharge state) the capacitor discharge or re-charge is 
being triggered. In this case, the wave form generator includes its own 
oscillator being of the blocking oscillator variety. However, one can 
readily see that such incorporation of the clocking operation may not be 
practical because it is advisable to control the periodicity of the bursts 
separately by a particular clock and without trying the repetition rate of 
the bursts to the ramp slope because that slope determines the rate of 
change in frequency of an acoustic wave burst and should be treated as an 
independent parameter. The resistances 35 and 36 in the respective high 
impedance charge or discharge circuit are drawn to be adjustable in FIG. 1 
which is indicative of the possibility and desirability of varying the 
slope of the ramp signals. 
FIG. 3c depicts a situation in which the wave form generator 25 is adjusted 
as indicated in FIG. 3a. However, the bias circuit 27 is adjusted to apply 
a strong negative value to summing point 26. Now, only the peak portions 
of the range become effective. It can readily be seen that by appropriate 
fine tuning or fine trimming of the bias 27, the frequency range of each 
burst is controlled as to the highest frequency produced. The clock causes 
discharge of the capacitor 30 when, for example, the output of the summing 
point 26 has reached a value causing VCFO 20 to produce an output of 1 
MHz. In this case then, only the lower frequency range from zero to 1 MHz 
is included in each acoustic burst. 
FIG. 3d illustrates a different situation resulting from biasing the 
summing point 26 to a more positive level. In this case, the output of 
summing point 26 will never drop to zero, but the ramp voltage retraces 
when the output of summing point 26 has dropped to a value for which the 
VCFO 10 produces a non-zero frequency such as, for example, 1 MHz. As the 
ramp slopes up, the frequency rises and the filter 21 cuts off any output 
when the 2 MHz level is being traversed so that the virtual ramp drops 
again to zero until the clock discharges the capacitors 30 and the ramp 
begins a new cycle. 
It should be noted that most of the pulses in FIG. 3 are shown to have 
about similar width. However, it can readily be seen that by changing, 
e.g. the slope of the ramp, the pulse width is varied therewith. If one 
wants to maintain the pulse length, but the slope of the ramp is still to 
be varied, one needs to use circuit 28, 29. Use of this circuit obviates 
the distinction between virtual and real ramps as the termination of a 
pulse through clamping action by circuit 28, 29 is directly effective and 
may supersede the filter cut off action if occurring at ramp signal levels 
below x. 
Upon adjustment of the delay, monostable multivibrator 29, one can vary the 
length of the pulse, particularly to obtain cut off below the level x if 
that is desired. In other words, an up sloping ramp signal can be clamped 
to ground potential before it reaches the level x. This is particularly of 
interest for a wave form, as per FIG. 3d. This wave form is to be used to 
obtain a more or less narrow band within each burst. The resistance 35 in 
the charge of the wave form generator circuit determines the slope of each 
pulse as between onset and end which, in turn, controls the band width of 
each burst. The selected bias 27 permits slicing a particular portion of 
the total band in that the low point of ech ramp (point 4) is determined 
by the bias and that, in turn, determines the lowest frequency of a burst 
produced. Without circuits 28, 29, the upper band limit will always be 2 
MHz. However, if the delay 29 is adjusted for an earlier termination of 
the ramp (by clamping the output of the summing point 26 to ground), the 
upper frequency limit of that band is adjusted therewith. In other words, 
a termination of each pulse, e.g. at times t, shifts the highest amplitude 
of each pulse as actually being produced to a lower level z, and the 
amplitude differential between y and z determines the frequency band 
actually being used upon selecting the ramp-slope-defining impedance to 
have a very high value, one can obtain a very narrow band in each chirp. 
It will be appreciated that the wave forms as per FIGS. 3c and 3d result 
from the same selection of impedances as in the case of FIG. 3a except 
that the bias 27 is shifted to a more negative or to a more positive 
value. Analogously, more negative or positive bias for a selection that 
causes a wave form as per FIG. 3b, results in the production of virtual 
signals and wave forms analogous to FIGS. 3c and 3d, respectively, except 
that the signal level decreases during each pulse corresponding to a 
gradual reduction in frequency of the transducer drive signal. 
FIG. 3e shows a pulse diagram which is modified as compared with FIG. 3a in 
that resistor 35 is adjusted to a rather low value. The charge curve of 
capacitor 30 follows the usual non-linear e function with negative 
exponent. The wave form still traverses the level c in which the cutoff of 
filter 21 becomes effective. 
FIG. 3f depicts a situation in which the selected resistor, for example, 37 
when selected for operative connection in charge circuit 31 has a 
non-linear impedance. It is assumed that this particular resistor has a 
characteristic in which its resistance is, in fact, negative for most of 
its effective range. The resistance value may level off for higher values 
or it may be advisable to limit current flow through the resistor 
otherwise. 
Actually, block 37 should be interpreted as an adjustable non-linear 
resistance network. The particular wave form of FIG. 3g results from a 
different pattern of resistances in the charge circuit and will be used to 
obtain a near constant output for the acoustic waves. The rate dV/df is 
small for low frequencies as well as for frequencies near 2 MHz, but high 
for frequencies of say about 1 MHz. The faster rate of change in the wave 
form of each pulse at levels for the production of frequencies near 1 MHz 
has the effect that the output of the transducer 10 is lower than in the 
steady or quasi stationary state. Thus, this wave shape provides for a 
transition from the solid curve to the dashed curve in FIG. 2. 
FIGS. 3h and 3i depict two different situations when the charge circuit for 
capacitor 30 is supplemented by the coil 39 and a stimulating pulse is 
applied for an adjustable delay prior to the respective next clock pulse. 
FIG. 3h in particular depicts a situation in which that delay 43 is 
adjusted to cover just above one half wave of an oscillation for a 
particularly adjusted value of the coil 39. At the end of such a half 
wave, the clock pulse sets the capacitor circuit to the zero level. Bias 
27 is shifted to a more negative level. The FIG. 3g, therefore, can be 
interpreted as producing a wave form in which the peak portion of a sine 
wave is used to produce a double ramp, i.e., a more or less gradual 
upswing followed by a gradual downswing so that in effect the frequency 
spectrum from zero to a peak frequency, being equal to or below 2 MHz, is 
run through twice for each pulse. 
FIG. 4b illustrates the frequency spectrum of a burst when received by 
transducer 60 in response to an acoustic burst produced by transducer 10 
in response to a wave form as per FIG. 3h. It should be noted that the 
spectrum is richer in higher frequencies as compared with FIG. 4a (wave 
form FIG. 3a). 
FIG. 3i depicts the situation in which the delay, provided by circuit 43, 
is somewhat larger to cover a full oscillation which, however, has a lower 
frequency, coil 39 having been adjusted accordingly. The biasing circuit 
27 has been adjusted so that most portions of the output of summing point 
26 is above level x. Thus, only the bottom peak of the negative half wave 
has values below the x level, so that only frequencies from a certain 
minimal, non-zero value, up to 2 MHz, are produced for and by each pulse. 
Turning now to the calibration operation for transducer 60 in that the 
latter has to respond to acoustic emissions of the object 50. For example, 
the internally developing cracks, i.e., the local relief of localized 
tension produces micro-noise having a rather low level and very high 
frequencies. Although acoustic emissions produced under different 
conditions can have a wide variety of frequency spectral types, it was 
found that a typical acoustic emission burst has a frequency spectrum as 
shown in FIG. 4c. This frequency spectrum has resulted from the detection 
of a known acoustic emission followed by frequency analysis by means of 
suitable, known instrumentation. This particular spectrum includes, of 
course, the frequency selectivity of the transducer 60. For purposes of 
calibration, it is desirable to duplicate that spectrum. Thus, acoustic 
signals are to be produced which simulate that spectrum. A good simulation 
is shown in FIG. 4d, showing the response of such a transducer to a 
particular acoustic signal which was generated by applying to the VCFO a 
voltage signal as shown in FIG. 3i. 
It can, thus, be seen that the frequency spectrum of the bursts as received 
by the pickup transducer 60 can be modified by modifying the wave form of 
the signal that drives the VCFO 20. In each instance the control signal 
for the latter results in a narrow band output by the VCFO whose output 
frequency varies over a wide range so that the narrow band itself is 
shifted over that wide range. That range is determined by the boundaries 
(amplitude limits) of the signal driving the VCFO. The various wave forms 
and contour modify the sequence (or direction) in which the various 
frequencies are run through. So far as calibrating operations are 
concerned, one can readily see, that visual inspection of a frequency 
spectrum and variations in the parameters determining the signal contour 
as provided by and in generator 25 permits matching of desired particular 
spectra. 
The invention is not limited to the embodiments described above but all 
changes and modifications thereof not constituting departures from the 
spirit and scope of the invention are intended to be included.