Volume measuring apparatus

The apparatus consists of a mechanical acoustical driver producing sound vibrations in a fluid, such as a liquid or gas, within an enclosed space. A pickup transducer positioned within the tank measures the sound vibrations of the gas within the tank and transmits the same to a lock-in amplifier and a low pass filter. The outputs of the lock-in amplifier and low pass filters are fed to differential amplifiers which amplify the difference between the dynamic fluid pressure changes caused by the sound vibrations and the static pressure. An acoustical resistor is positioned between the pickup transducer and the wall of the enclosed space for providing a fluid passage way therethrough. The acoustic resistor in conjunction with a nonlinear circuit element mounted at the output of the differential amplifiers linearizes the output of the apparatus providing accurate measurements of the material at both extremes of either very small volume or very large volume in the enclosed space. Another embodiment of the invention incorporates a container for a fluid and a pickup transducer positioned therein. An acoustical driver produces sound waves therein which are measured by the pickup transducer and transmitted to a preamplifier and from there to a spectrum analyzer. In place of a spectrum analyzer, a fast fourier transform machine may be substituted. The spectrum analyzer or fast fourier machine detects amplitude changes caused by a fluid leak in the enclosed space.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates generally to an apparatus for measuring the volume of 
an incompressible material confined within a space, and more particularly 
to an apparatus using an acoustical method for making such a measurement. 
2. Description of the Prior Art 
Various methods for measuring the amount of liquid in a storage space are 
currently being used. Probably the most common of these methods is an 
apparatus which incorporates a float designed to rest on the surface of 
the liquid in a tank or chamber. The position of the float (through 
electrical or mechanical means) is used to ascertain the total volume of 
the liquid in the chamber. One disadvantage with this type of apparatus is 
the instability of the float level resulting from movement of the tank 
which may be within a motorized vehicle. Movement of the tank causes the 
mass of liquid within to move to one side (and up one side) of the tank 
upon acceleration or deceleration of the vehicle. Consequently, movement 
of the tank results in a change in the level of the liquid at various 
locations within the tank, thereby altering the position of the float 
without a change in liquid volume. Thus, this type of prior art apparatus 
may require a damping or averaging meter to compensate for the effects of 
movement of the tank. 
In a device where the level of the liquid is used to determine the volume, 
the correlation between the level of the liquid and volume of the liquid 
must be ascertained in order to provide such a measurement. The float 
position range must then be calibrated in order to provide an precise 
measurement of the volume of the liquid. However, even accurate 
calibration may not overcome the inaccuracies inherent in this form of of 
measuring device. If the float is at one side of the tank, the liquid may 
have a meniscus of which may prevent accurate measurement of the liquid 
level. This meniscus can also vary according to the type of liquid or the 
purity of the liquid contained in the tank. Moreover, different types of 
liquids typically have their different surface curvatures caused by 
different surface tensions. Thus, accuracy of the surface level 
measurement of such liquids depends to a large extent on its surface 
curvature and on where the float measurement is made. Thus, a different 
reading will be obtained depending on whether the float is positioned at 
the center of the liquid surface or at the edge near the side of the tank. 
For measurement of a solid or a powdered substance, volumetric measurements 
can involve even greater difficulties. Clearly, a powdered or granular 
solid does not ordinarily have a level surface--particularly if this 
material is constantly being depleted from the tank or added to the tank. 
Thus, a float system of measurement is impractical with solids. 
Measurements of the volume of solids and liquids having large surface 
irregularities have been accomplished in the prior art by ascertainment of 
the specific weight of the liquid or solid to be measured and the 
ascertainment of the weight of the chamber. The weight of the entire 
chamber and material contained therein is then weighed and related to the 
specific weight of the material in order to arrive at the volume of the 
material. The accuracy of this method depends to a large extent on the 
consistency of the specific weight value of the material and the accuracy 
of the weight measurement of the tank. In this regard, it must be noted 
that the specific weight of a material may vary substantially according to 
the temperature of the material. Moreover, it is not always practical to 
obtain an accurate weight measurement of some types of tanks. The location 
of such tanks may make weighing infeasible, or the tanks may be rigidly 
secured to another fixture. The inherent inaccuracies of float type of 
measurement system becomes even more acute when in a zero gravity 
environment such as in open space. In zero gravity environment, the shape 
of a liquid and a solid may be constantly changing. Pockets of liquid and 
gas may be interspersed throughout the storage chamber thereby preventing 
any accurate measurement of the surface level. 
It must also be noted that in a zero gravity and zero acceleration 
environment, the surface level and shape of the liquid is determined by a 
variety of factors. Thus, a determination of the shape of the liquid and 
its position may be very difficult. Thus, the complexity of ascertaining 
the shape and location of the liquid may make prior art volume measuring 
devices unreliable as well as impractical. 
Other prior art devices for measuring the volume of a liquid in a tank 
include various sensors for ascertaining the location of the surface level 
of the liquid. In one such prior art device, an acoustic signal is 
reflected from the surface of the liquid to a receiving sensor. 
Measurement of the time it takes to arrive at the receiving sensor makes 
possible a measurement of the location of the surface level of the liquid. 
However, as pointed out hereinabove, meniscus of the liquids, their 
surface curvature variations and movement of the liquid are also pertinent 
with this prior art system as well. Consequently, this of the float type 
measurement described hereinabove. method of measurement has most, if not 
all, of the disadvantages 
Another prior art system for measuring the volume of a noncompressible 
liquid or solid in tank uses an acoustical means to measure the pressure 
of a fixed volume of gas in the tank. The pressure is inversely 
proportional to the gas volume for a constant quantity of gas at a 
constant temperature. Consequently, a measurement of the gas pressure will 
indirectly be a measurement of the gas volume. Moreover, a pressure change 
cause by a change in volume caused by the acoustic diaphragm is 
proportional to the volume of the gas in the tank. In a storage tank of a 
fixed and invariable size deducting the gas volume from the total volume 
of the tank will result in a measurement of the liquid or solid volume in 
the tank. 
One prior art device uses an acoustic speaker and two transducers to 
measure the pressure changes of the gas. One transducer is situated within 
the chamber containing the material to be measured and the gas, and the 
other transducer is situated in a reference cavity which contains only the 
gas. Use of the reference cavity tends to neutralize any static pressure, 
as per PV/T=Gas Const; this is any variation not caused by the movement of 
the speaker; thus, the reference cavity tends to compensate for pressure 
variations due to temperature variations, mixture of another gas within 
the chamber static pressure changes, etc. The reference cavity is 
connected to the chamber by means of a small passage way. However, an 
obvious disadvantage of this prior art system is that a liquid to be 
measured within the chamber may also leak into the gage and/or reference 
cavity thereby altering the total volume of the material in the chamber, 
changing and introducing inaccuracies into the measurement. Pressure 
changes result from acoustic vibrations in the gas accomplished by means 
of the speaker diaphragm which is driven also by a transducer. 
However, a primary disadvantage with this prior art system incorporating an 
acoustic speaker is its susceptibility to diaphragm distortion caused by 
splashing of the liquid within the chamber. The liquid may soak into the 
diaphragm causing distortion in the diaphragm's frequency of vibration. In 
addition, the liquid may also splash onto the diaphragm thereby adding to 
its weight and also consequently adding to the load onto the 
electromagnetic driver for the speaker; since the amplitude of vibration 
of the diaphragm is load-dependent, the added weight of the liquid on the 
diaphragm causes an undesirable alteration in the volume change (and 
concomitant pressure change) of the gas produced by the vibrating 
diaphragm. This alteration in amplitude introduces an inaccuracy in the 
measurement of the material volume. The liquid may also distort the shape 
of the diaphragm thereby making the reference cavity volume not a 
constant; since this prior art system bases its volume measurement on the 
equation: 
##EQU1## 
where k=a constant a lack of a known constant value of V.sub.R reduces the 
accuracy of this measurement. 
It must also be pointed out that since the acoustic speaker is operated at 
its resonant frequency, the liquid on the diaphragm will also alter 
.DELTA.P (and .DELTA.V). 
The use of an acoustic speaker makes the system very sensitive and fragile; 
since the speaker is driven at resonant frequency, the liquid 
contamination and mechanical and/or acoustic vibration could make the 
system grossly inaccurate since the speaker must be responsive to only one 
sonic frequency of vibration. 
The diaphragm is mounted between the reference cavity in the chamber so 
that displacement of the diaphragm results in a corresponding change in 
volume of the reference cavity as well as a change in volume of the 
chamber. Vibration and noise interferences are eliminated by means of a 
suitable pass band filter. The operating frequency of the diaphragm is 
preferably less than the resonance frequency of the liquid gas mixture. 
It must also be noted that operating this prior art system at its resonant 
frequency renders the amplitude of the response to the driver frequency 
and/or driver energy nonlinear i.e. at resonance small incremental changes 
in energy input to the speaker result in disproportionately large 
increases in the amplitude of vibration of the speaker and consequently 
the change in pressure and volume of the gas. Thus, the nonlinear response 
of the speaker introduces gross inaccuracies in the measurement. 
A disadvantage with such prior art acoustic measurement systems is the 
difficulty of obtaining a stable frequency of vibration of the diaphragm. 
A wide range of sound frequencies emitted therefrom introduces 
inaccuracies in the final measurements. Indeed, vibration of the diaphragm 
may induce vibrations in other parts of the system. Moreover, vibration 
problems are not completely eliminated by the pass band filters. Instead, 
vibration problems are extant and are likely to introduce error in the 
final measurement. 
One of the primary disadvantages is that the reference cavity requires a 
passage way connecting it to the chamber; this passageway is not large 
enough to quickly equalize static gas pressures between the chamber and 
the cavity. However, enlargement of the passage way introduces leakage of 
liquid and/or solids into the passageway and therefore from the chamber 
into the cavity. Both of these occurrences introduce various inaccuracies 
in the measurement. It is also pertinent to note that gas diffusion from 
the chamber to the reference cavity or vice versa typically takes an 
inordinately long period of time. A primary disadvantage of this prior art 
acoustic measuring system is that theoretically, as the tank fills the 
pressure signal approaches infinity whereas the volume of the material to 
be measured merely approaches a certain maximum value. Conversely, as the 
tank approaches empties, the pressure signal approaches a certain value 
nonlinearily. Thus, the relationship between the tank volume and the 
pressure signal is a curve rather than a straight line. This nonlinear 
aspect of the system introduces gross inaccuracies and difficulties in 
measurement when the tank is either close to empty or close to full. 
Moreover, the complexity of the system introduces many ways in which the 
system may break down or compromise accuracy. 
SUMMARY OF THE INVENTION 
The present invention provides a simple apparatus for acoustically 
measuring a volume of liquid or solid within an enclosed chamber. 
Another aspect of the present invention incorporates an improved acoustic 
driver which provides a constant amplitude independent of frequency 
response to energy input. 
Another aspect of the present invention incorporates an improved acoustic 
driver in the apparatus which is impervious to frequency and amplitude 
response alteration due to contact with liquid in the chamber or cavity of 
the apparatus. 
Another aspect of the present invention incorporates an improved acoustic 
driver in the apparatus which is substantially vibration free. 
Another aspect of the present invention provides an apparatus for 
acoustically measuring volume of a liquid or solid in a chamber to a high 
degree of accuracy. 
Another aspect of the present invention provides an acoustical volume 
measuring apparatus which has a linear output from zero volume of material 
to be measured to maximum volume of the material to be measured within the 
tank. 
Another aspect of the present invention provides an apparatus for 
acoustically detecting leakage of a gas, liquid, or solid from an enclosed 
space. 
The present invention thus provides for a very simple apparatus for using 
acoustics to measure the volume of a solid or liquid within a chamber. One 
embodiment of the invention uses only one transducer to pick up the sound 
vibrations within the chamber. The acoustical driver is operated at very 
low frequencies in order to neutralize the effects of heat, and 
contamination of the gas on the acoustical measurements. At low 
frequencies, the effect of the specific heat on the pressure change 
relationship to volume change is minimized. 
The present invention uses a lock-in amplifier to selectively amplify a 
signal within a very narrow band width. The lock in amplifier is precise 
in rejecting unwanted signals caused by vibration or noise and eliminate 
the need for a pass band filter. The use of a lock-in amplifier thus 
enables a more accurate measurement of the volume of the material. 
Another important feature of the present invention is the addition of an 
acoustic resistor and nonlinear circuit element which enables the 
linearization of the acoustic output in relation to the pressure of the 
fluid, such as a liquid or gas. Thus, the addition of these elements 
changes the acoustic response in relation to the pressure which is 
ordinarily a curve into an approximately straight line. However, in place 
of the acoustic resistor and nonlinear circuit element, a logarithmic 
amplifier may also be used. The logarithmic amplifier converts the 
exponential relationship between pressure change and volume change into a 
linear relationship. This eliminates inaccuracies both at points at where 
the tank is approximately empty and where the tank is approximately full. 
In summary, the present invention provides for a very accurate and reliable 
apparatus for acoustically measuring the volume of a material within the 
chamber. Moreover, the accuracy is maintained at both extremes of the 
volumetric measurements.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to the drawings, there is shown a preferred embodiment of the 
invention generally designated by the numeral 10 according to the present 
invention. The apparatus 10 consists of both mechanical and electrical 
components. The apparatus is illustrated in diagrammatic form for 
simplicity and clarity. A sound source or acoustic driver 12, which may be 
any one of the acoustical drivers depicted in FIG. 6, emits sound into an 
enclosure 16. The enclosure 16 has a portion, designated as 18, containing 
the fluid, such as a liquid or gas; and also a portion, designated as 20, 
containing a liquid or solid, the volume of which is to be measured. There 
is preferably no partition between the portions 18 and 20. A pickup 
transducer 14 is appropriately positioned in the portion 18 of the 
enclosure 16 containing the fluid, such as a liquid or gas; portion 18 is 
also referred to as the ullage volume portion 18 of the enclosure 16. The 
transducer 14 may be merely a microphone or other suitable device for 
changing mechanical energy of the sound waves of the sound source or 
acoustic driver 12 into electrical energy. An acoustical resistor 22 is 
connected between the transducer 14 and the enclosure 16. The acoustical 
resistor 22 is preferably merely a tube providing a connection between the 
outside and inside of the enclosure 16. The embodiment of FIG. 1 is 
provided with the acoustical resistor 22 where there are no significant 
changes in static fluid pressure and fluid mixtures in the ullage volume 
18 of the enclosure 16. 
A lock-in amplifier 24 is connected to the electrical output of the 
transducer 14. The lock-in amplifier 24 has a fixed gain and the output of 
the lock-in amplifier 24 is in turn connected to a differential amplifier 
28. The output of the transducer 14 is also channeled to a low pass filter 
26. The low pass filter filters out all of the signal above a certain 
desired value whereas the lock-in amplifier only allows a signal in a very 
narrow band width. Thus, the low pass filter 26 processes a signal which 
is lower in frequency than that processed by lock-in amplifier 24. 
Consequently, the low pass filter processes static pressure changes which 
are typically slower than the pressure changes caused by the sound source 
or acoustical driver 12 and therefore have a lower frequency. The output 
of the low pass filter 26 is transmitted to differential amplifier 30. The 
other input terminal of differential amplifier 30 receives from a desired 
reference pressure output. The output of differential amplifier 30 is fed 
into the input of differential amplifier 28, the output of which is 
communicated to resistor 32. The output from resistor 32 is bifurcated 
into a nonlinear circuit element and an ordinary direct readout which may 
be digital or mechanical. The readout 36 produces a response curve whereas 
the output from the nonlinear circuit element 34 produces an output which 
is linear. 
FIG. 2 shows the response of the embodiment of FIG. 1: 1) without the 
nonlinear circuit element and the acoustic resistor; 2) with the acoustic 
resistor; 3) with the acoustic resistor and nonlinear circuit element 
combined. It can be seen through FIG. 2 that the acoustic resistor and 
nonlinear circuit element provide a response which is a substantially 
straight line. Consequently, the response of the embodiment of FIG. 1 with 
the acoustic resistor and nonlinear circuit element enables a measurement 
to be made when the tank is both nearly empty and nearly full. This 
provides an apparatus which is capable of accurately measuring the volume 
at both ends of the scale. 
The embodiment of FIG. 3 is similar to that of FIG. 1 except that the 
outputs of the low pass filter 126 and lock-in amplifier 124 are fed to a 
divider 138. The divider 138 expresses the outputs from the lock-in 
amplifier and low pass filter in terms of a ratio. The numerator of this 
ratio is the output from the low pass filter 126 which is essentially a 
measurement of the slowly varying static pressures within the enclosure 
116; the denominator is the output from the lock-in amplifier 124 which is 
a measurement of the dynamic pressure changes produced by the sound source 
or acoustical driver 112, which may be any one of the acoustical drivers 
depicted in FIG. 6. Since the volume of the mass to be measured is 
inversely proportional to the dynamic pressure of the fluid in the ullage 
volume 118 of the enclosure 116, the output of the divider 138 provides 
the ratio: 
##EQU2## 
The output of the divider 138 is fed into the differential amplifier 140 
which also receives input E.sub.T which is a desired signal which is 
proportional to the total volume of enclosure 116 (i.e. a calibration 
signal). 
The same transducer 114 can measure both dynamic pressure variations and 
static variations and feed the same to one divider circuit. The output of 
the differential amplifier 140 is fed into a direct readout indicator 136. 
FIG. 4 show the output of system in FIG. 3 plotted against the volume of 
the liquid of solid to be measured. As is desirable, the graphical 
representation shows a linear response. 
FIG. 5 shows a volume measuring apparatus which may be any one of the 
acoustical drivers depicted in FIG. 6. 
In FIG. 5 a transducer 214 changes the acoustical vibrations in the 
enclosure 216 to electrical energy and communicates the electrical output 
to a lock-in amplifier 224 which in turn communicates its output to a 
divider 238. The communication means are preferably just ordinary 
electrical wiring generally designated as 239. Where the acoustical driver 
212 used is one which contains one or two reference cavities a pick up 
transducer 244 may be positioned in one or both reference cavities to 
detect acoustical vibrations in each respective reference cavity. Pressure 
switches 215 may be installed adjacent to the passage way to the reference 
cavity so as to equalize pressure and specific heat ratio much more 
quickly. An acoustic resistor 222, preferably merely a tube, provides a 
connection between the fluid in the ullage volume 218 and each reference 
cavity. The pick up transducer 244 in each respective reference cavity 
communicates the output to an electrical filter 242. The output from 
filter 242 is communicated to the divider 238, first passing through an 
AC/DC converter 246. The output of the divider 238 provides a ratio in 
which the output of the reference transducer 244 is the numerator and the 
output of the ullage volume transducer 214 is the denominator. Due to the 
following relationship the resultant output is devoid of any indication of 
static fluid pressure variations: 
##EQU3## 
The output of the divider 238 is communicated into a differential 
amplifier 240. The other input of the differential amplifier 240 receives 
the court signed E.sub.T proportional to the total volume of the enclosure 
216. The output of the differential amplifier 240 is thus proportional to 
the difference between the enclosure volume and the ullage volume. The 
output of amplifier 240 is communicated to an indicator 241 in an 
appropriate digital or mechanical readout form as desired. The output thus 
is a direct readout of the volume of the material to be measured 220. 
FIG. 6 contains several embodiments of the acoustic driver or sound source, 
each of which may be used in the apparatus as described in FIGS. 1, 3, 5 
and 8. 
The mechanical acoustical drivers of FIGS. 6(a) and 6(b) are similar except 
that the mechanical acoustical driver of FIG. 6(b) contains a diaphragm 
621 which seals off each reference cavity 613 from the ullage volume 
chamber 618. As shown in FIG. 6(a), however, the mechanical acoustical 
driver 512 is adequately functional without the diaphragm. Consequently, a 
detailed description of the mechanical acoustical driver of FIG. 6(b) 
equals a description of the mechanical acoustical driver of FIG. 6(a) 
minus the diaphragm. 
The mechanical acoustical driver of FIG. 6(a) and 6(b) have two reference 
cavities 513 and 613 respectively which enable the passage of the ullage 
volume liquid, 518 in FIG. 6(a) and 618 in FIG. 6(b) between these 
reference cavities and the enclosure containing the ullage volume fluid 
and the material whose volume is to be measurered. A prefereably motor 
driven cam 515 in FIG. 6(a) and 615 in FIG. 6(b), makes contact with 
preferably two pistons 617. Which produce volume metric displacements in 
each reference cavity 613 that are naturally, substantially 180.degree. 
out of phase with the volumetric displacements produced in the enclosure 
containing the ullage volume fluid and the material whose volume is to be 
measurered. 
A diaphragm 621 positioned along each piston 617 provides a subsantially 
frictionless seal and seals off each reference cavity 613 from the ullage 
volume chamber fluid. Springs 519 in FIG. 6(a) and 619 in FIG. 6(b), bias 
each piston 517 in FIG. 6(a) and 617 in FIG. 6(b), and accompanying 
diaphragm 621 in order to provide a faster and more positive action. 
However, other suitable biasing means may also be used. 
Cam lobes 522 in FIG. 6(a) and 622 in FIG. 6(b) are positioned preferably 
at opposite ends of the motor driven cam 515 in FIG. 6(a) and 622 in FIG. 
6(b), in order to substantially eliminate backlash, thereby moving the 
piston 517 in FIG. 6(a) and 617 in FIG. 6(b), axially outward 
approximately simultaneously. The substantially simultaneous movement of 
the pistons, 517 in FIG. 6(a) and 617 in FIG. 6(b), significantly reduces 
undesired noise and vibration caused by the driver 512 in FIG. 6(a) and 
612 in FIG. 6(b). Moreover, the undesired noise and vibration may be 
further reduced by coating the pistons, 517 in FIG. 6(a) and 617 in FIG. 
6(b), with teflon or other suitable material. 
Consequently, the mechanical acoustical driver, 512 in FIG. 6(a) and 612 in 
FIG. 6(b), is virtually unaffected by liquids and has a precisely 
controlled amplitude and frequency of response. Its response to frequency 
input is linear; a response which is optimal for acurate measurement of 
the volume of the material to be measured. Additionally, the mechanical 
acoustical driver can produce a signal which provides a constant amplitude 
independent of frequency response to energy input into the acoustic 
driver. 
Also, a pickup transducer, 544 in FIG. 6(a) and 644 in FIG. 6(b), may be 
positioned in each reference cavity for measuring pressure changes in the 
fluid which has entered each reference cavity. 
FIG. 6 (c) constitutes another embodiment of a mechanical acoustical driver 
712. It has preferably two reference cavities, each designated as 713, 
which enable the passage of the liquid between these reference cavities 
and the enclosure containing the ullage volume 718 and the material whose 
volume is to be measured. These cavities also have the same function and 
volumetric displacement effects as each of the reference cavities depicted 
in FIGS. 6(a) and 6(b). 
A motor driven cam 715 actuates preferably two diametrically opposed 
bellows 717 which are biased preferably by tension or compression springs 
719 so as to provide a faster and more positive action. However, other 
suitable biasing means may also be used. 
Both bellows 717 move axially outward at approximately the same time, 
produce minimal friction during their operation, and act as a 
substantially frictionless seal in the manner similar to the diaphragms 
depicted in FIGS. 6(a) and 6(b). Also, a pickup transducer 744 may be 
positioned in each reference cavity for measuring pressure changes in the 
fluid which has entered each reference cavity 713. 
As with the drivers in FIGS. 6(a) and 6(b), the mechanical acoustical 
driver 712 is virtually unaffected by liquids and has an amplitude and 
frequency of response which may be precisely controlled. Its response to 
frequency input is also linear; the optimum response for accurate 
measurement of the volume of the material to be measured. Additionally, 
the acoustic driver can produce a signal which provides a constant 
amplitude independent of frequency response to energy input into the 
mechanical acoustical driver. 
FIG. 6(d) depicts a pneumatic acoustical driver 812 with double bellows 
817. It may also have two reference cavities, each designated as 813, 
which enable the passage of the liquid between these reference cavities 
and the enclosure containing the ullage volume 818 and the material whose 
volume is to be measured. Here, a pneumatic device actuates the double 
bellows by transferring high pressure fluid from an enclosure or tank 814 
into the double bellows 817; thereby producing in them substantial 
simultaneous axially outward and retractive movement. The double bellows 
817 operate without producing substantial friction. The above pneumatic 
device comprises a high pressure fluid tank 814, an electric pressure or 
solenoid or piezoelectric valve 816 operated at preferably infrasonic 
frequency, and passageways for transferring the above fluid to the above 
valve 816 and from the valve 816 to the double bellows 817. FIG. 6(e) 
depicts a pneumatic acoustical driver 912 with a diaphragm 921. Here, a 
pneumatic device actuates a diaphragm 921. A spring 919 biases the 
diaphragm 921, thereby facilitating substantially uniform expansion and 
contraction of the diaphragm 921. 
As in FIG. 6(d) the pneumatic device comprises a high pressure fluid tank 
914, an electric pressure or solenoid or piezoelectric valve operated at 
preferably infrasonic frequency, and passageways for transferring the 
fluid to the above valve and thereon to the diaphragm. The pneumatic 
device further operates similarly to that depicted in FIG. 6(d). 
FIG. 6(f) depicts an electrodynamic acoustical driver 1012. This driver 
comprises a closed loop system; and includes a power amplifier 1018 and a 
signal amplifier 1030 which electronically operate on the electrodyamic 
core 1019 so as to produce a relatively constant volumetric displacement 
of the bellows 1027. 
More specifically, an oscillator 1016, electronically connected to the 
power amplifier 1018, provides a driving signal for the power amplifier 
1018. The output of the power amplifier 1018 is electronically 
communicated to the primary or driver coil 1022 contained in the 
electrodynamic core 1019; and thereby drives the driver coil 1022. 
Electric current in the driver coil creates movement of the driver coil 
1022 and the sensing coil 1021 in a magnetic field due to magnet 1020 in 
accordance with the well-known Lens law. 
The sensing coil 1021 moves in the same magnetic field as the driver coil 
1022, measures the displacement of the bellows 1027 and communicates this 
displacement to the signal amplifier 1030. 
The output from the signal amplifier 1030 is electronically channelled back 
into the input channel of the signal amplifier 1030, and is electronically 
fedback into the power amplifier 1018. The above channelled output 
proceeds through a damping network and signal conditioner 1032. The above 
feedback proceeds through an alternating current to direct current 
converter 1034. 
The act of feeding back output from the signal amplifier 1030 into the 
power amplifier 1018 produces a relatively constant volumetric 
displacement of the bellows 1027. In particular, any changed displacement 
of the bellows 1027 is automatically measured by the sensing coil 1021 and 
communicated to the signal amplifier 1030, thereby producing a change in 
the output of the signal amplifier 1030. This change is in turn fedback 
into the power amplifier and activates a gain control 1036 such that the 
output from the power amplifier can be adjusted so as to maintain the 
relatively constant volumetric displacement of the bellows 1027. 
FIG. 7 is another embodiment 310 of the invention providing improved 
resolution and accuracy. One of the results of this system is to provide a 
means whereby at any given ullage volume dynamic fluid pressure output 
equals reference pressure output. This is accomplished by means of 
feedback systems 356. 
Enclosure 316 has an ullage volume 318 and a liquid volume 320 as in the 
previous embodiments. A transducer 314 is disposed so as to pick up 
dynamic fluid pressure changes in the ullage volume 318. The output of the 
transducer 314 is communicated to a lock-in amplifier 324 and to a low 
pass filter 326 as in the previous embodiments. The lock-in amplifier has 
an output which is communicated to a differential amplifier 328. The 
output of the low pass filter 326 is fed into differential amplifier 330. 
The other input of the differential amplifier 330 receives static pressure 
variation measurements from transducer 314 as in the other embodiments. 
The output of differential amplifier 330 is communicated to the other 
input terminal of differential amplifier 328. The output of differential 
amplifier 328 is fed into differential amplifier 350. The other input 
terminal of differential amplifier 350 receives a desired reference signal 
representing the total volume of the enclosure 316 or the pressure change 
in a fuel enclosure 316. The output of the differential amplifier 350 is 
communicated to a power amplifier 352 and also back into the pressure 
signal side of the differential amplifier 350. An oscillator 354 is 
connected to the lock-in amplifier 324 and amplifier 352 and serves as a 
driving signal for the power amplifier 352 and reference signal for the 
lock-in in amplifier 324. The output of the power amplifier 352 is 
directed both to a suitable readout indicator 341 and to the driver 312. 
The output of the power amplifier 352 thereby drives the acoustics source 
and tends to equalize the pressure output from the lock-in amplifier 324 
to the reference pressure. 
FIG. 8 shows another embodiment 410 of the invention which is designed to 
measure leakage from an enclosed space. In this embodiment of the 
invention the enclosure 416 does not have to be rigid as in the other 
embodiments. This leakage detection system 410 is applicable particularly 
in the case of a spacecraft, space station, or astronaut suit. A constant 
volume sinusoidal driver 412 provides a constant volume displacement at a 
specified frequency. A pressure transducer 414 detects the change in 
pressure within the enclosed volume. Whenever there is a leakage in the 
enclosed volume, the amplitude of the acoustic signal is reduced and there 
is a distortion in the sinusoidal acoustic pressure wave shape. In this 
embodiment, the output of transducer 414 is communicated to a preamplifier 
462 which is then fed into a spectrum analyzer or fast fourier transform 
machine 464. Theoretically, the system can detect even very small leaks in 
a very large enclosed space. 
It can also readily be seen that compliance (for example, of a lung) can be 
measured using the applications of the principles described hereinabove. 
The compliance of air enclosed in a lung cavity of 10 liter capacity is: 
##EQU4## 
where .gamma.=1.41; V.sub.o =10 liters; and P.sub.o =14.7 psi. 
It is to be understood that the above described embodiments are merely 
illustrative of some of the many specific embodiments which represents 
applications of the principles of the present invention. Clearly, numerous 
and varied other arrangements may readily be devised by those skilled in 
the art, without departing from the spirit and scope of the invention.