A Soldier's Microphone/Hydrophone-Part II
(Experimental Results)

1  Prototype Microphone Mechanical Design

Illustrated in Fig. 1 is the prototype microphone which was built and tested during this study. Primarily because of time constraints of the abbreviated summer program, it was not possible to build a sophisticated instrument. The expectation that a crudely fabricated prototype would nevertheless yield useful results proved to be true.

The static electrodes shown in Fig. 1 were made of 1/16" printed circuit board, clad with copper on both sides. Not shown in the figure are the external copper circles which were grounded to improve shielding. The 1" diameter circular PC board central electrodes were formed using a die punch and a hammer. Once stamped, insulator strips were produced by filing away copper along a diameter; and a near uniform distribution of approximately 75 holes of 500 mm diameter each were drilled in each electrode. They were then re-flattened with the hammer on a smooth hard surface. Each electrode was then pressed back into the hole from which it came, the gap spacing being maintained by friction. The rss deviation from flat of each electrode was estimated to be no better than 100 mm. The gap spacing was established by manually pushing at various points around the circumference, while viewing with a magnifying glass. The nominal gap was visually estimated to be 500 mm. This estimate was reasonably supported by measured values of the capacitance.

The subscripts, T and B of Fig. 1, are for purpose of identifying the application place of electrical drive. External cross connection (to form 2 equipotentials, characteristic of SDC devices) is established by soldering an external wire between 1T and 1B and likewise between 2T and 2B.

The diaphragm was fabricated from aluminized mylar, of 10 mm thickness. A straight insulator strip of approximately 1 mm width was produced in the otherwise uniform distribution of aluminum, of thickness 0.02 mm. This was accomplished by dipping a toothpick in concentrated sodium hydroxide and then moving it, in light contact with the metal, along a straight edge. After washing, the film was then stretched in a ``crochet" like hoop, made of plexiglass turned in a lathe (the newly formed insulator strip passing along a diameter of the hoop). In the stretched state, the membrane was superglued (non-metallic side) to the non-metal bearing (top) surface of the outer ring which supports the lower static electrode of Fig. 1. Note that the top and bottom support rings for the static electrodes are oppositely positioned with respect to their single side copper surfaces. The copper of the bottom ring is not indicated. That of the top ring is shown (bold line) because it is the means for making electrical (soldered) contact with the membrane semi-circular electrodes. It can be seen from Fig. 1 that electrical connection to the diaphragm electrodes is via mechanical contact between the aluminum of these electrodes and the bottom layer of copper on the upper outer ring that supports the top electrode The drive wire pair were soldered at 3 & 4. It may be useful for the reader to compare this microphone to the SDC pressure sensor (Figures 10 & 11, pages 22-23, part I.) For the prototype, the mechanical contact was maintained by light pressure from external screw/nuts.


Picture Omitted
Figure 1: Illustration of the prototype SDC microphone.

2  EXPERIMENTAL RESULTS

The data of this document were obtained with the microphone illustrated in Fig. 1, as supported by the Symmetric Differential Capacitive Control Unit sold by Tel-Atomic Inc of Jackson, Michigan[]. This electronics package is too expensive for extensive fielding, so consideration of alternatives became a natural part of the present study. A possible beginning electronics configuration, which showed promise, is discussed later in section 4.

In the figures which follow, temporal data is not provided; however, the study was also concerned with transient time records, derived from shocks and acquired with a storage oscilloscope. These will also be described later in section 2.5.

2.1  SDC high frequency response

Partly because of their experience with fluidic amplifiers having similar symmetry, the acoustics personnel headed by Steve Tenney at ARL recognized that the SDC pressure sensor might be convertible to a useful microphone. Since pressure sensors are normally concerned with very low frequencies, as compared to those measured with microphones; a first challenge was to extend the frequency range of SDC operation. This required two modifications: (i) decreasing the physical dimensions of the transducer (specifically the diameter of the mylar membrane diaphragm), and (ii) increasing the cutoff frequency of the low pass filter that is used with its synchronous demodulator. The latter modification was trivial-simply changing the size of the capacitor that shunts the feedback gain resistor in the output stage operational amplifier. To accomplish the former, however, required major changes to traditional manufacturing techniques. Methodologies for construction of the crude prototype were described earlier; a major challenge for any future work will be the identification and perfection of assembly line techniques for mass production of fieldable microphones.

The Part I theoretical predictions, pertaining to frequency response, have been reasonably validated; i.e., the circular ``drumhead" diaphragm should resonate at a frequency which is directly proportional to the tension with which it is stretched and is inversely proportional to its radius, a. These proportionalities were demonstrated as follows. The first phase measurements of the study were performed with the 2" pressure sensor, connected to the sealed enclosure of a loudspeaker with a short section of tygon tubing. The tubing was necessary because the diaphragm of the pressure sensor does not couple to the system it is monitoring via multiple holes, as in Fig. 1. It could be compared with the follow-on prototype, because both were produced from the same 10 mm mylar. Because of the expected proportionalities, the prototype microphone was made smaller, and its diaphragm was tensioned as greatly as was conveniently possible in the homebuilt ``crochet" hoop. Although tensions in the two were not measured (either absolutely or in a relative sense), it is thought that the tension in the microphone is greater than that of the pressure sensor by a factor of about 2.

The resonance frequency of the SDC microphone was found to be in the neighborhood of 1 kHz, whereas that of the pressure sensor was about 300 Hz. These numbers are consistent with the earlier statements and with the expectation that the mechanics of vibration (described in Part I) should be closer to that of a ``drumhead" than a ``plate".

Figure

Figure 2: SDC microphone high frequency cutoff.

Shown in Fig. 2 are data that were taken by positioning the prototype microphone close to an inexpensive midrange speaker that was driven by a frequency adjustable sinewave oscillator. The drive voltage was held constant as the frequency was varied from 200 to 2000 Hz. Not shown in the graph are the responses that were seen above the ``zero" at 1600 Hz. Sharp secondary resonances at 1750 and 2200 Hz were observed. All these results are in reasonable agreement with the Part I treatment of drumhead vibration.

2.2  Spectra

The speaker used to generate the data of Fig. 2 was not suited to low frequency measurements. For SDC response measurements down to 1 Hz, the microphone was placed in the ARL anechoic chamber. The frequency domain graphs of Figures 3 through 6 were generated using Fast Fourier Transform (FFT) algorithms, that are part of the well known LabView (National Instruments) software. The ARL computer with which these data were acquired and then anaylyzed, is MAC rather than PC based. Because the LaTEX desk top publishing software with which this document was written, is PC based; the laser printed figures were scanned, rather than directly imported. By contrast, other figures of this document were directly integrated post script (PS) or encapsulated PS files. The EPS files derive from monitor displayed graphs generated by QuickBasic, which were screen captured using PCXLAB, by Frendsen. The PS files were produced with TEXCAD, a computer aided drawing package which is common to many users of LaTEX shareware produced by Eberhard Mattes.


Figure 3: Microphones' response at 1 Hz.

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Figure 4: Microphones' response at 2 Hz.

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Figure 5: Microphones' response at 5 Hz.

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Figure 6: Microphones' response at 10 Hz.

(This page left blank for replacement with separate figure 6) Sound fields were generated with high quality loudspeakers, driven by a powerful McIntosh vacuum tube, push pull amplifier. Both speakers and microphone were located inside a sophisticated anechoic chamber (manufactured by Eckel-Cambridge, MA). The dichotomy of the equipment collection was striking. On the one hand, test instruments and the reference B & K microphone were state of the art; whereas the prototype SDC microphone was almost as crude as could be built and yet yield significant results. This dichotomy is mentioned for the following reason. It is rarely possible to take a sensor which performs extremely well in one arena and adapt it to another, where design considerations are necessarily quite different. Consider, for example, the B & K microphones. They have been without equal for most applications, largely because of the excellence of their manufacture. Consequently, they have become the primary world standard for acoustic measurements during the last half-century. As with any sensor, however; their regions of acceptable performance are limited. The low frequency cutoff of currently manufactured instruments is one which is especially important to envisioned army applications involving infrasound. This point should become clear to the reader in the discussions which follow.

2.3  SDC - B&K comparisons

From the theory of Part I, it is predicted that the SDC microphone should not experience low frequency cutoff, in stark contrast to the conventional condenser microphone. Already well known were the resulting severe limitations that can result when working with inexpensive electret microphones. Reasons for the usually large value of the low frequency cutoff are discussed in Part I of this document. Even though the 1/2" B & K microphone, against which the SDC prototype was compared, is not so severely limited; nevertheless, the extent of limitation had not been previously determined. This was true even though the microphone has served as a standard for acoustics testing at ARL. Lack of data for frequencies below 5 Hz resulted because of changes in the B & K product line. If the 1/2" ARL reference microphone were connected to an earlier B & K electronics package that is carrier based (high frequency); then the low frequency cutoff should be consistent with the manufacturer's specified 0.01 Hz for this configuration. The electronics currently produced by B & K and provided with the microphone, yield a much higher cutoff of 5 Hz (refer to ref. 2, Part I). The data of Figures 3 - 6 are consistent with this value.

In all of the Figures 3 through 6, the SDC prototype response is provided in the top graph; and the 1/2" B & K response in the bottom one. All plots are log-log, with the ordinate units being dB below 1 V. Thus, it is seen that the background noise for the SDC microphone is 30 mV in the range from 1 -50 Hz. The SNR of the B & K microphone is seen to be less for frequencies below 10 Hz. Although it had been previously thought that the 1/f character of the B & K microphone response was acoustical in nature, the present comparisons show that this cannot be the case. Instead, the 1/f noise derives from the B & K electronics. The distinctly different frequency trends for the two microphones is due to differences in detection technique. In particular, the reduced low frequency noise of the SDC microphone is the direct result of synchronous demodulation; which is described in Part I.

In each of Figures 3 - 6, a monochromatic drive was applied to the loudspeaker woofer. The sizeable full width half maximum (FWHM) of the respective acoustic ``lines" evident in the figures, is a reflection of short record collection times, and not due to a loss in signal generator monochromaticity.

As can be seen from the SDC graphs, the woofer efficiency degrades significantly for frequencies below 10 Hz. [From studies performed with SDC pressure sensors (such as the whirling catheter calibration scheme mentioned in section 5.4 of Part I), it can be confidently stated that the loss of signal at low frequencies is due to the loudspeaker and not the microphone.] Moreover, the speaker's harmonic distortion is seen to be significant at the low frequencies. In Fig. 4, for example, both microphones show an unusually large 3rd harmonic that is only 10 dB below the fundamental, even though the 2nd harmonic is more than 15 dB down. In the 10 Hz Fig. 6 case, the 2nd through 5th harmonics are all quite large. It should also be noted that there is a pronounced 60 Hz pickup in all of the SDC cases. This was largely due to the manner in which the SDC microphone was attached to the electronics hardware. For sake of convenience, the two part chassis box was opened, and the microphone was attached with a screw and nut to one of the two components. In the future, most of this pickup can be eliminated by employing a better mechanical mount.

For all frequencies below 10 Hz, it is seen that the SDC microphone outperforms the B & K microphone. In fact, somewhere in the neighborhood of 2 Hz, the B & K signal is no longer above noise. Contrariwise, the B & K unit outperforms the prototype for all frequencies above about 10 Hz. It's superior SNR at high frequencies was expected, for the following reason.

2.4  SDC prototype limitations

It was noted earlier that the prototype microphone was crudely built with a very wide gap spacing of 500 mm. Additionally, the drive voltage applied to the SDC microphone in these studies was only 10 V peak to peak. Because the sensitivity of any capacitive microphone is proportional to the magnitude of the electric field in the gap, there is a resulting dramatic reduction in the instrinsic sensitivity, as compared to the B & K instrument. The amount of this reduction can be appreciated by considering the nominal parameter values for the B & K units-20 mm gap spacing, with a bias voltage of 200 V. Thus the electric field is greater than that of the SDC unit by a factor of 500  54 dB. Any SDC microphone that should become a viable candidate for fielding must have reduced the gap spacing significantly below the 500 mm value of the prototype. By means of standard processing techniques, such as etching; it should be straightforward to reduce the gap by an order of magnitude. As demonstrated by the author during his doctoral work with ultrasonic capacitive microphones, this task can be greatly simplified by polishing the electrode surfaces until they're nearly optically flat in the visible. The microphone may prove to be acceptably sensitive at this point, without having to go to higher drive voltages. In the unlikely event that still greater sensitivity at high frequencies is needed, an inexpensive transformer may be added. A commercial transformer costing less than $10 (price per unit in small numbers) has been routinely used to provide a seven-fold increase in drive voltage with the TEL-Atomic electronics.

2.5  Temporal data

It was mentioned earlier that some time traces were taken with a storage oscilloscope. Although it would have been possible to use an IEEE488 standard hookup to produce records from which figures could have been generated, time constraints did not permit this.

In these shock studies, a pellet gun was used with scotch tape placed across the breech opening, where the pellet is normally inserted. Loud bangs from this gun, subsequent to trigger pulls, were studied with the microphone. In lieu of this schock tube device, ordinary vigorous hand claps are capable of much the same signature, except that the levels are lower and the hands eventually begin to complain. An especially useful source of shocks that the author contemplated, but didn't get around to trying, may be an ordinary hand-cranked Wimshurst machine. The breakdown of air (simulated lightening/thunder), when the Wimshurst operates with its largest capacitors, is quite loud and abrupt. In use, one must be careful to avoid electromagnetic pulse damage to the computer acquisition system. The author discovered this need while using a Wimshurst machine to study field dependent mechanical strains of dielectrics placed between capacitor plates. The mechanisms of charge storage on dielectrics, as influenced by defects (pertinent to electret microphone design), has not been carefully studied. Understanding the long term stability of these states could be important to future improvements of a variety of devices such as electret microphones.

There are several noteworthy results of the shock studies. One, the diaphragm of the SDC prototype microphone is underdamped, as expected. Reduction of the gap spacing from 500 mm to 50 mm should increase the damping to a more desirable level. The free decay period of about 1 ms is consistent with its measured resonance frequency of 1 kHz, discussed earlier.

It was also found, as expected, that the shock signature depends significantly on environment, particularly echo features. Multiple ringing can be readily seen when the shock bounces several times around a room with hard walls and/or ceilings and floors. The value of an anechoic chamber where these reflections are virtually eliminated, becomes quickly appreciated.

3  Hydrophone

3.1  Introduction

The development of a low frequency hydrophone was motivated by work performed by Michael Scanlon, of the ARL acoustics group (Sensors Directorate). He is the inventor of a recent patent that was assigned to ARL[]. Not only has he shown that a hydrophone placed inside a fluid-filled bag is useful for SIDS monitoring; it could also become commonplace on the battlefield. Credence for such a statement is realized by recognizing that the stethoscope has been used for many decades by physicians to assess health of a patient. Whereas the frequency response in the case of the conventional stethoscope is limited by the quality of the doctor's ears, a properly built hydrophone opens new vistas, because of sensitivity to infrasound. Mike has shown, for example, that there are low frequency ``signatures" associated with complex pulmonary/cardiovascular interactions. For example, there are significant differences in time/spectral plots, depending on whether the patient's breath is held. Also, skipped heartbeats and murmurs can be readily detected, at least in some cases. A full appreciation for the value of this technology must obviously await extensive testing by physicians.

The sensor used in the SIDs monitor work was a water-proofed electret type. As such, it comprises two membranes: (i) one that separates the air cavity of the microphone proper from the water, and (ii) the standard diaphragm of the sensor. The extra membrane (water separator) is a cause for reduced sensitivity; and also, the low frequency cutoff of the microphone is greater than desired. Thus it was natural to consider an SDC alternative to this Knowles' unit.

The SDC hydrophone built by the author during this study is illustrated in Fig. 7. All of the semi-circular electrodes were cut with scissors from brass shim stock of thickness 250 mm. Their radii are 0.7 cm. The static pair, 3 and 4, were cut with small protruding ``ears" for solder attachment to the coaxial cable which connects the hydrophone to the TEL-Atomic support electronics input. These were superglued to the housing, which threads into the water bottle. A slit was cut through the flat base of this cylindrical housing, to freely pass the cross coupling links of the moving members of the hydrophone (1T, 1B, 2T, and 2B). Note the difference between this design and that of the microphone. In the microphone, the central electrode pair were the moving ones (diaphragm bowing); whereas, here the central pair are fixed, and the outside set do the moving. Also note that in the microphone case, the drive equipotential pairs were formed by solder attachment of external wires. For the hydrophone of Fig. 7, the cross connections are by means of the links. These rectangular links are about 1/3 a diameter wide and positioned on opposite sides of center, so that they're electrically separated. The SDC drive signal is applied from a coaxial cable by soldering to the topside of 1T and 2T.


Picture Omitted
Figure 7: Edge view illustration of the prototype SDC hydrophone.

The narrow rectangular strips (both ears and links) were cut from the same brass stock as the electrodes. They were formed as an integral part of the lower (2B and 1B) set. The top moving electrodes (1T and 1B) were soldered to these strips in the last step of fabrication. The strips provide the structural rigidity with which the electrodes maintain parallelism. The rubber pad shown was cut out of a thin black rubber glove. It provides restoration to the moving electrodes when water pressure is reduced. To provide a water seal, silicone was used to fill the region inside the housing that surrounds the plunger. This silicone also adds to the restoring force. The remaining unlabeled ``four rectangles" shown in the figure are actually two center slotted circular disks of thin insulating fiber glass to which the moving electrodes were glued. The plunger was turned from plexiglass and glued to the bottom of the sensor. It should be noted that (for purpose of clarity) Fig. 7 is not drawn to scale. The spacing between all adjacent electrodes is roughly 1/2 mm in the unstrained state. An increase of pressure on the plunger side of the hydrophone causes the 1T - 3 and 2T - 4 gap space to increase, at the same time as the 2B - 3 and 1B - 4 gap spacings decrease. Thus the device is a full bridge sensor, a hallmark feature of the SDC sensor technology. (Actually, the symmetry is slightly degraded by insignificant dielectric constant mismatches.)

3.2  Experimental results

Shown in the top trace of Fig. 8 are the results of an experiment in which the author sat quietly in the ARL anechoic chamber, with the hot water bottle containing the SDC hydrophone, placed lightly between his left arm and chest. This spectrum was obtained with the LabView FFT software mentioned earlier. For reference purposes, the background is shown in the lower trace. It was generated by placing the bottle at rest by itself inside the chamber.

One can quickly glean a number of interesting conclusions from Fig. 8. The line at 1.3 Hz (upper trace) corresponds to heart beat (78 per min). In the vicinity of 0.2 Hz (broad ``line") is evidence of respiration (12 per min). The sharpest line in the spectrum is the one whose frequency is 7 Hz. It has been long known that the body exhibits resonances in the vicinity of 10 Hz, of which this is probably an example. It is undoubtedly the means whereby even a deaf person may ``feel" some sounds.

From Fig. 8, one may also readily determine the upper frequency cutoff of the hydrophone. The rolloff at about 15 Hz is consistent with the relatively large mass of the moving parts of the sensor. For production purposes, this mass needs to be reduced (to yield a cutoff near 50 Hz), and the sensitivity should be increased by decreasing the gap spacing between electrodes. For improved sensitivity, an increase of the drive voltage from +5 V could be accomplished using a transformer (as mentioned relative to the SDC microphone).

Whereas Fig. 8 was obtained from a fairly short record ( < 1 min), some other data were collected over longer intervals using a B & K spectrum analyzer operating in an averaging mode. Shown in Fig. 9 is a case in which the hotwater bottle was simply laid on top of the author's chest (not in the anechoic chamber) Fig. 10 is the associated background, taken with the bottle at rest on a chair. Although the 1.3 Hz fundamental frequency of his heartbeat is not readily apparent in this case, the 2nd through 4th harmonics are way above noise, particularly the 4th harmonic. Additionally, the broad ``body resonance" region centered at 10 Hz appears to accommodate harmonics 6 through 10. With regard to line widths, there are evidently major differences, in general, according to the duration of the analyzed interval. In this case the properties do not derive from the uncertainty principle (which causes line broadening), but rather from physiological variations that appear to correlate with periodicities yet unstudied. The implications should be of great interest to the medical community, particularly physiologists. For the author, these results are not surprising; since years ago he looked at the time delay between the R-wave of the EKG and the arrival time of arterial pulses at various points in the body. In this work he was able to show that these times (associated with a highly nonlinear system) depend on stroke volume of the heart [].


Figure 8: Sample response of the SDC hydrophone.

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Figure 9: Response measured with an analogue spectrum analyzer.

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Figure 10: Background associated with Fig. 9.

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4  Electronics

A primary factor in the high cost of existing commercial electronics support for the SDC sensors has been the use of a sine wave drive. Although the overall cost was reduced, and performance improved, by the integration of the Signetics NE5521 integrated circuit into the TEL-Atomic package; nevertheless the cost has remained too high for extensive fielding of an SDC microphone. Thus, the author was led to consider digital circuitry. Two critical factors had to be addressed: (i) inexpensive bipolar square wave generation and (ii) switching circuits to provide synchronous demodulation. John Speulstra and Art Harrison provided ready solutions to both these problems. For the drive, one uses a simple opamp oscillator feeding a flip-flop. For the demodulator, a CMOS switch can be configured with an opamp, to do what otherwise has taken a larger number of active components. Their design is shown in Fig. 11. The filters at all power points (``inductor" & 2 capacitors, each case) may not be necessary to filter out noises, particularly when using battery power. The inductor is in each case a small toroidal ferrite with 3 turns of hookup wire.


Figure 11: Prototype electronics support for SDC sensors.

(This page left blank for replacement with separate figure 11) To test the electronics, an SDC position sensor was fabricated from PC boards. The ``cardinal" electrode dimension of the sensor was 2.4 by 3.6 cm, so that the maximum range of linearity is +1.8 cm. The spacing between adjacent electrodes is 0.2 cm. This sensor is known from other studies to be very linear []. The results of a linearity test with it are shown in Fig. 12. (Note that the negative range exceeds the positive range by 1/2 cm; this is due to stray capacitance unbalance in the electronics-readily fixed.) Although the sensitivity is low, it is seen that the linearity of the electronics is outstanding. The gain of the instrumentation amplifier (AD620) had to be kept small to avoid unpleasant saturation effects, for large deviations from the null position. Time did not allow, but the author is confident that the sensitivity can be made adequate by following the last indicated stage of Fig. 11 with an additional opamp (probably only one required).

Figure

Figure 12: Test of electronics linearity.

5  Conclusions

This study has demonstrated that the SDC technology is capable of filling an important battlefield sensing void-that of infrasonics. The importance of certain acoustic signatures, from sounds that are both airborne and intrinsic to the human body, has come to be appreciated only recently. The seminal contributions to this field are the result of research done by personnel associated with the Army Research Laboratory at Adelphi, MD. Were cost not a factor, some commercial microphones might fill a limited number of categories of this void. Other categories would remain unfilled, such as those involving robustness.

Various configurations of SDC microphones and/or hydrophones are envisioned for addressing presently known challenges of battlefield deployment. The envisioned instruments are expected to be inexpensive, robust, and suitable for use by soldiers in a variety of applications. In some cases, a single microphone may be all that is required; others might use a large number of sensors in an array.

The preliminary work of this report involves measurements performed with three prototypes. These prototypes addressed both mechanical and electronic issues via: (i) an SDC microphone, (ii) an SDC hydrophone, and (iii) an electronics support package for both. Further refinements are required, largely ones involving issues of mass production.

The most cost effective SDC microphones for initial fielding are likely to be of ``macro" rather than ``micro" type. This is expected in spite of recent research trends in the direction of micro-electro-mechanical- systems (MEMS). The SDC technology has been recognized for several years as one of the best candidates for MEMS integration on silicon; nevertheless, conventional production methodologies should be the first ones evaluated. Only then can the ``payoff" of miniaturization be quantified. This recommendation is based partly on the known limitations of a popular commercial device, whose price has dropped because of MEMS type processing in high volume- the electret microphone of hearing aids. These microhones do not respond at all frequencies of interest to the army, partly because of their small size and resulting small capacitance. Since all miniaturized capacitive sensors are subject to the challenges of femtoFarad scale, there is merit to first considering larger instruments for which this constraint does not apply.

6  Acknowledgements

The author is appreciative of the support provided by Steve Tenney, Mike Scanlon, and Steve Post, of the Acoustics group, Sensors Directorate, at the Army Research Laboratory, Adelphi Maryland. (This work would not have been funded without Mr. Tenney's recognition of the potential value of the SDC pressure sensor as a microphone.)

Steve Post assembled the hybrid electronics package described in this document and also produced the associated figure. His assistance with other hardware tasks is much appreciated, as are the ``extra mile" contributions of Mike. The pellet gun shock tube was loaned by Jerry Gerber, who also provided the author with useful shock references.

Two others who deserve special recognition are Art Harrison and John Speulstra, who are primarily responsible for the new electronics design. Being of a different ARL group than acoustics, the author was much impressed with (i) their interest in this work, in spite of their own pressing schedules, (ii) their frequent assistance in spite of those pressures, and (iii) their outstanding abilities.

References

[]
This company markets physics demonstration and laboratory equipment which uses SDC sensors. Instruments which they sell include: (i) Computerized Cavendish Balance, (ii) Multipurpose Chaotic Pendulum, (iii) Pressure Transducer, and (iv) Accelerometer.
[]
Michael V. Scanlon, ``Sudden Infant Death Syndrome (SIDS) Monitor and Stimulator", U. S. Patent No. 5,515,865. Other pending patents, relevant to battlefield use, are under review.
[]
During the early 1970's at the University of Mississippi, the author measured propagation speeds of arterial pulses using an EKG and a pulse sensor. As inferred by known influences of posture and/or the valsalva maneuver, these measurements showed that the speed is a function of stroke volume.
[]
The planar electrode SDC position sensor used for the linearity tests has been referred to as the author's ``crucial" invention. It works on the basis of area variation rather than the gap changes of the pressure sensor. It is described in the patent (ref. 9, Part I).


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