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.
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.
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".
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.
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(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.
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.
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.
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.
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.)
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 .
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(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).
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.
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.