THREE AND A HALF YEARS AGO in "The Amateur Scientist" (October, 1960) Frank Watlington described how make an inexpensive hydrophone. With such an instrument one can enter the recently discovered world of natural underwater sound: the hiss of breaking waves, the rattle of pebbles carried along by a current and the remarkable variety of noises made by fishes and

aquatic organisms. Now Watlington, a specialist in underwater sound whose avocation is listening to whales and porpoises off Bermuda, has devised two new hydrophones that are easier to construct and that should make the study of underwater sound more accessible to amateurs. To give only one example of such a study, a snorkel diver a confederate at the surface can maneuver a hydrophone up to a sound-making organism and record its sounds for the purpose of identifying their source in "broadcast" listening. Perhaps the hydrophone will join the camera a standard adjunct to the explorations of the skin diver who prefers not to kill fish.

One of Watlington's new hydrophones is designed to operate at substantial depths; the other, which is more sensitive, to work in shallow water. Watlington writes: "Hydrophones for converting variations of sound pressure into equivalent variations of electric current are of three types: electrodynamic, magnetostrictive and piezoelectric. The electrodynamic type operates on the principle of a conventional electric generator. A coil of wire, actuated by a diaphragm in contact with the water, moves in a fixed magnetic field. A voltage proportional to the velocity of the diaphragm is induced across the terminals of the coil. Instruments of this type are satisfactory but difficult to construct. Hydrophones of the magnetostrictive type take advantage of the fact that the magnetic properties of certain materials, such as nickel, are altered by mechanical stress. A coil of wire surrounding a core of such material picks up a proportional voltage when sound waves stress the core. Although magnetostrictive hydrophones are easier to make than the electrodynamic types, the coil must be wound by means of a bobbin-a tedious operation. Piezoelectric hydrophones are based on the property of certain materials to acquire electric charge when they are stressed, an effect that was first described by Pierre Curie in 1880. A number of minerals show the effect, but barium titanate, an inexpensive ceramic, is commonly used for making hydrophones. Barium titanate can be molded in any shape and is fired much like clay. Both tubes and disks of the material are used for hydrophones. When a tube is stressed radially, its outer and inner surfaces become oppositely charged. When a disk is fixed at its edge and pressure is applied to one face, its two faces also become oppositely charged. Electrical connection to barium titanate is made through silver electrodes that are deposited on the material during manufacture. Numerous U.S. manufacturers supply the parts. Mine were obtained from the Clevite Corporation (3631 Perkins Avenue, Cleveland 14, Ohio).

"The deepwater hydrophone uses a tube of barium titanate one inch long with an outside diameter of 1/2 inch and an inside diameter of 1/4 inch. The part comes from the manufacturer fully finished, with ground ends and silver electrodes on the outer and inner surfaces. Flexible copper leads must be soldered to the electrodes, an operation that requires some care because the silver is in the form of a thin deposit that can be damaged by excessive heat. Make the joint with rosin-core solder applied with an electric iron that draws no more than 25 watts. Acid flux will corrode the electrodes. Apply the well-heated iron to the silver just long enough to cause the solder to flow. The tube must be closed at the ends by two caps of Lucite or a similar plastic that are drilled for a brass retaining screw as shown in the accompanying drawing [Figure 1 ]. The caps must make a snug fit with the inner surface of the tube and the ends. Before these parts are assembled insert the screw through one cap and then solder the lead from the inner surface of the tube to the screw. Next apply a thin coat of silicone grease to the mating surfaces of the tube and the caps. Most dealers in radio parts stock silicone grease. Complete the assembly by tightening the nut just enough to hold the end caps solidly. Then cut off the screw flush with the top of the nut and solder the output leads to the end of the screw and the outer electrode as shown in the illustration.

"The assembly can be tested by connecting the leads to either a high-gain speaker system or an oscilloscope. A: gentle tap on either end with the handle of a screw driver should produce a sharp click from the speaker and a pip on the oscilloscope. Sound pressure deflects the wall of the tubing inward against the cushion of trapped air and sets up stresses that appear as fluctuations of voltage at the electrodes.

"For service as a hydrophone the assembly must be sealed and specially housed to keep fluid from leaking into the air space, to protect the unit against corrosion and to prevent water from . short-circuiting the leads. The seal can be made with common beeswax. Simply hold the unit by the leads and dip it into the melted wax. Apply two or three coats, allowing each coat to harden before applying the next. Let the unit remain in the wax about three minutes during the first dip. Make succeeding dips quickly to prevent preceding coats from melting. I prefer a more durable sealing material, such as epoxy resin, and I sealed my unit with a product known as Araldite 6020,-which is manufactured by the Ciba Corporation. This epoxy resin is mixed with No. 951 hardener in the ratio by weight of 10 parts resin to one part hardener. Two coats were applied. On standing at room temperature for 24 hours the material sets as a hard, glassy coating of considerable strength and excellent electrical insulating properties. Like all epoxy resins, Araldite is toxic and must be used with care. Prolonged contact with the skin results in a rash that resembles the one caused by poison ivy. Wear rubber gloves and work with the material in a well-ventilated room. Fortunately epoxy compounds are soluble in water before they harden. Smears can be removed from the skin by scrubbing with strong soap and hot water.

"For additional protection the sealed unit is potted in a plastic container completely filled with castor oil. A plastic squeeze bottle of the kind used for dispensing mustard or catchup makes an ideal housing. The flexible sides transmit sound readily to the oil and the tapered spout provides a handy orifice for attaching the microphone cable to the pickup unit. The microphone cable should be mechanically strong and of the two-conductor type that has a braided shield, such as Belden No. 8422 microphone cable.

"The tapered nozzle of the squeeze bottle is snipped off at just the right spot to make a tight fit with the cable. The cable is then pulled through the spout and the outer covering and the braiding are skinned to expose about six inches of the leads. Strip about half an inch of insulation from the tips of the leads and splice them to the pickup unit. Insulate the splices with a few inches of plastic 'spaghetti.' Lash the cable an inch or so above the joint with a dozen turns of thin twine to form a stop large enough to prevent the cable from slipping through the spout when it is pulled. Next fully immerse the squeeze bottle and cap assembly in a deep bowl of castor oil. Dislodge all bubbles that may adhere to the apparatus; even a small amount of air trapped inside the housing will lower the efficiency of the hydrophone. When the bubbles have been removed, and while the apparatus is still fully submerged, screw the cap securely in place. Remove the assembly from the oil, wash it thoroughly with a strong detergent, rinse it, dry it and apply several layers of plastic-backed tape to the joint between the spout and the cable and between the bottle and the cap. Finally skin the remaining end of the cable and terminate the leads with connectors that mate with the amplifier [see Figure 2]. The completed hydrophone should then be tested as described earlier.

"The barium titanate tube is designated by the Clevite Corporation as part No. 16-8125-5, priced at $5. It withstands a pressure of 10,000 pounds per square inch and has a sensitivity of approximately -106 decibels relative to one volt per microbar. This means that if a pressure of one microbar (about a millionth of an ounce per square inch) is applied to the cylinder, its output voltage, measured with a high-impedance voltmeter as the only load, should be five millionths of a volt. Greater sensitivity can be achieved by using a larger tube of barium titanate. Clevite also sells for $10 a tube of lead zirconium titanate, designated part No. 16-24125-5 PZT; it measures one inch long and 1 1/2 inches wide and has a sensitivity of -91 decibels relative to one volt per microbar.

"Still greater sensitivity can be achieved by using an entirely different construction technique, one that employs four piezoelectric units in the form of disks cemented together in pairs to form 'bimorph' assemblies. The hydrophone of this type that I made was found by measurement to have a sensitivity of -81 decibels relative to one volt per microbar. Although the unit is designed for use in shallow water, it has a fail-safe feature. It becomes inoperative at a critical depth but is not damaged by the pressure at that depth.

"The increased sensitivity of bimorph hydrophones arises from the fact that a pair of cemented disks develops a greater potential difference when stressed than a single disk of the same size does. When a single disk is stressed, its faces acquire a charge of characteristic polarity: one face is positive and the other negative. A bimorph is made by cementing two disks together with the faces of like polarity in contact. A bimorph hydrophone requires two such assemblies, one with positive faces cemented in contact and the other with negative faces cemented. The pairs are then combined with a thin spacing washer sandwiched between them, as shown in the accompanying illustration [at left]. Opposing pressure applied to the outer faces of the sandwich causes the bimorphs to flex inward and a charge of opposing polarity to appear on their outer faces. A voltage proportional to the stress appears across leads connected to the outer faces. The maximum deformation of the disks is limited by the thickness of the washer. The washer of my assembly was cut from brass shim stock .004 inch thick. It allows each disk to flex only .002 inch, well below the point of fracture. The sensitivity of the unit drops when the disks flex enough to touch in the middle but does not fall to zero because the separation remaining near the edge allows some additional movement.

"The disks of my unit are 1 1/4 inches in diameter. Prior to assembly they are thoroughly cleaned with a strong detergent to remove grease and then I joined as bimorphs by means of a conducting cement, such as Hysol No.4238 epoxy resin and its companion No.3475 hardener. The same cement is used for simultaneously joining the bimorphs to the spacing washer. Apply a thin film to all surfaces, taking care to prevent the cement from entering the air space between the bimorphs. Heavy pressure should not be applied to the stack during the hardening period. Simply place the coated elements on a flat surface and, put a small weight-a pound or so-on top. After the cement has hardened for about 24 hours, solder leads to the outer faces as shown in the illustration. Avoid burning the silver electrodes. Test the unit with an appropriate speaker system or oscilloscope, then seal with beeswax or, preferably, Silastic RTV No. 731 (a product of the Dow Corning Corp.) . The unit is then potted in a plastic container filled with castor oil as previously described.

"The electrical output of all these hydrophones is small, of course. Appropriate circuits must be employed to conserve this output. Piezoelectric units are high-impedance devices. They deliver maximum power only when their terminals are connected to a load of equal impedance. I achieved an adequate impedance match between the hydrophones and amplifier by means of a step-down transformer connected as shown in the accompanying diagram [Figure 4].

"The choice of amplifier is dictated by both the sensitivity of the hydrophone and the availability of power for driving the amplifier. The requirements of the deepwater hydrophone can in general be met by a conventional highfidelity phonograph amplifier equipped with a preamplifier. For field work the 110-volt, 60-cycle power can be derived from a battery-operated inverter. The more sensitive hydrophones work satisfactorily with inexpensive transistor amplifiers such as the basic five-transistor push-pull audio amplifier (PK-544) that is sold for $7 by the Lafayette Radio Electronics Corporation of New York City. Two amplifiers are used in series.

"The motion of a hydrophone through the water causes extraneous noise that must be minimized. The drifting of a boat from which observations are made, currents in the water and wave action combine to cause trouble. The difficulty can be met, at least in part, by suspending the microphone cable in loops from airplane shock cord, particularly at points of attachment to the boat, the supporting buoy and at the instrument itself" [see illustration above].

Charles J. Merchant, a mathematician at the University of Arizona, submits the following description of a sundial for indicating standard clock time that can be made in less than an hour. "A sundial," writes Merchant, "even when it is perfectly constructed and correctly installed, generally indicates a time substantially different from standard clock time. This often leads people not familiar with the beautiful intricacies of sun time to the erroneous conclusion that a sundial is an inherently inaccurate device. Sundials that indicate a time correctly to within one minute can be constructed with no great difficulty; with refinements they can be accurate to within a few seconds.

"The difficulties with sun time versus standard time stem from two sources. The eccentricity of the earth's orbit and the obliquity of the ecliptic cause the sun to gain or lose as much as a minute a day over considerable periods of time, with accumulated inaccuracies of plus or minus 15 minutes at certain times of the year. The correction for this variation is known as the equation of time. When this correction is applied to the reading of a sundial, the result is local mean time. Local mean time, however, is the same as standard time in the U.S. only in those cities whose longitude is 75, 90, 105 and 120 degrees. In all other localities standard time differs from local mean time by a constant amount depending on the longitude of the place. This second cause of a sundial's apparent inaccuracy is known as the longitude correction.

"Two corrections must therefore be applied to the reading of a conventional sundial in order to derive standard time: the equation of time, which varies from day to day, and the longitude correction, which is constant for a given place.

"Numerous methods, some of considerable ingenuity, have been devised for making a sundial indicate standard time directly. My sundial accomplishes this by means of a circular computer. The face of an equatorial-type dial is rotated by various amounts depending on the setting of a pair of disks. When the device is properly adjusted, it indicates standard time correctly to within better than five minutes. It operates only during the spring and summer months, from the vernal equinox to the autumnal equinox; during the other six months of the year the sun lies below the equatorial plane. A set of disks could be calibrated for this interval, but they have not been included with this model.

"The dial was designed to be cut out and mounted on thin cardboard, using a non-wrinkling cement such as Gripit. Rubber cement does an excellent job, but it is not permanent. After they have been mounted on the stiff backing the parts are carefully cut out. The base is then cut off for the latitude of the place where it is to be used and bent at right angles along the broken lines [see above]. When properly mounted on a baseboard and placed on a level surface, the face makes an angle with the horizontal equal to the colatitude of the place.

"The disks are then assembled on the face. A needle, pushed through the center mark from below, serves both as the means of assembly and as the gnomon. The gnomon should be as exactly perpendicular to the face as possible!

"The larger of the two disks, the correction disk, is placed face up on the needle first, then the smaller of the disks. Finally a small piece of cardboard, to act as a retaining washer, is pressed down on the needle.

"To operate the sundial the correction disk is first rotated so that the longitude of the place is opposite the arrow on the face that indicates the local time zone. (The abbreviations are self-explanatory: CST means Central Standard Time, MDT means Mountain Daylight Time, and so on.) For example, when the dial is to be used in New York City during the period of Eastern Daylight Time, the correction disk is turned so that 74 degrees, the longitude of New . York City, is at the arrow marked EDT. This disk will require further adjustment. It can be fixed to the base plate with a small piece of drafting tape. The tape must not interfere with the movement of the smaller disk, however. This disk must be adjusted every few days.

"The correction disk carries a date scale that is graduated nonlinearly to correct for the equation of time. The outer edge of the dial plate also carries a date scale, but this scale is graduated linearly. When the dial plate is rotated so that a given date on the dial plate coincides with the same date on the correction disk, the time scale is automatically rotated by the amount necessary to correct for the equation of time on that date.

"Finally, the dial is set on a level surface-in the sun-with the gnomon pointing exactly north. The dial then indicates correct standard or daylight time. For instance, assume that the dial is to be used in New York City on July 10. The latitude of New York City is 41 degrees. The base support should be cut for this angle. The longitude of New York City is 74 degrees. On July 10 Eastern Daylight Time is in effect. The correction disk is therefore rotated so that longitude 74 degrees is at the EDT arrow. The correction disk should be taped to the face plate. The smaller disk is then turned so that July 10 coincides with the July 10 date on the larger disk. The dial is next placed on a level surface in the sun with the gnomon pointing exactly north. The shadow of the gnomon now indicates correct Eastern Daylight Time for New York City.

"It should be noted that this dial will indicate correctly even in those cases where a city operates under an incorrect standard time zone. Certain cities in eastern Indiana and western Ohio, for example, operate on Eastern Standard and Eastern Daylight Time even though they are well within the Central Standard Time zone. In these cities follow the rule of setting the longitude of the city to the arrow representing the time zone under which it operates. The dial will indicate the correct clock time.

"Although the dial is calibrated only for the time zones of the continental U.S., it can be used anywhere in the Northern Hemisphere. Merely add or subtract from the longitude of the place that multiple of 15 degrees which results in a longitude within plus or minus 7.5 degrees of 90 degrees, and then use the resulting longitude with the arrow for CST or CDT, depending on whether standard or daylight time is in use. All longitudes must be converted to longitude west of Greenwich, however. Thus longitudes east of Greenwich should be subtracted from 360 degrees to give west longitude."

Bibliography

SONICS. Theodor F. Hueter and Richard H. Bolt. John Wiley & Sons, Inc., 1955.

SUNDIALS: HOW TO KNOW, USE, AND MAKE THEM. R. Newton Mayall and Margaret L. Mayall. Charles T. Branford Company, Publishers, 1958.

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