Method of calibrating microphones

May 20, 1952 j. m. kendall 2,597,005

METHOD OF CALIBRATING MICROPHONES

Original Filed June 4, 1943 7 Sheets-Sheet 1

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J. M. KENDALL

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May 20, 1952 j. m. kendall 2,597,005

METHOD OF CALIBRATING MICROPHONES

Original Filed June 4, 1943

Sheets-Sheet 2

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May 20, 1952 j. m. kendall 2,597,005

METHOD OF CALIBRATING MICROPHONES

Original Filed June 4, 1943 7 Sheets-Sheet 3

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May 20, 1952 j. m. kendall 2,597,005

METHODOF CALIBRATING MICROPHONES

Original Filed June 4, 1943 7 Sheets-Sheet 4

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May 20, 1952 j. m. kendall 2,597,005

METHOD OF CALIBRATING MICROPHONES

Original Filed June 4, 1943 7 Sheets-Sheet 5

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May 20, 1952 J. M. kendall 2,597,005 . METHOD OF CALIBRATING MICROPHONES

Original Filed June 4, 1943 7 Sheets-Sheet 6

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May 20, 1952 j. m. kendall 2,597,005

METHOD OF CALIBRATING MICROPHONES

Original Filed June 4, 1943 7 Sheets-Sheet 7

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Patented May 20, 1952

2,597,005 ' Λ

UNITED STATES PATENT OFFICE

2,597,005

METHOD OF CALIBRATING MICROPHONES

James M. Kendall, Coral Hills, Md., assignor to Geophysical Research Corporation, New York, N. Y., a corporation of New Jersey

Original application June 4, 1943, Serial No. 489,669. Divided and this application July 1, 1944, Serial No. 543,180

Claims. (Cl. 73—1) (Granted under the act of March 3, 1883, as amended April 30,

This invention relates to underwater microphones and more particularly to a method of and apparatus for calibrating microphones employed for detecting, indicating, and measuring underwater sounds for the purpose of making seismic surveys in areas covered by water or for the purpose of studying underwater sound phenomena.

This application is a division of an application of James M. Kendall, Serial No. 489,669, filed June 4, 1943, now Patent No. 2,582,994, issued January 22, 1952, which is a continuation-in-part of application, Serial No. 435,614, filed March 21, 1942, now abandoned, for Seismic Surveying Equipment.

Many areas in which seismic surveys are conducted are covered with water. In surveying such areas, it is customary to mount the recording apparatus on boats. The seismic detectors usually used are microphones of a special type which are adapted for low frequency operation and are known in the art as geophones. The geophones may be of the same type as are used in land surveys but they are sealed up water-tight and lowered to the bottom of the water body. If the bottom is of firm, solid material, this procedure usually works quite well. However, in the case where the bottom material is soft muck or ooze, a geophone resting thereon does not always give satisfactory results. In such cases, the practice usually has been to push the geophone down through the soft material until it rests on firm material. This is an awkward and difficult procedure in shallow water and becomes practically impossible when the water is very deep.

In one form of the invention, a geophone or wave detector of any suitable type such as used on land is enclosed in a water-tight casing. The dimensions of the casing are so proportioned that the effective density of the assembly is slightly greater than the density of water. The geophone or wave detector is mounted at or close to one end of the container so as to make the center of gravity of the device close to that end. As the device almost but not quite floats, it will rest on the bottom but will exert practically no pressure thereon. Therefore, it will respond to any wave motion in the water surrounding it because its density is practically the same as that of the water. The wave motion in the water will move the detector just as it would move the water displaced by the detector, if the detector were not there, and the wave field will not be substantially distorted by the presence of the detector. This is true provided the wave length of the wave motion is large as compared

1928; 370 O. G. 757) with the dimensions of the casing. The velocity of seismic waves in water is approximately 5000 feet per second so that the wave length of a seismic wave of frequency equal to 200 cycles per second is 25 feet. The greatest linear dimension of the casing can usually be made of the order of one foot. Since the frequencies recorded in seismic prospecting are always less than 200 cycles per second, the response of the wave detector will be independent of wave length. As a matter of fact, by suitable design, the device can be made small enough so that frequencies up to at least 2000 cycles may be recorded.

For higher sound frequencies up to 10,000 cycles per second, the wave length of the sound begins to approach the dimensions of the microphone and the microphone no longer partakes of the particle motion of the water in responding to the sounds. It is then necessary to reduce the size of the microphone considerably but it has been found that when the microphone is made relatively small, bubbles which collect thereon produce inaccuracies in the microphone response by reason of the fact that the bubbles are compressible and each bubble covers a proportionately larger percentage of the microphone surface area that is true in the case where a large microphone is employed. It is, therefore, necessary to shape the microphone so that it can be easily debubbled.

The small microphone should also have a shape which easily lends itself to mathematical analysis for the purpose of computing the corrections which are necessary when the microphone is subject to sounds in the higher frequency ranges having a wave length approaching the dimensions of the microphone. It is also necessary that the shape of the microphone be such that it is fairly easy to compute corrections when the mass of the moving parts of the microphone differs from the mass of the water which the microphone displaces and thereby produces distortion in the wave field. The shape of the microphone case should be such as to provide as rigid a structure as possible thereby to eliminate as far as practicable resonance effects within the frequency range in which sound measurements are to be made.

A second form of the invention has been devised to conform to the above-mentioned prerequisites. The second form comprises a microphone which has a substantially spherical casing comprised of two hemispheres having a watertight joint therebetween and is, therefore, capable of being easily debubbled because it presents nd

2,597,005 recesses within which bubbles may adhere. Since a sphere is symmetrical about all its axes, it lends itself tp relatively simple mathematical analysis for the purpose of making the aforementioned corrections. A sphere is the most rigid structural shape obtainable and a microphone casing having a spherical shape will therefore have the least tendency to resonate at frequencies within the range to be measured.

One hemisphere of the spherical· microphone casing has rigidly mounted thereon and extending into the interior of the casing a coil which is associated with a magnetic structure mounted within the casing and flexibly supported by the hemisphere upon which the coil is mounted thus providing a magnetic field in which the coil moves. The coil is provided with terminal leads which pass through packing glands in the microphone casing for connection to indicating apparatus located exteriorly thereof. The weight of the casing and the elements rigidly mounted thereon is made substantially equal to the weight of the volume of water displaced thereby when the microphone is suspended in a body of water. The weight of the magnetic structure which is flexibly supported within the casing increases the overall weight of the complete microphone structure considerably over the weight of the volume of water displaced but this produces no effect on the ability of the casing to partake of the particle motion of the water without substantial distortion of the wave field thereof. By reason of the flexible mounting of the magnetic structure within the casing, the inertia of the magnetic structure causes it to remain stationary as the casing moves in response to underwater sounds and the motion of the coil in the stationary magnetic field generates a voltage proportional to the velocity of the motion of the sphere and, hence, proportional to the particle velocity of the fluid.

To permit the casing to move freely within the body of water, a flexible suspension is provided therefor comprising a circular brass ring having hooks placed at intervals in its circumference. The spherical casing is provided with similarly spaced hooks and flexible bands are arranged to extend from the hooks on the ring to the respective hooks on the casing. The ring may be supported from the surface of the body of water in any suitable manner.

It' is sometimes desirable to measure the acoustical impedance of the bed of a body of water. To secure such measurements, the velocity microphone is placed on the bed of the body of water with the axis of the coil normal to the bed and the response of the microphone to sounds is measured simultaneously with the acoustic sound pressure at substantially the same location, From these measurements, the acoustical impedance of the bed at that point may readily be determined.

The construction of the microphone permits determination of the absolute sensitivity thereof by the use of a simple calibrating device and method. To determine the absolute sensitivity of the microphone, the two hemispheres are separated and the hemisphere on which the coil and magnetic structure are mounted is affixed to a calibrating device which permits measurement of the change in flux linkages of the coil when the magnetic structure is moved a known amount with respect to the coil. The change in flux linkages is measured by a fluxmeter and the movement of the magnetic structure may be measured by either a microscope or a micrometer, a method being employed which insures accurate measurement thereby. A simple formula permits the translation of the two measurements into the absolute sensitivity of the microphone stated in microvolts/dyne/cm.².

As pointed out hereinbefore, a velocity microphone when constructed in accordance with the invention, does not substantially disturb the wave or sound field into which it is introduced and a measurement of the true field pressure is therefore obtained except at the high frequencies at which the wave length is comparable to the microphone diameter. This makes it possible to obtain the field response of a pressure microphone by placing it at a point at which the free field produced by an underwater loudspeaker has been calibrated by the velocity microphone, and the velocity microphone therefore may be utilized as a standard for the calibration of pressure microphones which are employed for the study of underwater sound pressures. The method of and apparatus for calibrating pressure microphones by employing the velocity microphone as a standard will be more fully described hereinafter.

The term “pressure” as employed herein refers to the acoustical sound pressures within a body of water and is in no way related to the hydrostatic pressure occurring in the water by reason of the hydrostatic head thereof.

One of the objects of the present invention is the provision of a new and improved underwater wave detector or microphone possessing all of the advantages of devices heretofore proposed for this purpose and in which the foregoing disadvantages have been eliminated.

Another object of the invention is the provision of an underwater microphone which responds to the particle motion of the water without substantially distorting the wave field thereof.

Still another object of the invention is the provision of a new and improved microphone which will aid in the accurate determination of the particle velocity of the bed of a body of water in the presence of sound waves for the purpose of determining the acoustical impedance of said bed.

An additional object of the present invention is to provide an underwater microphone which is adapted to be easily debubbled and has a simple geometrical surface which readily lends itself to mathematical analysis.

A further object of the invention resides in a novel and improved arrangement whereby underwater sounds having frequencies up to and beyond 10,000 cycles per second may be easily measured.

Still another object is the provision of an underwater velocity microphone which is sufficiently accurate in its response to serve as a primary standard for the calibration of other underwater microphones.

Another of the objects of the present invention resides in the provision of a novel method of and apparatus for determining the absolute sensitivity of a velocity microphone.

Still another object of the invention is to provide a novel method of and apparatus for calibrating an unknown microphone by employing a velocity microphone as a primary standard.

Still other objects, advantages and improvements will become apparent from the following detailed description taken in connection with the accompanying drawings in which:

2,597,005

Fig. 1 is a diagrammatic view partly in section Of one form of the apparatus in accordance with the invention;

Fig. 2 is a view in elevation of a preferred form of the invention and illustrating a spherical velocity microphone and the manner of supporting same;

Fig. 3 is a view in section of the microphone of Fig. 2 taken substantially on the line 3—3 thereof;

Fig. 4 is a view taken on the line 4—4 of Fig. 3 and illustrating the arrangement for centering the magnetic structure of the microphone with respect to the coil carrying element;

Fig. 5 is a perspective view of an apparatus suitable for determining the absolute sensitivity of a velocity microphone;

Fig. 6 is an enlarged view partly in section and partly broken away of a portion of the apparatus of Fig. 5;

Fig. 7 is a diagrammatic view of the apparatus employed for calibrating underwater microphones;

Fig. 8 is an enlarged view of a portion of Fig. 7 and illustrating the manner in which certain of the apparatus is supported;

Fig. 9 is a view taken on the line 9—9 of Fig. 8;

Fig. 10 is a diagrammatic view of the electrical apparatus employed for the purposes of calibration and utilizing a velocity microphone as a standard; and

Fig. 11 is a view illustrating the apparatus of Fig. 10 with the velocity microphone replaced by an unknown microphone.

Referring now to the drawings in which like numerals of reference are employed to designate like parts throughout the several views, there is shown in Fig. 1 a complete system according to one embodiment of the invention. As indicated in this figure, the seismic operations are conducted from a boat 10 in which are provided the necessary recording and control equipment which is not shown in detail. Another boat, not shown, is equipped with devices for lowering and firing the explosive charge by which the seismic waves are generated.

The wave detector or geophone 11 is located at one end of a cylindrical casing or tank 12, the size of which is dependent upon the weighted size of the geophone. The size and material of the casing are such that the effective density of the assembly is just slightly greater than that of the water covering the areas to be surveyed so that the assembly rests on the bottom without exerting any appreciable pressure thereon. A twoconductor insulated cable ί 3 leads from the geophone through the side of the casing near its lower end to the recording equipment 14 contained in the boat. This cable serves to conduct electrical impulses from the geophone to the recording equipment and may also be used for lowering the geophone to the bottom and raising it therefrom, or a separate cable may be provided for this purpose.

In the use of this device for seismic surveying, it is lowered to the bottom and because of the arrangement of the geophone at one end, such end contacts the surface of the bottom without exerting appreciable pressure thereon. The assembly will respond to any wave motion in the water without substantial distortion of the wave field. Seismic waves are generated in the usual manner and the geophone detects the waves which pass up through the earth into the water and produce wave motion therein corresponding to the wave motion .produced in the earth. Electrical impulses corresponding to the wave motion are impressed by the geophone on the recording equipment in the usual manner. The condition of the bottom has no effect on the response of the geophone since it is fully responsive to the wave motion in the water.

Referring now to Figs. 2 to 11 inclusive and more specifically to Figs. 2, 3 and 4, a preferred embodiment of a device for measuring underwater sounds is illustrated and comprises a microphone designated generally by the numeral 15, the microphone having a substantially spherical shell 16 about 2½ inches in diameter which is made of Duralumin or any other suitable light weight metal. The shell is divided into two substantially hemispherical sections 17 and 18 connected to each other as by threads on the male portion 19 of the section 17 and the female portion 21 of the section 18 to form a water-tight joint therebetween at the machined abutting faces 22 of the two hemispherical sections.

Screwed into the section Π is a thin cylindrical coil form 23 made of plastic or any other suitable material about five-eighths of an inch in diameter on which is wound a coil 24 which, by way of example, may comprise about 250 turns of #39 Formex magnet wire. The coil 24 is provided with terminal leads 25 and 26 supported within the section 17 as by an adhesive 27, the leads 25 and 26 being joined to insulated conductors 28 and 29 respectively which pass through watertight packing glands 31 and 32, respectively, arranged in the hemipsherical section 17. The packing glands 31 and 32 may be of any conventional construction but the metal portions thereof should preferably be made of a light weight metal such, for example, as Duralumin.

Flexibly supported within the hemispherical section 17 as by the soft rubber blocks S3 is a magnetic structure designated generally by the numeral 34 and comprising a cylindrical soft iron pole member 35 having a circular aperture 36 therein, the coil 24 being located within the aperture 36 which is slightly greater in diameter than the coil so as to provide a small amount of clearance therefor. The rubber blocks 33 are affixed to the pole member 35 by a suitable adhesive and support the pole member 35 centrally of a cylindrical seat 37 within the hemispherical section (7. _t

The cylindrical pole member 35 has attached thereto as by screws 38, a circular soft iron plate 39 having a circular aperture 41 therein within which is welded as indicated at 42 a cylindrical extension 43 of a permanent magnet 44. The magnet 44, which is preferably made of Alnico, is square in cross-section and has the end 45 reduced to a circular form to provide a surface to which a cylindrical soft iron pole piece 46 is welded as at 47.

The pole piece 46 has a diameter slightly less than the internal diameter of the cylindrical coil form 23 so that it is freely movable within the coil form in juxtaposition to the coil 24. To properly center the pole piece 46 with respect to the coil form 23 and yet permit free movement therebetween, a flexible rubber disc 48 having four arms 49, as shown in Fig. 4, is clamped to the top of the pole piece 46 as by a screw 51 and a washer 52, the arms 49 lightly gripping the interior of the coil form 23 and thereby flexibly centering the pole piece 46 with respect thereto.

For supporting the microphone 15 in a body of water so as to be substantially free of mechani«/B97.O06 cal restraint, the shell 16 has embedded therein a plurality of eyes 53 arranged-in the same plane and spaced symmetrically about the circumference of the shell. A brass ring 54 about 10 inches in diameter is provided with symmetrically spaced eyes 55 and flexible supporting means such as rubber bands 5S are arranged between the respective eyes 53 and 55, the shell i 6 being substantially free to move in a direction normal to the plane of the ring 54. The conductors 28 and 29 are clipped to the ring 54 by clips 57, the ring being provided with an upstanding member 58 through which the conductors 28 and 29 pass to form the cable 59 held to the member 58 as by a clamping nut 61. The conductors 28 and 29 are provided with sufficient slack between the shell 16-arid the ring'54 thereby to permit the shell to have substantial freedom of movement.

The spherical shell 16 and all the parts rigidly affixed thereto, such as the coil form 23, the coil 24, the leads 25 and 26 and the packing glands 31 and 32, are so designed that the weight thereof will substantially equal the weight of the volume of water displaced thereby when the shell is submerged in water. For all underwater sound frequencies well above the resonant frequency determined by the mass of the magnetic structure and the flexible support therefor, the inertia of the magnetic structure 34 prevents its moving appreciably. The shell 16, being substantially free of mechanical restraint by reason of its flexible connection to the magnetic structure and to the supporting ring 54, will have a motion substantially identical with the fluid particle motion and will not distort the wave field produced by the underwater sounds. The motion of the coil 24 in the stationary magnetic field which extends between the pole member 35 and the pole piece 46 generates a voltage proportional to the velocity of motion of the shell 16 and which is, therefore, proportional to the particle velocity of the fluid.

The spherical shape of the shell 16 possesses several advantages. As hereinbefore pointed out air bubbles which collect on the surface of a submerged microphone are a source of inaccuracy because of their compressibility thereby preventing the sphere from truly responding to the fluid particle motion. A sphere is easily debubbled because of the absence of recesses therein within which bubbles may collect. Such bubbles which may collect thereon when the microphone 15 is submerged may be removed therefrom by wiping with a wet cloth or by forcing a stream of water thereagainst and, since the bubbles are easily visible, it is a simple matter to discern whether the shell 16 of the microphone is entirely free of bubbles.

Another advantage possessed by the spherical shell is that its simple symmetrical geometrical shape lends itself to accurate mathematical calculation for any departure from the true fluid particle motion which may be caused either by any small difference which may exist between the weight of the shell 16 and the weight of the volume of water displaced thereby or by the fact that the wave length of the sound being measured is relatively short and therefore begins to approach the diameter of the microphone shell. The manner in which such calculations are made will be apparent to those skilled in the art and forms no part of this invention.

It is necessary to determine the absolute sensitivity before any underwater measurements made by the microphone can be given their proper interpretation. The manner in which the absolute sensitivity ‘of-the microphone can be determined will now be described in connection with-the calibration apparatus required for this purpose arid illustrated in Figs; 5 and 6.

' The calibration apparatus - comprises a fixture

Shown generally by the numeral 62 and a microscope shown generally by the numeral 63. The fixture 62 has a base 64 supporting at one end thereof as by screws 65, an upstanding plate 66 10 having therein a threaded circular opening 67 adapted to receive the male portion 49 of the shell 16. Also mounted on the base 64 and fastened thereto in any suitable manner is an upstanding plate 68 having a pair of clamps 69 15 adjustably fastened to the upper edge thereof as by screws 71 each adapted to be screwed into any one of a plurality of threaded holes 72. The clamps 69 are adapted to adjustably fasten a micrometer 73 to the upper edge of plate 68.

The base 64 also has fastened thereto a,s by machine screws 75 and nuts 70 a flat plate 76 and an angle member 77, the latter having a circular aperture therethrough for receiving a freely slidable shaft 78. The shaft 78 has thread25 edly connected to one end thereof a screw element 79 which is adjustably held in any desired position with respect to the shaft 78 by a locknut 81. The opposite end of the screw element 79 has rigidly fastened thereto by the pair of 30 lock-nuts 82 a coupling bar 83 which is adapted to be fastened to the plate 39 of the magnetic structure 34 of the microphone by several of the screws 38 which have been previously removed for this purpose. The rod 78 also has mounted 35 thereon a collar 84 which is adjustable along the shaft and may be clamped in any position thereon by the knurled set-screw 85, the collar 84 being placed between the measuring faces Of the micrometer 73. A removable spring 86 is ar 40 ranged to bias the collar 84 away from the member 77 for a purpose which will become more apparent as the description proceeds.

The microscope 63 is provided with a base 87 upon which the fixture 62 is clamped by any con45 ventional means in a position such that the objective lens 88 of the microscope is centered On the shaft 78 regardless of the adjustment of the shaft. The microscope is also provided with a filar micrometer eyepiece having a cross-hair ₆₀ adjustment dial 89 which, when properly calibrated, is employed to make very exact measurements in a well known manner.

The manner in which the calibration apparatus is employed to determine the absolute sen55 sitivity of the microphone 15 will now be described. The hemisphere 18 of the microphone is removed and the male portion 19 of the hemisphere 17 is screwed into the opening 67 of the plate 66. Two diametrically opposite screws 60 38 are removed from the magnetic structure 34 and the coupling bar 83 is attached thereto by means of the same screws 38. The spring 86 is removed and the set-screw 85 loosened to permit the collar 84 to slide freely on the shaft 78 and 65 permit the magnetic structure 34 to assume its normal position within the hemisphere 17 as determined by the unstressed condition of the rubber blocks 33 which support the magnetic structure within the hemisphere. The measur70 ing faces of the micrometer 73 are separated by a distance equal to the width of the collar 84 plus 0.015 inch, the micrometer faces being locked in this position, and the collar 84 is clamped to the shaft 78 by the set-screw 85 substantially 75 centrally of the distance between the micrometer

2,597,005 measuring faces or about 0.0075 inch from either face. This distance is not critical as the exact measurements will be made by the microscope 63.

The spring 86 is again placed between the member 77 and the collar 84 thus biasing the collar against the movable measuring face of the micrometer and simultaneously moving the magnetic structure 34 approximately 0.0075 inch from its normal position. A fluxmeter (not shown) is connected to the conductors 28 and 29 for measuring the flux linkages produced when the magnetic structure 34 is moved 0.015 inch with respect to the coil 24. Aluminum dust is sprinkled on the shaft 78 directly below the objective lens 88 of the microscope and the crosshair of the microscope is centered on one side of a selected particle of aluminum dust. A reading of this position is taken on the microscope scale and cross-hair adjustment dial 89.

The collar 84 is manually moved against the bias of the spring 86 until it strikes the stationary measuring face of the micrometer and thus causes a displacement of approximately 0.015 inch of the magnetic structure 34 to the other side of the normal position thereof with respect to the coil 24. The cross-hair of the microscope 63 is again centered on the same side of the same particle of aluminum dust on the shaft 78 and another reading on the scale and the cross-hair adjustment dial 89 is taken. The fluxmeter reading is also taken and the foregoing procedure is repeated a number of times to secure a good average result.

The absolute sensitivity of the microphone for plane waves may now be determined from the following formula:

„ i?X.4X10j

RXDXB where

S=absolute sensitivity in microvolts/dyne/cm.². K=fluxmeter sensitivity in linkages/division. A=fluxmeter deflections in divisions.

B=displacement in microscope divisions. JD=cm./microscope division.

R=specific acoustic resistance of water.

=1.437X10⁵ dyne sec./cm.³.

If a microscope is not available, fairly accurate results may be obtained by employing the readings of the micrometer 73. When these readings are substituted for those of the microscope, two of the factors of the formula above must be redefined as follows:

B=displacement of micrometer in inches. 7)=2.54 cm./inch.

It will be noted that the above formula applies only when determining the absolute sensitivity of the microphone for plane waves. However, the microphone must be operated at a relatively small distance from the sound source in order that pressures under investigation will be appreciably greater than those of background noises and reflections. Hence, the sound waves at this distance will be spherical rather than plane waves. The relation of particle velocity for plane waves to that for spherical waves, at points where the pressures are equal, is z&gt;„_71 + (c/ra;)² v_a 1 where t&gt;2=particle velocity for plane waves. Vs=particle velocity for spherical waves. r—distance between sound source and detector. c=velocity of sound in water.

W = 27r/.

/=frequency in cycles per second.

Referring now to Figs. 7 to 11 inclusive, an apparatus and method is disclosed for calibrating a pressure responsive condenser .type microphone employing the microphone 15 as a primary standard.

The microphone calibrations are conducted in a tank 91 or other body of water which is substantially free from currents, waves and noise. The water should be at least fourteen feet deep and the tank must be at least equally wide and long to prevent serious reflection of sounds from the surface of the water or from the bottom and side walls of the tank.

The velocity microphone 15 to be employed as a standard is suspended by adjustable links 92 fastened at one end to the brass ring 54 and at the other end to a wooden support 93 by any suitable means. Similarly mounted by means of adjustable links 84 about one foot away from the microphone 15 is an underwater loudspeaker having a supply cable 96 connected thereto, the microphone 15 and the loudspeaker 95 being so placed with respect to each other that the axis of the microphone coil 24 is in alinement with the axis of the loud speaker diaphragm.

The wooden support 93 is suspended at the proper depth within the tank 91 as by a pair of links 97 fastened at one end thereof to the support 93 by any suitable means and joined together at the other end by a fixture 98 fastened to a raising and lowering cable 99, the two ends of which pass over pulleys 101 fastened to the ceiling 102. The ends of the cable 99 are adapted to be wound around individual wall cleats 103 which permit adjustment of the microphone and loudspeaker at any desired position within the tank 91. The microphone cable 59 and the loudspeaker cable are connected to the calibrating apparatus, shown generally by the numeral 104, which rests upon a table 105.

The calibration apparatus i 04 is more fully illustrated in Figs. 10 and 11 and comprises an oscillator 106 which is adjustable by means of a control knob ! 07 through a frequency range of 100 to 10,000 cycles per second, this being the range of sound frequencies through which it is desirable to calibrate a pressure microphone. The output of the oscillator 106 is adapted to be connected by the transfer switch 108 either across a resistor 109 in series with the microphone 15 or to an amplifier 111 having a volume control H2. An ammeter 113 is provided to indicate the value of the current being supplied to the resistor 109 by the oscillator 106 which is provided with a volume control I! 0 for varying this current. The output of the amplifier HI is connected to the loudspeaker 95 through the cable 96, a voltmeter H4 being connected thereacross.

The cable 59 of the microphone 15 is connected to a preamplifier I i B, the output of which is connected to an amplifier i 16 having a volume control 117 and thence to a rectifier 118. The output of the rectifier i 18 is connected to a recording instrument 119 provided with a pen 121 and a cylindrical drum on which is carried a recording chart 122. The cylindrical drum is provided with a central shaft 123 having a pulley (not

25597,005 shown) fixed on one end which is driven by a belt 124 operated from a pulley (not shown) which is fixed to a shaft- i 25 geared to the frequency control knob ί 07 of the oscillator ί 06.

The apparatus so far described is employed for &lt; the purpose of calibrating the output of the loudspeaker at a point in the water one foot away; that is, at the point at which the standard velocity microphone 15 is located. Having once done so, it is a: simple matter to calibrate an unknown 1 pressure' microphone by replacing the standard microphone 15 by the microphone to be calibrated and recording its response to the loudspeaker 05 under a similar set of conditions.

Referring now to Fig. 11, the apparatus of Fig. 1 10 is illustrated as it appears after the unknown microphone shown generally by the numeral ί26 has been substituted for the standard microphone IS, the microphone 126 being supported by the wooden' support 93 (Fig. 8) by links in a similar 2 fashion to that employed in supporting the microphone 15 and in exactly the same position with respect' to the loudspeaker 95. The unknown microphone in the present example comprises a condenser transducer i 27 connected in series with 2 a resistor 128 to the‘ conventional built-in preamplifier 12 9, the output of which is connected by the cable 131 to the amplifier IIS in a manner similar to the arrangement of Fig. 10. The resistor 128 is adapted to be connected by the 3 cable 131 to'the oscillator iOS in the upper position of the switch 108.

The method whereby the pressure microphone 126 is calibrated with respect to the standard microphone 15 will now be described. Let it be ³assumed that the standard microphone ί 5 and the' loudspeaker^- S5 have been submerged to a proper .'depth' in the tank- 91 as illustrated in Fig. 7 and the apparatus 104 has'been connected as illustrated in Fig. 10. The pen 121 is raised from ⁴the'chart i 22 and the knob' ί 07 is adjusted to set the oscillator 106 at its lowest frequency generating'position of 100'cycles per second, the oscillator meanwhile being permitted to remain unenergized. The switch 108 is placed in its upper 4 position thereby disconnecting the loudspeaker 95-frOm the oscillator and connecting the oscillator across the resistor 109 in circuit with the microphone IS; The oscillator 106 is now energized and its volume control 110 is adjusted un- 5 til the ammeter 113 reads a current value which will produce for purposes of illustration about 50 microvolts across the resistor 109, this current value remaining fixed' during the subsequent measurements. The pen 121 is now replaced on 5 the chart 122, the volume control 117 having previously been adjusted’ to a value which experience has taught will give- a satisfactory trace on the chart.

The frequency control knob 107 is now slowly c rotated through the entire range of the oscillator 106 from 100 to 10,000 cycles per second, the belt 124 slowly operating the chart 122 in synchronism with the control knob 107. Simultaneously, the pen 121 will trace on the chart 122 c the'response of the elements 115, 116, 118 and 119 for each frequency supplied thereto by the oscillator through its connection to the resistor 109 thereby calibrating these elements at the various frequencies, it being understood that the 7 chart 122 has' properly marked thereon indicia representative of the frequencies supplied by the oscillator 106.

The switch 108 is now moved to· its lower position thereby disconnecting the oscillator from re- 7 sister 109'and-connecting it to the loudspeaker 95and tlie= volume control 112 is· adjusted until a satisfactory input as indicated by the voltmeter 114 is supplied to the loudspeaker. The fre5 quency control knob Γ 07 is again rotated whereby a second trace is placed on the chart 122; the latter trace being representative of the response at each frequency of the microphone 15.

From the two traces on the chart 122 and the 0 absolute sensitivity of the microphone 15 deter? mined with the apparatus of Figs. 5 and 6 as hereinbefore described; the calibration of the loudspeaker output' in- dynes/cm.² at a point one foot away for-any or all frequencies may easily 5 be determined) For any-particular frequency, the calibration of the loudspeaker output in dynes/cm.² equals the ratio of the magnitude of the-second trace at that frequency to the magnitude of the first trace at the same frequency 0 multiplied by the number of-microvolts placed across the resistor 109, the resultant being· divided by the absolute sensitivity of the microphone 15 -in microvolts/dyne/cm.².

To calibrate an unknown pressure microphone ;5 126 for the range of^! frequencies from 100 to 10,000 cycles per second, the support 93 (Fig. 7) is raised from the tank 91 by the cable 99-and the unknown pressure microphone is substituted for the standard microphone 15 in the manner 0 shown diagrammatically in Fig. 11. The pres? sure -microphone and loudspeaker are submerged in the tank; it being understood that the pressure microphone 126 is placed in exactly the same space relation to the loudspeaker 95-as the stand5 ard microphone bore to it. The· volume controls 110, 112 and 117 are allowed to remain in exactly the same positions to which they are adjusted in making the calibration; of the loudspeaker output.

The switch 108 (Fig. 11) is placed in its upper position and a first trace is made on the? same record chart 122 preferably in a different colored ink than: was employed in: making the earlier traces, the trace beingtrnade by rotating the frer quency control knob i 07 as heretofore. The read^a ing of the ammeter 113· is noted as it will differ considerably from the-reading noted in Fig. 10, the value of the resistor 12 ff· being considerably higher than that of the resistor. 109 by reason of ₀ the much greater sensitivity, of the condenser transducer 127 over that of the velocity microphone 15. By'multiplying: the reading of the ammeter 113 by the resistance of the resistor-128, the microvolts applied thereacross may be deter₃ mined. The switch 108 is now moved to its lower position and a trace in still another colored ink is made upon the chart 122 by rotating the frequency control knob 107'.

From the latter two traces taken in connecq tion with the calibrated-loudspeaker output derived from the first two traces, the calibration of the-pressure microphone 126 at any frequency may readily be determined. For any particular frequency, the. calibration of the pressure micro5 phone is microvolts/dyne/cm.² equals the- ratio of the magnitude: of the second pressure microphone trace at that frequency to the magnitude of the first pressure microphone trace at the same frequency multiplied by the number of microvolts 0 applied across resistor 128, the resultant being divided by the-loudspeaker output calibration at the same frequency in dynes/cm.².’

It will, of course, be understood that each of the hereinbefore mentioned corrections which may be applicable under a particular set of con2,597,005 ditions is applied in determining the calibration of the pressure microphone 126 thereby to eliminate inaccuracies produced when the wave lengths of the sounds being measured approach the diameter of the microphone 15, or the weight of the shell 16 is different from that of the volume of water displaced thereby, or the distance between the loudspeaker and the microphones during measurements is relatively small.

The velocity microphone of this invention may also be employed for determining the acoustical impedance of the bed of a body of water. The ring 54 is disconnected from the links S2 and the ring, with the microphone i 5 still mounted therein, is laid horizontally upon the bed of the body of water at the point at which the acoustical impedance is to be determined. The ring 54 performs the function of maintaining the axis of the coil 24 normal to the bed of the body of water so as to place the coil in a position wherein it will respond to the particle motion at the interface between the water and the bed by reason of the fact that the microphone shell 16 has practically the same density as the water.

A pressure type microphone, such as the microphone I2S of Fig. 11, is supported adjacent the microphone 15 and both microphones are subjected to sounds of various frequencies, so that both the acoustical velocity and the acoustical pressure of the sounds may be simultaneously recorded on suitable instruments located at the surface of the water. The acoustical impedance of the bed of the body of water at any particular frequency can be determined from the formula

<img file="US2597005A_D0013.tif" />

where

Z=acoustical impedance P=acoustical pressure V=acoustical velocity

Briefly stated in summary, the present invention contemplates the provision of a new and improved underwater sound wave detector or microphone adapted to respond to the fluid particle motion of the water without substantial distortion of the wave field thereof. In the preferred embodiment, the microphone is so shaped that it may be readily debubbled and lends itself to mathematical analysis for the purpose of calculating the corrections which are necessary when there is either a difference in the mass of the microphone from the mass of the water which it displaces or when the wave lengths of the sounds to be detected approach the dimensions of the microphone. The invention further contemplates the provision of a novel method of and apparatus for determining the absolute sensitivity of the microphone of the present invention and also a new and useful method of and apparatus for employing the microphone of the instant invention as a standard for calibrating other microphones.

Although, in accordance with the provisions of the patent statutes, this invention has been described in concrete form with reference to a preferred embodiment thereof which gives satisfactory results, it will be understood that this form is merely illustrative and that the invention is not limited thereto since alterations and modifications will readily suggest themselves to persons skilled in the art without departing from the true spirit of this invention or the scope of the annexed claims.

Hie invention herein described and claimed may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.