US 2102668 A
Description (OCR text may contain errors)
Dec. 2l, 19.37.
S, BALLANTINE PIEZOELECTRIC LOUDSPEAKERl 2 Sheets-Sheet l Filed June 14, 1933 lll/111111111111 Dec. 21, 1937.
S. BALLANTlNE PIEZOELECTRIG LOUDSPEAKER 2 Sheets-Sheet 2 Filed June 14, 1933 N n m M v. a fr WL wb wn.. r. v N1. 4,
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Patented Dec. 21, 1937 PIEZOELECTRIC LOUDSPEAKER -Stuart Ballantine, Mountain Lakes,fN. J., assignor, by `mesme assignments, to Radio Corporation of America, New York, N. Y., a corporation of Delaware Application June 14, 1933, Serial No. 675,834
This invention'relates to electroacoustic apparatus for the conversion .of electrical energy' into sound energy, and more particularly to electroacoustic apparatus of the type employing a piezoelectrically active crystal assembly.
An object of the invention is to provide electroacoustic apparatus of simplified constructionl and in which the active element is a piezoelectrically active diaphragm comprising a plurality of units, each unit comprising two crystal plates which have a torque response when a potential is impressed across the electrodes associated with the crystal plates. An object is to provide a loud speaker of the horn type, and driven by a piezoelectrically active crystal assembly of the type stated. A further object is to provide a loud speaker'of the type stated, and in which the circuit or circuits supplying power to the speaker compensate for inequalities in the sound pressure-frequency response characteristic of the reproducer elements, thus rendering the combined characteristic of the electrical supply circuit and the reproducer elements more nearly uniform over the band of frequencies which is to be reproduced.
A further object is to produce a loud speaker of the above type which is particularly adapted for the reproduction of the higher audible frequencies.
' These and other objects of the invention will taken with the accompanying drawings in which: Fig. 1 is a longitudinal central section through a loud speaker embodying the invention, the sectionl being taken substantially on line I-l oi Fig. 2;
Fig. 2 is a transverse section substantially on line 2-2 of Fig. 1;
y Figs. 3a and 3b are somewhat schematic views, in perspective, of crystal shear sensitivity;
Fig. 4 is a similar view of the crystal plate diaphragm system of the speaker shown in Fig. 1;
Fig. 5 is a curve showing the sound -pressure- .frequency characteristic of a small piezoelectric loud speaker designed for the reproduction of the higher audio frequencies;
Figs. 6a, 6b and 6c are circuit diagrams of' the loud speaker and associated circuits for supplying audio frequency voltages thereto, the circuits including inductance for altering the sound pressure-frequency characteristic; and h Fig. 7 is a curve sheet showing a series of curves which illustrate the compensating eifect be apparent from the following speciiication when plate systems having obtainable with a supply circuit such as shown in Fig. 6b.
In the drawings, the reference numeral l identifies a crystal plate diaphragm assembly which is clamped to the base of the horn 2 by means of screws 3 which bear against the annular metal ring 4, a compressible gasket 5 being arranged between ring 4 and the crystal assembly, proper clearance between the crystal faces and the horn base -being provided by the annular spacer 6. The spacer ring B may be made of rubber or of any other suitable material capable of forming a seal to prevent the escape of air from the throat chamber 1. 'I'he throat of the horn is ofuniform diameter out to the tip of the insert 8 whichvis approximately bullet-shaped to provide the `de sired exponential law of the increase in the crosssection of the horn.
The insert 3 provides an annular throat passage at the base of the horn, and thus reduces the average length of the paths from the rvarious parts of the crystal diaphragm to the throat,
thus bringing the phases of the air currents due The crystal diaphragm [comprises iour square crystal plates cemented together at their edges. to form a square assembly, as illustrated in Fig. 4. 'I'he method of operation of this assembly will be best understood by rst considering the simpler crystal plate systems shown in Figs. 3a
The plates A are cut from homogeneous Rochelle salt crystals with the at faces of the crystals normal Ato the electrical or a axis and the sidesparallel to the b and c axes. Electrodes B of metal foil are cemented to the faces of the y crystals and, as is well known, the application ofa potential difference between the electrodes tends to strain the crystal. When the cornersy of the crystals are held immovable, the application of direct current potentials o! the polarities indicated will tend to move the upper edges of the crystals in shear in the directions indicated by the arrows adjacent those edges.
ward'the other tends to move backward, the
result will be a twisting or torque motion of the upper part of the assembly when the corners 1:, il are clamped. When one of the upper corners e is also clamped, the free corner d will move at right angles to the plane of the crystal plates, as shown by the arrow D. This method of securing 4torque sensitivity fromjshear plates is described by C. Baldwin Sawyer in The Proceedings of the Institute of Radio Engineers", Nov.
Y 1931. page 2025.
The crystal plate diaphragm of the present invention comprises four units of the type shown in'Fig. 3, these units being cemented together at their 4edges to form the squa're assembly which is illustrated in Fig. 4. When this assembly is clamped to the horn base by the ring l and the screws I, the corners z, 1,' and z of each doubleplate unit of the composite diaphragm are sub- V stantially prevented from moving and the application of a potential diiference between the electrodes causes the free corners d to move in and out inunison in a direction normal to the plane of the assembly. 'I'he Awhole assembly thus acts as a diaphragm and functions in the same manner for the generation of sound as the ordinary type of horn loud speaker.
ToV prevent the undesired escape or entrance of air into the throat chamber, the union oi' the four crystal plates at their edges must be care- Y fully made so as to remain air tight. If cement is used, it is preferably of a plasticized resin type, such as cellulose nitrate cement containing castor oil, which will remain resilient after setting to avoid any constraint upon `the free motions of the plates. It is not necessary to use cement,y as the spaces between the plates may be ca1ked,
but, in rgeneral, the use of cement results in a better assembly mechanically.
As noted above, the gasket 5 is compressible to permit adjustment of the screws 3 to obtain that clamping pressure which affords the 'maximum and Ysmoothest frequency response.
The four plates or double plates of the diaphragin are u,connected in parallel, as shown in Fig. 4, care being taken that the connections are so made/that the motions of the four units are in phase. The clearance between the throat and the surfaces of the crystals was of the order of 0.010 inch. v
The performance of a loud speaker of this type and designed for the reproduction of the higher audible frequencies is shown by curve E of Fig. 5. The data for this curve was obtained by measuring the pressure inthe sound wave emanating from the horn for applied voltages of different frequencies. across the crystals for all frequenciesV and the pressuresV were measured on the axis of the horn at a'distance of three feet fromvthe outer end.
'The dimensions 'of the crystals were so chosen that elastic resonance was obtained at a frequency approximately equal'to the upper limit of the frequency range to be reproduced. As shown by curve E, the design was such that the resonant frequency was about 8000 cycles.
For many purposes, it is desirable to have a relation between sound pressure and applied voltage which is substantially uniform over an ex- Theslame voltage was maintained- 2,1 oa,ess
tendedl range of frequencies. This characteristic maybe obtained with a horn having the inherent characteristic oi' Fig. 5 by an appropriate design of the circuit which transmits' the voltage to the loud speaker.
vAs shown in Fig. 6a, the generator G or source of power supply is represented as having an internal resistance R, andthe circuit completed between the terminals of the generator G and the4 loudspeaker C includes the series inductance L which tunes the circuit-'to a frequency lower than the resonant frequency ofthe loud speaker. With this' arrangement, the sound pressure response will beesubstantially uniform for all frequencies between the resonant frequency ofthe circuit and the resonant frequency of the horn.
' When it is desired to secure an impedance match with a generator of arbitrary internal resistance, the speaker Cmay be/connectedto the supply circuit through a transformer T, Fig. 6b, which transformer corresponds to the transformer lof Fig. 1. With this arrangement, the tuning inductance L may be inserted in the primary, as shown.
or in the `secondary circuit in series with the horn. It may` also be built into the transformer, for example. as leakage inductance.
The circuit may also be resonated by means y of a shunt inductance L', or equivalent reactance, as shown in Fig. `6c.
The eiect of the series tuning inductance L upon the sound pressure characteristic is illustrated graphically in Fig. 7, the several curves being identified by legends L=0.35 mh, etc., to indicate the value, in millihenries, of the inductance effectively included in the supply circuit when the data for a particular curve was obtained.
The electrical supply circuit may also be tuned to a frequency higher than the resonant frequency of the loud speaker. In this case theresponse tends to become more uniformabove the diaphragm resonant frequency rather than below it.
Some control over the sound-pressure frequency characteristic may also be obtained without -the tuning inductor L by adjusting the effective capacity reactance of the loudspeaker relative to the internal resistance of the supply source. The most convenient way todo this is by means of. a transformer inserted between the supply source and loud speaker. Increasing the secondary-primary turns ratio will in general cause an increase in the effective crystal capacity and a greater slumping oi! of voltage across the crystais with frequency.
The above data is given merely as an illustration of the operating characteristic of.one particularembodiment, and it is to be understood 'that the'invention is not limited to any particular frequency range of operation. By appropriate design of the diaphragm and the associated electrical system, the frequency range of reproduction may be locatedat another section of the audio frequency range, and it may be increased or decreased in width in accordance with the design requirements for a particular construction.
1. In an electromechanical transducer, ,a flat assembly of four piezoelectrically active plateshaped elements arranged in edge alinement and with corners of the said elements substantially meeting at a common central point of said assembly, each of said elements comprising a pair of superposed crystal plates having-an `electrode between their adjacent faces and electrodes at arcanos' their outer faces, and circuit connections between the several electrodes of said elements toproduce like deiections oi all elements atsaid common point when voltages are impressed across said electrodes or, alternatively, to produce like electrical responses between connected electrodes of all elements `when pressure is applied to said common point of the assembly, and means introduced between the contiguous edges of said elements to prevent leakage of air past said contiguous edgessaid 'means consisting oi a resilient cement joining the adjacent edges of said elements to each other.
2. The invention 'as claimed ln claim l, in com` bination with means engaging marginal areas at opposed faces of said at assembly of elements for limiting movement oi the said engaged areas. 3. 'A loud. speaker comprising a horn, a diaphrafgm comprisinga plurality of piezoelectrilcally active crystal plates secured across the throataof said horn, and sealing means between and cooperating withsaid diaphragm and horn to form an air-tight throat chamber.
4. A loud speaker comprising a horn, a dia phragm comprising a plurality of piezoelectrically active crystal plates at the throat of the horn,
a gasket cooperating with said throatand horn the throat of the horn, a piezoelectrically' active diaphragm positioned adjacent the throat oi' the horn,a gasket between said diaphragm and`horn base, and means clamping said diaphragm-against said gasket, whereby said gasket cooperates with 11:; said 4diaphragm and horn 'base to'iorm an airtight throat chamber.
' 7. Aloud speaker as claimed in claimt, where-Y in said diaphragm comprises a plurality of vplateshaped crystal units' havingM their contiguous edges cemented to each other; each unit being piezoelectrically responsive normally toits planeA trdei'ine "an annular opening into said throat chamber, and means comprising -ribs projecting radially from one of said members for' centering said insert member within said horn member.
9. In a loud speaker, a pizoelectrically active diaphragm, a horn provided with a throat chamhorn member, a vpiezoelectrically. active diaphragm and means mounting the same atthe base o! the horn to' form an air-tight throat chamber, said diaphragm and horn member having an oven-all resonant sound-pressure' frequency characteristic, saidresonant frequency being locatedat the Aupper end of the range of irequencies to be reproduced.
l1. :A piezoelectric loud speaker` comprising a horn member, a piemelectrically active diaphragm and means mounting the same at the base oi' the horn to 'form an air-tight throatv chamber, said diaphragm and horn member having an over-all resonant sound-pressure'frequency `characteristic, ,said resonant frequency being f located above '1,000 T