Abstract:
The present invention provides a completely self-contained intraoral larynx comprising a power source, tongue activated controls, power saving signal generation circuitry, acoustic and audio amplifiers, and an intra-oral speaker. These components are contained within an otherwise conventional denture or dental prosthesis. The device produces a high amplitude sound while minimizing current drain from the power source. Laryngectomized patients require only short term training by a speech pathologist to use this device. Intelligible speech is usual within an hour of such training, and facility rapidly improves. The psychological barriers, post-operative physical limitations, aesthetic concerns and limited gestural communication traditionally associated with the use of extra-oral larynges is eliminated by the present invention.

Description:
This a continuation of application Ser. No. 438,376, filed Nov. 1, 1982 which in turn is a continuation-in-part of Ser. No. 249,140, filed Mar. 30, 1981 issued as U.S. Pat. No. 4,473,905 on Sept. 25, 1984. 
    
    
     BACKGROUND 
     The present invention relates to the field of sound producing prosthetic devices for use by laryngectomized patients. 
     Many devices have been suggested for providing speech capability to laryngectomized patients. One approach involves the provision of a mechanical or electromechanical device to create pitches or tones which are modulated to produce speech. In Richard L. Goode&#39;s review entitled &#34;Artificial Laryngeal Devices In Post-Laryngectomy Rehabilitation&#34;, which appeared in The Laryngoscope (pages 677-689, 1974), two basic types of artificial larynges, transcervical and transoral larynges, are disclosed. Transcervical larynges are electronic, hand-held, battery-powered, vibrating devices that are placed on the neck to produce voice. While transcervical devices have experienced considerable commercial success, they must be hand held on the neck while speaking, produce an &#34;electronic&#34; type of speech, leak unwanted sound from the neck, need battery changes, and require a place on the neck that will allow efficient transfer of sound vibrations. Transoral laryngeal devices are electronic or pneumatic devices in which the sound enters the mouth through a tube. While such units produce a loud sound and may be used in early post-operative stages by patients with thick neck tissue who cannot use transcervical devices, transoral devices are noticeable in use, require placement in the mouth, and are prone to saliva blockages. Additionally, transoral devices may result in articulation which is disturbed by the placement of the tube near the tongue and lips, this results in speech which is less intelligible than that produced by transcervical devices. 
     The desirability of including all of the operative components of an artificial larynx on an intraoral prothesis has long been recognized. See for example U.S. Pat. No. 2,862,209, Col. 1, lines 61-64. Unfortunately, the art has heretofore failed to disclose any complete intraoral device: 
     One early attempt at an &#34;intraoral&#34; device is the &#34;Tait larynx&#34;. In 1974 Goode, supra. described the Tait larynx, &#34;no longer manufactured&#34;, as comprising an intraoral earphone attached to a patient&#39;s denture or dental retainer, an external power source, external oscillator and external on-off switch. In the Tait device a wire was passed from the intraoral earphone to these external components. While the sound produced by the Tait device is at least comparable to that produced by other transoral larynges, the presence of a wire coming out of the user&#39;s mouth makes the Tait device unacceptable to most laryngectomees. For a disclosure of a similar transoral device see U.S. Pat. No. 2,862,209. 
     Pichler eliminated any need for transoral wiring by suggesting that a wireless induction system could be used to transmit sound from a pocket oscillator driven primary coil worn around the neck to an intraoral secondary coil associated with an intraoral earphone. However, Pichler&#39;s device has apparently never been placed on the market. See Goode, supra. 
     In an attempt to improve upon the Pichler device, Goode suggests that a miniature pulse generator capable of being mounted on a denture or dental retainer can be used to drive a small, waterproof, low impedence, modified insert earphone. Power is provided to the pulse generator by a hand-held, external radio frequency generator operating at 100 kHz and using a 15-volt rechargeable battery pack, carried in the pocket, with a battery drain of 780 mW. When the transmitter is held to the cheek, the transmitter coil lies about 1.5 cm from a miniature, tuned receiving coil in the denture. 
     A wireless electro-larynx consisting of a receiver/speaker concealed in an ordinary denture or prosthesis, and an external transmitter worn under the patient&#39;s clothing has been suggested by Zwitman et al. See &#34;Development and Testing of an Intraoral Electrolarynx for Laryngectomy Patients&#34;, Zwitman et al, Journal Of Speech And Hearing Disorders, XLIII, 263-269 (May, 1978B). In the Zwitman et al device, a 1 millimeter thick receiver is housed in the center of the denture and surrounded by a receiving coil which is used to pick up an incoming differentiated transmitter-generated pulse which is then amplified. This amplified signal is used to change the state of a bistable multivibrator circuit, which then activates an astable multivibrator. This astable multivibrator then begins to oscillate at a repetition rate (fundamental frequency) of 70 Hz to produce an audible buzz. Current is permitted to pass through the receiver for only 0.3 msec periods, which are long enough to facilitate the generation of intelligible speech, while conserving battery life. A plastic tube is attached to the speaker of the Zwitman et al device which extends medially and slightly upward past the mid-line of the palate to a point just short of the opposite side of the dental prosthesis. This tube is intended to transmit sounds to the posterior region of the oral cavity to provide maximum resonance, and to prevent the tongue from occluding the speaker aperature. Two rechargable batteries are used to power the receiver/speaker for up to five continuous hours. Since the standby current drain is low, Zwitman et al reported that intermittent use of the unit permits it to function for an entire day before recharging is necessary. 
     In &#34;A Modified Intraoral Electro-larynx&#34;, McRae et al, Archives of Otolaryngology 105: 360-361, 1979, and in &#34;The Design of a Wireless-Controlled Intraoral Electro-larynx&#34;, Knorr et al. Journal of Bioengineering 1: 165-171, 1977, other designs of intraoral electro-larynges are disclosed. 
     More recently, in work conducted by Mr. Kenneth Stern, a self-contained, intra-oral artificial larynx has been suggested, and Stern&#39;s described circuitry breadboarded. This circuit generates a squarewave of 125 Hz which is used as a clock for a five stage binary counter. Outputs of the counter are logically interconnected through exclusive-or&#39;s, amplified, and used to energize a speaker to produce a buzz. By using bi-lateral switches, most of the circuitry is not powered in order to extend battery life. 
     Stern has suggested the desirability of providing a tongue operated switch for power control, and of providing a speaker housed in the mouth which produces varied output frequencies which simulate changes of pitch; however Stern has failed to provide designs for these components. 
     As seen from the above, the desirability of providing a completely self-contained, intraoral artificial larynx has long been recognized. Nonetheless, there is a long felt need for a simple, intraoral, tongue-controlled larynx which may function without recharging over extended periods of time, and which facilitates the generation of clearly audible speech. 
     SUMMARY OF THE INVENTION 
     The present invention provides a novel, completely self-contained intraoral artificial larynx. This larynx is an ultra thin, miniature device which may be mounted on or within a dental appliance and is wholly contained in the mouth. Based on an estimate of 3 hours per day of continuous speaking, the larynx of the present invention may be used for up to 30 days without battery replacement or recharging. 
     The preferred embodiment of the present invention comprises a power source, tongue activated controls, power saving signal generation circuitry, an audio amplifier, an intraoral sound source, and an acoustic amplifier which is tuned to the output of that speaker. Power saving signal generation circuitry is designed to produce a high amplitude signal which drives the speaker to produce an &#34;audio flicker&#34;. In the preferred embodiment, the tongue activated controls comprise contacts located on the palate which may be touched by the tongue to activate, deactivate, or temporarily disable the artificial larynx. In order to prevent accidental activation or deactivation, the on-off contact must be touched for preselected periods of time to produce the desired activation or deactivation of the device. Occlusion of the speaker with saliva or water is prevented by locating the speaker at one end of an acoustic horn, encapsulating the speaker, and by disposing a liquid barrier means, such as a microporous film or screen, over the speaker and/or horn orifice to prevent fouling of the speaker with mouth fluids. This barrier means is installed such that its attenuation of sound is minimized. 
     Laryngectomized patients require short term training to use the device of the present invention, and this device can be conveniently worn for extended periods of time. Unlike prior art devices which have relied upon extra-oral components, the device of the present invention is worn entirely within the mouth and is incapable of visual detection. A unique combination of tongue controls for permitting convenient intermitent use, novel power saving circuitry, and tuned acoustic amplification permit the device of the present invention to be used for far longer periods between battery replacement or recharge then has heretofore been possible. 
     In an alternate embodiment of the present invention, the artificial larynx is completely sealed, and includes a solar cell for recharging a self-contained power source, such as a rechargable battery. In this embodiment, the larynx is made so that the power source may be recharged by placing it under a light source when it is not being worn. 
     In other alternate embodiments the circuitry of the preferred embodiment is simplified, sound output is maximized, and current leakage and acoustic hum are minimized. 
     Accordingly, a primary object of the present invention is the provision of an entirely self-contained, intraoral larynx. 
     A further object of the present invention is the provision of an intraoral larynx which is entirely tongue controlled. 
     A further object of the present invention is the provision of a tongue controlled intraoral switching device. 
     Another aim of the present invention is the provision of power saving signal generation circuitry for use in driving an artificial larynx. 
     A further object of the present invention is the provision of an artificial larynx which resists fouling by saliva or water. 
     A further aim of the present invention is the provision of an artificial larynx having a tuned acoustic amplifier for maximizing the amplitude of sound generated by the electronic tone generator. 
     These and further objects of the present invention will become apparent from the following, more detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagramatic plan view of the top surface of a preferred embodiment artificial larynx, diagramatically illustrating the speaker, battery, solar cell, acoustic horn, mesh and tongue contacts which are contained within an otherwise conventional dental prosthesis; 
     FIG. 2 is a block diagram illustrating the operative components of the preferred embodiment artificial larynx of the present invention, an alternative embodiment being illustrated with dotted lines; 
     FIG. 3 is a circuit diagram of the preferred embodiment represented in FIG. 2; 
     FIG. 4 is an enlarged cross-section of the terminal portion of an alternate embodiment horn over which is diposed a microporous barrier; and 
     FIG. 5 is a circuit diagram of an alternate amplifier circuit. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     While specific forms of the present invention have been selected for the purposes of illustration, one of ordinary skill in the art will recognize that various departures may be made to the examples set forth herein without departing from the scope of the present invention, which is defined more particularly in the appended claims. 
     The intraoral artificial larynx of the present invention generally comprises a prosthetic means for mounting the larynx within the oral cavity, a power source mounted on the prosthetic means, signal generation means mounted on the prosthetic means for generating preselected electrical signals, and speaker means mounted on the prosthetic means for converting said signal into acoustic energy. Referring now to FIG. 1, the prosthetic means, designated generally 100, will be seen to generally comprise a conventional palatal denture or dental prosthesis. Such a prosthesis usually comprises means for anchoring the prosthesis in the mouth cavity, such means in FIG. 1 being dental wires 102, 104 and 106. The body 108 of the dental prosthesis is formed from conventional dental prosthetic materials, such as an acrylic polymer, which is shaped to fit comfortably against the roof of the wearer&#39;s mouth. This body is preferrably formed to encapsulate all of the electrical components of the preferred embodiment larynx, and is molded to contain an acoustic horn 110, which is the preferred acoustic amplification means for amplifying acoustic energy generated by the aforementioned speaker means. As shown in FIG. 1, the preferred acoustic horn 110 is arcuate, having its minimum diameter at its juncture with speaker 112 and its maximum diameter at its terminus 110a at the rear of the dental prosthesis 100. In order to maximize achieved amplification, horn 110 is generally circular at its point of coupling to speaker 112 and gradually becomes elliptical as it approaches its terminus 110a 
     In the embodiment illustrate in FIG. 1, a gentle curvature of the horn has been selected. Alternatively the horn may double back on itself so that this component is located entirely on one side of the mid-line of the prosthesis. In this instance a metal (stainless steel) spiral or spring may be used in the bend of the horn to maintain the (150 &#34;) lumen of the horn. 
     In forming the horn for this alternate embodiment, a length of heat settable 1/8&#34; I.D. tubing (available from Chemotron, Inc.) may be formed into its desired shape using a (silicone coated) conical jig inserted into one end of the tube and a speaker form jig into the other. With the stainless steel spring disposed at the point of bend, the horn may be bent over on itself, fastened or wrapped in that position, and heat set, as for example through immersion in boiling water. Once set, the jigs may be removed from the horn and the speaker or speakers fitted into the proximal end of the horn. If desired, the terminal end of the horn may have been sealed with a barrier means as described hereinafter. Additional liquid tight seals between these and/or other components may be provided by dipping them into an air dry solution of trichlorethane. Such a solution is commercially available under the trade name &#34;Dip It&#34; from Plasti-Dip International, of St. Paul, Minn. 55113. 
     The length of acoustic horn 110 is carefully selected to maximize the audible output of tones generated by speaker 112. A tuning process, an example of which is described more fully hereinafter, should thus be performed which includes testing the desired circuitry with acoustic horns of varying lengths and configurations until maximum amplitudes are achieved for given frequencies. For the preferred embodiment for the present invention, horn 110 should have an acoustic length of between about 1.50 and 2.75 inches, preferably about 1.75 and 2.25 inches. 
     The larynx of the present invention is controlled by bridging anterior contacts 114, or posterior contacts 116 with the tongue. As is described more fully hereinafter, the touching of either of these pairs contacts acts to complete a ground path in the artificial larynx. Anterior contacts 114 provide a pause control which instantaneously interrupts the generation of sound by the larynx for as long as the contacts are bridged. The posterior contacts 116 provide an on-off control which must be touched by the tongue for a pre-selected period of time. By requiring prolonged tongue contact of posterior contacts 116, inadvertant switching between the off and on positions is effectively prevented. 
     In an alternate embodiment, the grounding contact of each pair of contacts may be common to the pause and off-on switches. Accordingly, only three adjacent contacts are needed in this embodiment. These contacts can then be conveniently placed at the anterior of the prosthesis with the grounding contact located between the on-off and pause contacts. 
     The efficiency of the sound circuitry makes it possible to use a low-voltage, low-current power source, such as batteries 118. For the embodiment shown in FIG. 1, these batteries may be entirely sealed within the dental prosthesis 100, and may be of the lithium, silver oxide, or nickel cadium type. In alternate embodiments replacement of such batteries may be peritted. It is presently preferred to provide a solar cell 120 which is located within the dental prosthesis adjacent to an area of transparent material in that prosthesis. The current drain of the larynx of the present invention is sufficiently low as to facilitate recharging of the larynx on a periodic basis by removing the larynx and replacing it under an artificial light source, such as incandescent light. 
     Special precautions are taken to prevent speaker 112 from becoming fouled with water or saliva. In addition to increasing the amplitude of sound generated by speaker 112, acoustic horn 110 aids in protecting speaker 112 from liquids contained within the mouth. This protection results from the location of the speaker at the proximate end of the horn, and may be further enhanced through the provision of a mesh 122 located at the speaker orifice. Applicants have found that introduction of saliva or water into acoustic horn 110 is unlikely, and that such amounts as may be introduced to acoustic horn 110 are generally shielded from speaker 112 by mesh 122. Mesh 122 does not appreciably interfere with sound transmission from the speaker 112 to horn 110. 
     Additional protection against flooding may be attained by covering the mouth of the horn 110 with a thin sheet of material which will prevent liquids from entering the horn but which will permit water vapor and air to pass therethrough. Hydrophobic microporous materials such as polytetrafluoroethylene sheets (half mil) sold under the tradename &#34;Teflon®FEP fluorocarbon film&#34; by American Durafilm Co, Inc. of Newton Lower Falls, MA, are suitable for this purpose. 
     Use of a microporous cover over horn 110 will, however, cause some attenuation of sound. This attenuation can be minimized to about 4 dB or less if care is taken to install the sheet as illustrate in FIG. 4. In this figure, the sheet 111 is shown wrapped over the end 110a of the horn and bound to an outer circumferential surface of that end using a binding, such as suture thread 113, to create a liquid tight seal with the horn. The portion of the sheet 111 disposed over the mouth of the horn is loosely draped into the horn mouth such that it extends from between about 1/4&#34; to 1/8&#34; into the mouth of the horn. This loose drape of the sheet material is sufficient to prevent sound attenuation, but not so great as to facilitate entrapment of liquids in the horn orifice. This microporous sheet, when used as described above, may supplement or replace the aforementioned mesh. 
     FIG. 1 diagrammatically illustrates representative mountings of solar cell 120, battery 118, sound generation circuitry 124, speaker 112, mesh 122, contacts 114 and 116, and acoustic horn 110. FIG. 2 provides further information concerning the operation of the preferred embodiment larynx of the present invention. In FIG. 2, further details of the sound generation and switching circuitry are provided. As a matter of illustrative convenience, batteries 118 and solar cell 120 are not illustrated in FIGS. 2 or 3. FIG. 3 is a circuit diagram providing the details of the circuit which is diagrammatically illustrated in FIG. 2. Portions of the circuit illustrated in FIG. 3 which correspond to blocks illustrated in FIG. 2 are surrounded by similarly numbered dotted outlines. 
     Referring now to FIGS. 2 and 3, the signal generation circuitry of the present invention is seen to comprise a signal generator 200 and differentiator 202. In the preferred embodiment, a square wave signal generator is utilized, the resultant signal of which is differentiated to produce an output signal comprising a plurality of spikes which are separated by long near-zero voltage time periods. In the preferred embodiment, the square wave signal generator comprises an astable multivibrator which generates a signal having a frequency between 0 and 20 KHz preferably 60-120 Hz. By differentiating the signal generator output, the frequency and maximum amplitudes of the positive and negative spikes produced thereby will, of course, correspond to the frequency and amplitude of the square wave signal. The periods of return to near-zero voltage between spikes of the differentiated signal are too short to be audibly resolved, and thus, an &#34;audio flicker&#34; is created when the output signal is fed to audio amplifier 212, and to speaker 112 to produce the output tone of the larynx. Thus, the amplitude and frequency of the apparent sound produced by the larynx has not been changed, while the duty cycle, and thus the power drain, of the output signal has been substantially reduced. A duty cycle control means is thus provided which is utilized to reduce the duty cycle of the output of the signal generation means by profitably utilizing the audio flicker effect. In the preferred embodiment, the duty cycle of the sound generation means is at least less than 10%, generally less than 5%, and most preferably less that about 1.0%. In fact, good results have been obtained using a duty cycle of about 0.85%. 
     The preferred embodiment artificial larynx further comprises a tongue activatable intraoral switching device. This switching device comprises an on-off circuit for activating or deactivating the device. This selective enabling and disabling function is accomplished by providing a timing means for timing the closure of a switch which, in the preferred embodiment, is accomplished by completion of a ground path through contacts 116. The desired grounding may be accomplished by bridging between adjacent contacts with the tongue, or by using the body as the system&#39;s ground whereby touching a single contact will accomplish a grounding of the system. In this embodiment, counter 204 counts in response to the output (φ) of the signal generation means when the on-off contacts 116 are grounded, but is inhibited (through R11 in FIG. 3) from counting when the on-off contacts 116 are not grounded. In order to turn on the artificial larynx, the tongue is held against &#34;on-off&#34; contacts 116 until counter 204 has been permited to count for a preselected period of time, after which an output pulse is provided to differentiator and inverter 206. The output pulse of counter 204 is thus differentiated and inverted in 206 in order to provide a distinct output to bistable multivibrator (flip-flop) device 208, which is caused to assume its &#34;on&#34; position. When in this position, flip-flop 208 provides a high level signal to &#34;and&#34; gate 210. If anterior &#34;pause&#34; contacts 114 are not grounded, and thus also provide a high level signal to &#34;and&#34; gate 210, then the differentiated output of signal generator 200 will be permitted to pass to high gain amplifier 212 and to speaker 112. When the user hears the signal from speaker 112, the tongue may be removed from on-off contact 116, and the flip-flop 208 will remain in its &#34;on&#34; position. 
     As mentioned above, pause contacts 114 are utilized to momentarily deactivate amplifier 212 and speaker 112. Under normal conditions, pause contacts 114 are open, and thus the signal to &#34;and&#34; gate 210 is high (through R12 in FIG. 3), permitting, in combination with a high signal from flip-flop 208, the passage of the differentiated output signal 202 to amplifier 212 and speaker 112. When the tongue is used to bridge pause contacts 114, the output drops to zero, and &#34;and&#34; gate 210 prevents the transfer of any signal to the amplifier and speaker. 
     It is anticipated that the switching device of the present invention may be utilized by a proficient user to closely approximate natural speech. The pause contact 114 may be utilized to stop any sound from being generated between sentences, or even between words. 
     In FIG. 3, a circuit diagram corresponding to the block diagram set forth in FIG. 2 is provided, each of the blocks of FIG. 2 being illustrated within dotted outlines surrounding its respective circuit components. Preferred components for use in constructing the preferred embodiment artificial larynx are as follows: 
     
         ______________________________________Compo-nent   Vendor/Part #     Description______________________________________R1     Mini-Systems MSR-2                    Resistor 2.2 Meg.R2     Mini-Systems MSR-2                    Resistor 270KR3     Mini-Systems MSR-2                    Resistor 18KR4     Mini-Systems MSR-2                    Resistor 1 Meg.R5     Mini-Systems MSR-2                    Resistor 1 Meg.R6     Mini-Systems MSR-2                    Resistor 100KR7     Mini-Systems MSR-2                    Resistor 1 Meg.R8     Mini-Systems MSR-2                    Resistor 1 Meg.R9     Mini-Systems MSR-2                    Resistor 100KR10    Mini-Systems MSR-2                    Resistor 2.7KR11    Mini-Systems MSR-2                    Resistor 10KR12    Mini-Systems MSR-2                    Resistor 10KC1*    Johanson 500R15W182KP                    Capacitor 0.0018 ufdC2     Johanson 500R11Y103PP                    Capacitor 0.01 ufdC3     Johanson 500R11Y103PP                    Capacitor 0.01 ufdC4     Johanson 500R11Y103PP                    Capacitor 0.01 ufdC5     Johanson 500S48N105PP4                    Capacitor 1.0 ufdD1,D2  Amperex LDD15T    Dual DiodeD3     Amperex LDD5T     Logic DiodeQ1     Amperex LDA405T   NPN TransistorQ2     Amperex LDA405T   NPN TransistorQ3     Amperex LDA405T   NPN TransistorQ4     Amperex LDA453T   PNP TransistorSP1    Know1es BK-1610   SpeakerIC1    Amperex LFG4011   Quad 2 input NANDIC2    Amperex LFC4020   14 Stage Binary CounterS1     --                Silver ContactsS2     --                Silver ContactsB1     Sanyo CR1220      Lithium Battery 3vB2     Sanyo CR1220      Lithium Battery 3v______________________________________ *If desired, a 0.01 ufd capacitor may replace this component. 
    
     One of ordinary skill in this art will readily appreciate that while certain signal generator of frequencies have been selected for use in the disclosed artificial larynx, other frequencies and/or dual frequencies may be provided to accomodate individual preferences. Similarly, connections to pins Q7 and Q8 of binary counter IC2 provide the aforementioned two second and four second on-off times. Other combinations of connections Q1-Q14 may be used to speed, slow, or otherwise alter the relative on-off times of the preferred embodiment device. 
     As mentioned above, it is preferred to provide an acoustic amplification means for amplifying acoustic energy within the oral cavity. It is further desired to tune the acoustic amplification means to the sound source to maximize the amplification. Accordingly, tests have been conducted to demonstrate the effectiveness of a horn to amplify the output of a speaker of the type preferred for use in the artificial larynx of the present invention. In one set of tests, the output of a BK1610 speaker was fed into a horn. The output from the horn was compared to the output of a similar BK1610 speaker, sound measurements being taken using a Scott type 451 (A weighted) sound meter. At 80 Hz, the speaker with horn generated a 57 decibel tone, by comparison to a 51 decibel tone generated by the speaker without a horn. At 120 Hz a 60 decibel output was achieved with the horn and a 53 decibel output without the horn. At combined input frequencies of 80 and 120 Hz, a 62 decibel output was achieved with the horn, and a 54 decibel output achieved without the horn. These changes are quite significant. A 6 decibel change indicates twice the pressure level, while a 10 decibel change indicates that the sound is perceived as being twice as loud. 
     In order to further investigate the effect of different horn lengths, a study was undertaken to determine the optimal length using a 11/2 volt circuit powering a BK1610 speaker. This speaker was placed at varying distances from the diaphram of a sound level meter, to ascertain the optimal length for an acoustic horn. Ambient sound accounted for 40 decibels under &#34;no signal&#34; conditions. Lengths of between 3/4 of an inch and 91/4 inches were tested, with maximum values of 98 decibels being obtained for 13/4 and 2 inch lengths. This testing is set forth in Table I: 
     
                       TABLE I______________________________________DECIBELS vs. HORN LENGTH______________________________________   95    9.25   |         |   95    7.5   96    7.25   |         |   96    5.5   95    5.0   94    4.75   |         |   94    4.0   95    3.75   |         |   95    3.0   96    2.75   96    2.5   97    2.25   98    2.0   98    1.75    96+  1.5   95    1.25   95    1.0   94     .75______________________________________ 
    
     Although the results set forth in Table I were obtained using a straight, cylindrical horn, the above-described tests were repeated using an arcuate, elliptical horn, and comparable results were obtained. It should further be noted that the circuitry used to drive the speaker in the tests refered to above was early prototype circuitry which failed to comprise any duty cycle control means. 
     One of ordinary skill in the art will appreciate that the voltage of the preferred embodiment device may also be varied, and that with such variations, some variation in loudness will be obtained. Using the preferred embodiment circuitry of the present invention, it has been found that a decibel output of between about 91 and 11 decibels may be obtained using different values for resistor R1O in section 212. Table II, which is set forth below, provides information concerning current drain for both conditions of `device on` and `device off` and the decibel output for different values of R1O. 
     
                                           TABLE II__________________________________________________________________________CURRENT DRAIN VS. LOUDNESS FOR DIFFERENT VALUES OF R10    DEVICE CURRENT               DEVICE CURRENT    `ON STATE` `OFF STATE`R10 (KILOHMS)    (ma)       (ma)       LOUDNESS (db)__________________________________________________________________________(SUPPLY VOLTAGE = 6.0 VOLTS)1        4.05       0.15       110.51.5      2.93       | 1102.2      2.18       | 1093.3      1.59       | 1084.7      1.21       | 106.55.6      1.05       | 105.56.8      0.91       | 1047.5      0.84       | 103.510       0.68       | 101.515       0.51       | 98.5(SUPPLY VOLTAGE = 3.0 VOLTS)1        1.54       0.14       1031.5      1.12       | 101.51.8      0.98       | 1012.2      0.84       | 1002.7      0.72       | 99__________________________________________________________________________ 
    
     As seen from the above, depending upon the loudness and the current drain desired, different values of amplifier resistor R1O may be provided to power the preferred embodiment larynx of the present invention. 
     In alternate embodiments of the present invention modification may be made to the above-described circuitry. One such embodiment is represented by the dotted line in FIG. 2, which facilitates elimination of the &#34;differentiate and invert&#34; component 206. In this embodiment the outputs of counter 204 are each fed to an additional NAND gate, which NAND gates also receive as inputs the output of differentiator 202. These NAND gates in turn separately feed the inputs of flip-flop 208, thereby eliminating the discrete components of the differentiate and invert block 206. 
     In a further alternate embodiment a filter capacitor is provided to eliminate perceived hum by the user when the device is in the off or pause condition. This filter capacitor may simply by connected between the power source (+V) and ground, and acts to eliminate any detectable &#34;leak through&#34; of the signal to the speaker while the unit is in the off or pause condition. A 10 ufd capacitor has been found suitable for this purpose, and may be mounted for removal with a replaceable or rechargable power source. 
     In still another embodiment of the present invention, the current passing to the tongue may be reduced by increasing the resistor value of resistors R11 and R12 to 680K, which reduces the current between contacts to less than 2 u amps during operation of the contact switches. 
     Those of ordinary skill in this art will recognize that the oscillator period of the signal generator 200 may be varied depending upon the value of resistor R2. For example, periods of 17 msec, 8 msec, 10 msec and 8 msec may be obtained using R2 values of 680K, 270K, 390K and 330K respectively. Presently, oscillator periods of between 10-17, preferably about 14, milliseconds are preferred by most patients. 
     Those of ordinary skill in this art will further recognize that the amplifier circuit 212 may be modified to achieve desired outputs at acceptable current draws. Both common collector and common emitter amplifiers are believed to be operative in driving the speaker means, however a common collector, direct coupled audio circuit (FIG. 5) comprising an NPN transistor (Amperex LDA-404), a silicon flyback diode (Amperex LDD-5) across the speaker terminals and a 1.5 ufd capacitor across the battery connections, is currently preferred. The flyback diode provides a cleaner signal at a lower current drain, while the capacitor enables the battery to supply the required current pulse to the speaker. Without this capacitor, the volume is substantially reduced. 
     Using the above describe direct coupled audio circuit, a 14 millisecond oscillator period, and a Fluke DVM8022A in series with a battery plus lead to measure the overall current, the following results were obtained: 
     
         ______________________________________CURRENT(mamps)   BATTERY    OUTPUTOn    Off     (volts)    (db)     COMMENTS______________________________________0.330         5.6        105      ND-NS 0.067   5.7                 ND-NS0.290         5.6        105      D-NS0.290         5.6        102      D-S______________________________________ NS = no microporous horn cover ND = no flyback diode D = with flyback diode S = with microporous horn cover 
    
     In accordance with one preferred embodiment, the speaker means of the present invention should comprise two phased speakers, wired in parallel, connected to the output of the amplifier 212. The use of two transducers, such as BK-1610&#39;s, BK-8212&#39;s or BK-1911&#39;s, has been shown to result in increased audio outputs. The outputs of single vs. dual speakers using common emitter (Type B) and common collector (Type A) (FIG. 5) type amplifier circuits were compared. The results are summarized below: 
     
         ______________________________________                 TOTALTYPE OF   NUMBER OF   CURRENT     OUTPUTAMPLIFIER SPEAKERS*   (mamps)     (db)______________________________________A         1           234         98A         2           244         102B         1           213         95B         2           216         99______________________________________ *using type 8212 
    
     As seen from the above, the use of dual speakers may offset db losses which may result from using microporous barrier means over the horn orifice. 
     While discrete electronic components are described above, one of ordinary skill in the art will readily appreciate that most of the disclosed circuit components can be included in a single semiconductor chip, and that such a chip is preferred for construction of the preferred embodiment of the present invention. 
     As seen from the above, a highly efficient wholly self contained artificial larynx is described which overcomes many of the disadvantages of prior art devices.