Pest Control Apparatus And Method

Abstract

A pest control apparatus and method for dispersing pests, e.g., rodents, from a designated area by the transmission into the area of high intensity ultrasonic sound waves having a primary frequency varying as a function of time and frequency modulating the sound waves with a secondary vibrato. The apparatus additionally provides periodic bursts of such ultrasonic power affording transducer rest periods maximizing the power handling capability of the transducer during "on" periods.

Description

BACKGROUND OF THE INVENTION

The invention relates to pest dispersing techniques using an apparatus providing ultrasonic sound waves such as disclosed in U.S. Pat. No. 3,636,559.

It has been demonstrated that various insects and animals react to and are repulsed by ultrasonic sound waves, and hence the use of such waves has been known for pest control. Different frequencies and sound intensities have been found to produce high coefficients of annoyance for different pests. An important promise for this type of technique is the control of mice and rats in warehouses, food processing plants, and the like. Rat control in such areas has presented a most serious and difficult problem due to rapid propagation of a rat colony or influx of a new colony to replace rats removed by poisoning or trapping.

It has been found that rat control may be successfully accomplished by the transmission, into areas in which rat control is desired, of high intensity ultrasonic sound. The rats' auditory system is maximally sensitive to ultrasonic sounds between about 20,000 and 50,000 cycles per second. Such ultrasonic power will cause rats to flee from the source of the sound; and rats will learn to avoid the areas in which they have been subjected to such ultrasonic exposure. Accordingly, only periodic use is required of the ultrasonic transmission such as during the night or other non-operational periods; although due to the elevated frequencies involved, plant personnel may, without injury, be exposed to the ultrasonic waves.

It has also been found that more effective rat control may be obtained by varying the frequency of the transmitted sound waves. U.S. Pat. No. 3,636,559, above referred to, suggests sweeping the frequency between approximately 18.5 kHz and 30 kHz and additionally periodically changing the amplitude of sound intensity.

SUMMARY OF THE INVENTION

I have found that the coefficient of annoyance may be significantly increased by using high intensity ultrasonic sound waves which not only vary in frequency as a function of time, but are additionally frequency modulated with a secondary vibrato. And it is an object of the present invention to provide and use such sound waves for rat control.

Another object of the present invention is to provide improved means for obtaining periodic bursts of high intensity ultrasonic sound with intervening rest periods for the transducer, enabling it to handle and transmit substantially higher power than could be obtained under continuous operation.

A further object of the present invention is to provide electronic circuitry and components affording the improved performance and unique rodent dispersal method above referred to efficiently and economically, with apparatus produceable at modest cost, yet designed for dependable, long life, and trouble-free operation.

The invention possesses other objects and features of advantage, some of which of the foregoing will be set forth in the following description of the preferred form of the invention which is illustrated in the drawings accompanying and forming part of this specification. It is to be understood, however, that variations in the showing made by the said drawings and description may be adapted within the scope of the invention as set forth in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Referring to said drawings:

FIG. 1 is a block diagram of pest control apparatus constructed in accordance with the present invention.

FIG. 2 is a schematic wiring diagram of certain of the components of the apparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The pest control apparatus of the present invention comprises briefly a tone generator 1 providing an output frequency as a function of applied voltage and a transducer 12 connected to the output of generator 1; a sawtooth generator 6 and gating means 7 therefor providing an interrupted slope sawtooth ramp output, illustrated as wave form 15 on FIG. 1, connected to the input of tone generator 1, the interrupted slope sawtooth ramp 15 having alternate segments, one segment 15a having a substantially constant voltage and the other segment 15b having a descending voltage and providing alternate tone generator outputs of substantially constant frequency and descending frequency respectively; and gating means 2 connected to gating means 7 and to tone generator 1 for chopping the output of the tone generator in synchronism with the interrupted slope sawtooth ramp output 15 so as to selectively pass tone generator outputs of descending frequency. This chopping of the signal transmitted to transducer 12 provides an interruption of the signal transmitted to the transducer thereby affording it rest periods conforming to the period of wave segment 15a and enabling the transducer to handle substantially greater power during the period of wave form 15b than could be obtained under continuous operation of the system.

As a further feature of the present invention, a D.C. amplifier 5 is connected between generator 6 and voltage controlled tone generator 1 and is provided with offset and gain controls more fully hereinafter described for determining the center frequency and the frequency range of the output of the tone generator. In the adaptation of the apparatus to the dispersal of rats, a frequency range of about 20 kHz to about 50 kHz is used.

As a further feature of the present invention, it has been found that the coefficient of annoyance is significantly increased by the superimposing on the basic or primary ultrasonic sound waves of varying frequency a secondary vibrato, which is preferably a random vibrato. This effect is obtained by the combination of a noise generator 8, amplifier 9, band pass filter 10, and clipping amplifier 11. Preferably, generator 8 provides a random "white" noise source, which is amplified by amplifier 9. Band pass filter 10 restricts the signal to a frequency range to the region of prime effectiveness. Experimentation to date on rat dispersal would indicate a range of about 200 Hz to about 2,000 Hz. As an alternative, a low pass filter which passes all frequencies up to about 2,000 Hz may be used. The clipping amplifier 11 provides a predictable amplitude of random noise, resolves the random amplitude signal into rectangular waves of known amplitude and rise time, and further introduces harmonic distortion which enhances the coefficient of annoyance. Noise source 8, amplifier 9, band pass filter 10 and clipping amplifier 11 may be of conventional design. Accordingly, these circuits are not here detailed. Also, the arrangement shown in the block diagram may be changed, as for example, the positions of amplifier 9 and band pass filter 10 and clipping amplifier 11 may be re-arranged. While the components 8-11 making up the random vibrato generator are illustrated separately in the block diagram, certain of the parts, such as amplifiers and filters, may be integrated.

D.C. AMPLIFIER

D.C. amplifier 5 with its multiple inputs and gain and offset controls is illustrated in FIG. 2. This stage is designed around any one of the commonly available integrated circuit operational amplifiers 90 which are equipped with nominally symmetrical differential inputs, I- and I+, provide open-loop voltage gains of 25,000 or better, are directly coupled and are internally compensated for suppression of Bodie-Nyquist oscillations. Stable and linear operation is achieved by feeding back a portion of the output signal present at its output O to the inverting input I- via an impedance, in this case resistance 91. Thus, any internal or external perturbation which would tend to cause the output signal at O to go in a positive direction is partially cancelled by the voltage fed back to the inverting input I-. With the noninverting input I+ grounded, the input impedance at I- assumes a very low value which is essentially independent of the characteristics of the input portion of the amplifier.

Input signals are applied to I- through summing resistors 92 and 93. Summing resistor 92 is connected to the wiper arm 95 of gain control potentiometer 94 having one end connected to input lead 101 from the ramp generator stage 6, and its other end connected to ground. By proper selection of values, substantially all of the input signal appearing at wiper arm 95 is developed across summing resistor 92 with minimal signal developed between I- and ground. Also by proper selection of values, the amplifier will yield a fractional voltage gain, but serve its prime function of providing the required current needed to drive the load connected to the output of the amplifier, thus, providing important buffering action. Potentiometer 94 provides gain control for the stage and therefore the madnitude of voltage applied to the voltage controlled tone generator and, accordingly, the frequency sweep range at the output of the generator. The offset control, for determining the center frequency of the apparatus, is provided by potentiometer 96 having its wiper arm 97 connected to summing resistor 93. One end of potentiometer 96 is connected by lead 98 to the negative side of the power supply, here -12 volts, while the opposite side is connected by lead 99 and voltage dropping resistor 100 to the positive side of the power supply +12 volts.

The random vibrato signal from clipping amplifier 11 is fed by conductor 102 to one end of potentiometer 103 having its opposite end connected to ground. The wiper arm 104 of potentiometer 103 is connected to the non-inverting input terminal I+ for controlling the magnitude of the vibrato frequency modulation of the primary frequency. In those instances where the vibrato modulation is not to be used, input terminal I+ will be grounded as indicated by dotted line conductor 106.

RAMP GENERATOR

Details of construction of the ramp generator stage 6 is illustrated in FIG. 2. The interrupted slope is obtained by applying a rectangular wave form signal from gating multivibrator 7 to a discharge circuit comprising transistor 31 and timing resistors 32 and 33, resistor 33 here being in the form of a potentiometer with its wiper arm 35 connected to ground for adjusting the amount of resistance in the circuit. Transistor 31 is an FET and by applying the proper potential to it, the transistor will be thrown out of conduction allowing capacitor 34 to retain its charge as of the time that the disabling voltage is applied to the gate of transistor 31 from the input conductor 37 connected to the output of the gating multivibrator stage 7. When the gating potential is removed, transistor 31 resumes its function as a constant current discharge device causing a linear decrease in voltage with respect to time at its drain 36. This decrease continues until the reset circuitry composed of transistors 38 and 39 re-establish the starting potential on capacitor 34 at which time the cycle resumes.

When power (+12 volts and -12 volts) is applied to this circuit, transistor 38 conducts heavily causing a current to pass from one plate of capacitor 34 through transistor 38 and sampling resistor 40. This current flows only as long as capacitor 34 is being dynamically charged. During this charging period, a voltage drop appears across resistor 40, thus lowering the potential at the collector of transistor 38. A fraction of this voltage is delivered to the base of transistor 39 by resistors 41 and 42 which form a voltage dividing coupler. During this period, the voltage at the base is less positive than the voltage developed at the emitter of transistor 39 by resistors 43 and 44, thus causing transistor 39 to be cut-off. Thus, the base of transistor 38 is biased into conduction by the current flowing to the +12 v. source through resistor 45. As capacitor 34 approaches full charge, the charging current flowing through resistor 40 decreases, thus raising the potential of the collector of transistor 38 and, through transistors 41 and 42, the base of transistor 39. Soon, the voltage at the base of transistor 39 becomes larger than that produced at its emitter, causing it to conduct. When this happens, a sizeable current flows from ground, through resistors 43 and 45 and transistor 39 causing the potential at the collector of transistor 39 to drop. This lowered potential is coupled directly to the base of transistor 38 which then ceases conduction completely. With little current flowing through resistor 40, transistor 39 conducts even harder, thus latching the reset circuit into its normal state. It will remain in this state until the discharge circuitry composed of transistor 31, and resistors 32 and 33 cause capacitor 34 to discharge sufficiently so that the emitter of transistor 38 falls slightly below the potential maintained on the base of transistor 38 by resistor 45, transistor 39 and resistor 43.

When the gate of transistor 31 is grounded, a current will flow from ground through resistors 33 and 32 and transistor 31. Field effect transistor 31 causes a constant current to flow even though the voltage at its drain is continually varying with time. This causes the voltage appearing on capacitor 34 to change at a uniform rate with time. The specific discharge current and thus the specific rate of change is determined by the setting of timing rheostat 33, a high resistance providing a low current and thus a long time between resets. The setting of rheostat 33 thus determines ramp frequency.

The descending ramp wave form so generated is coupled to the gate of transistor 46 which acts as a buffering amplifier. It provides no voltage gain, but does yield current gain such that an ordinary amplifier may be coupled to the circuit without loading the sensitive wave form generating circuit.

The current amplified wave form generated at the source of transistor 46 is coupled to the summing resistor 47 which then feeds the inverting input of operational amplifier 49, which input is connected by resistor 51 and rheostat 52 to the negative side of the power supply -12 volts. Rheostat 52 provides wave form centering. Feedback resistor 48 in combination with amplifier 49 establishes a voltage gain of four along with a sizeable current gain. The phase reversal induced by amplifier 49 causes the ramp wave form to appear at terminal "A" as on ascending wave form. This amplified and inverted wave form is coupled back to the second plate of capacitor 34 so as to synthetically magnify the value of capacitor 34, thus reducing its size and cost. With amplifier 49 operating at a gain of four, capacitor 34 assumes a magnified value five times its true capacitive size.

The ramp 15 may be interrupted anywhere in its descent by the application of a negative potential to the gate of transistor 31. If a square wave possessing the proper D.C. levels is applied to the gate of transistor 31, the ramp will be broken into a series of flat and sloped segments 15a and 15b which may be termed an interrupted slope or a "waterfall" wave form as illustrated at 15 in FIG. 1. Preferably the segments 15a and 15b are of equal duration of approximately one-half second.

GATING MULTIVIBRATOR

The gating multivibrator stage 7 generates two symmetrical rectangular wave forms in opposite phase and displaced with respect to one another in their D.C. levels. During that period of time when the gating multivibrator causes the gated output buffer to be in the conducting mode, interface buffer transistor 66 is in the non-conducting state, allowing the end of resistor 83, shown in the gated output buffer 2, to float free. This allows resistor 75 to inject a negative current into the base of transistor 76 so that both it and the buffering transistor 77 are conducting and operable. During the same time, the ramp gate wave form 53 is applied to the gate of transistor 31 grounding it and causing it to conduct. When the gating multivibrator changes state, the potential at the base of transistor 76 rises to about 0.8 volts thus cutting it off and cutting off transistor 77, thus causing transistor 77 to cease functioning as an amplifier. During this same period, the gating multivibrator 7 feeds a potential of about -8.2 volts to the gate of transistor 31, thus cutting it off and causing the ramp to stop its descent and maintain a fixed voltage until the gating multivibrator again changes state re-keying transistor 31. The out-of-phase wave forms are generated to be compatible with the transistor types employed in the ramp generator and the gated output buffer.

In this circuit, transistors 58 and 59 are connected in the classic collector-coupled configuration to form a free-running multivibrator. Capacitors 60 and 61 along with resistors 62 and 63 form the frequency determining network. Since the time constant established by capacitor 60 and resistor 62 is identical with that established by capacitor 61 and resistor 63, the resultant wave form will be a symetrical square wave. Load resistors 64 and 65 develop the voltages required for the sustaining of oscillation. To prevent loading of the circuit, transistors 66 and 67 are included as buffer amplifiers. For example, when transistor 58 is in the conducting state, a current flows from ground through transistor 58 and resistor 64 whereupon it divides at the junction of the base of transistor 66 and return resistor 68, most of the current flowing through the base of transistor 66 to the emitter of transistor 66 and then back to the power supply via the +12 v. distribution line. This aforementioned current flow between the base and emitter of transistor 66 causes the collectoremitter circuit of transistor 66 to conduct heavily, thus causing the collector of transistor 66 and whatever load that may be connected to it to assume a potential of +12 volts. When the multivibrator changes state, transistor 58 stops conducting, thus causing the cessation of current flow through resistor 64, transistor 66 and resistor 68. At the instant of transition, resistor 68 sweeps away the accumulated charges from the base of transistor 66 returning them to the power supply. This insures that transistor 66 will quickly and positively become non-conducting when the aforementioned change of state occurs. In this state, the collector of transistor 66 assumes whatever potential is dictated by the circuitry connected to it. Transistor 67 behaves in a similar fashion except that since it is associated with the current flowing through transistor 59 and resistor 65, its conduction cycle is reversed in phase with respect to transistor 66, i.e., when transistor 67 is "on" transistor 66 is "off" and vice-versa.

The wave form generated at the collector of transistor 67 is similar to that found at the collector of transistor 66, but the requirements of the driven load associated with transistor 67 are not compatible with the D.C. levels so generated. To solve this problem, the wave form developed at the collector of transistor 67 is processed by a level-shifting network composed of resistors 82, 71 and 72 along with diodes 73 and 74. The output of this network delivers a wave form that is essentially zero-negative in character rather than the positive-zero wave form as seen at the collector of transistor 66.

VOLTAGE CONTROLLED TONE GENERATOR

This circuit consists of a pair of resistance coupled, common emitter amplifiers connected back upon one another in cascade. The collector load resistors 116 and 117 are returned to ground while the emitters of transistors 118 and 119 are powered from a negative supply. Base resistors 120 and 121 provide the forward starting bias for transistor 118 and transistor 119 and, in combination with coupling capacitors 112 and 113 determine the operating frequency by influencing the discharge rate of these capacitors.

Assume initially that the mixing and control amplifier 90 delivers a steady potential of +10 volts with respect to ground to the common junction of resistors 120 and 121. Thus, when the -12 volt supply is applied to the emitter of transistors 118 and 119 approximately equal current will flow through resistors 116 and 117; the bases of transistors 118 and 119 being forward biased by the combined sum of the -12 supply voltage and the +10 control voltage via resistors 120 and 121. Because the transistors are not matched, a slightly higher current will flow in one with respect to the other. Assume in this instance that transistor 118 conducts more heavily than transistor 119. This imbalance causes a larger voltage to appear across resistor 116 than 117. This slightly greater starting transient is coupled via capacitor 122 to the base of transistor 119 tending to reduce its conduction more effectively than the similar transient induced into the base of transistor 118 by capacitor 123. The transient coupled by capacitor 122 tends to cancel the bias furnished by resistor 120 so that the conduction of transistor 119 is reduced. This reduces the voltage drop across resistor 117 which reduction is coupled to the base of transistor 118 by capacitor 123 which in turn aids the bias provided by resistor 121 to the base of transistor 118. This causes transistor 118 to conduct more heavily, further increasing the voltage drop across resistor 116. This new perturbation adds to the original transient which appeared across resistor 116 further decreasing the conduction of transistor 119. This circulative regenerative action continues until transistor 119 is fully cut off and transistor 118 is saturated. The above series of events takes fewer than three microseconds to accomplish. When this first of the two stable states has been achieved, the induced negative charge on the upper plate of capacitor 122, which maintains transistor 119 in the cut-off state, begins to discharge through resistor 120, the rate of discharge being determined by the capacitance of capacitor 122, the resistance of resistor 120 and the magnitude of the forward bias voltage provided by amplifier 90. After a specific period of time, as determined by the aforementioned parameters, the upper plate of capacitor 122 will have completely lost its negative charge (with respect to the -12 supply line) and acquired a slight positive charge. When this charge reaches a potential of +0.6 volts, transistor 119 will begin to conduct slightly. This conduction produces a small voltage drop across resistor 117 which is coupled via capacitor 123 to the base of transistor 118. This injected charge from capacitor 123 opposes the forward bias supplied by resistor 121 causing transistor 118 to become less conductive. The reduced conduction of transistor 118 reduces in turn the voltage across resistor 116 causing an even higher forward bias to appear at the base of transistor 119 through the capacitive induction of capacitor 122. This induced charge causes transistor 119 to become more thoroughly forward biased at which point the circulative regenerative action takes hold causing the circuit to quickly change states. Due to the conductive behavior of the emitter-base junction of transistor 119, capacitor 122 quickly accumulates a new normalized state of charge from the -12 supply line via resistor 116 and the base-emitter junction of transistor 119. In this second stable state, transistor 118 is cut off while transistor 119 is in heavy conduction; the cut-off bias being furnished by the accumulated charge present on the lower plate of capacitor 123. During this interval, resistor 121 is charging the lower plate of capacitor 123 in a positive direction which, after a prescribed period of time, will cause transistor 118 to start conducting slightly again. As soon as the renewed conduction of transistor 118 is reflected by a voltage drop across resistor 116, the circulative regenerative action again takes hold and the circuit returns to its first state, causing the charge on capacitor 123 to be re-normalized by the base-emitter conduction of transistor 118 and the conduction of resistor 117 and 124. As long as power is supplied to the circuit, it will continue to oscillate back and forth between these two stable states. Because resistors 120 and 121 have the same value along with capacitors 122 and 123, which are identical, the time period of the two stable states will be identical. This means that the current flowing through sampling resistor 124 will be broken up into rectangular pulses of equal on and off time. The signal appearing across resistor 124 is fed to the gated output buffer for further processing.

In the previous example, the junction of resistors 120 and 121 was held at a fixed potential of +10 volts with respect to ground. This condition caused capacitors 122 and 123 to discharge quickly each time a change of state occurred. The operating frequency under that condition was about 50,000 cycles per second. By lowering the potential applied to the junction of resistors 120 and 121, the discharge rate of capacitors 122 and 123 is reduced. This in turn causes the frequency to become lower. By bringing the potential applied to resistors 120 and 121 to a level of -5 volts, the frequency drops to about 14,000 cycles per second. Due to the basic nature of the circuit, a linear relationship exists between the control voltage applied to the junction of resistors 120 and 121 and the resultant frequency of the wave form appearing across capacitor 122. This permits the application of a ramp wave form whose voltage varies linearly with time, to be applied to the junction of resistors 120 and 121 via amplifier 90 which wave form will cause a linear variation in frequency with respect to time of the voltage controlled tone generator, the limits of change being determined by the setting of the gain and offset controls associated with control amplifier 90.

MODULATOR GATE

Modulator gate 2 functions as an off and on switch, thus allowing the signal generated in the voltage control generator 1 to pass intermittently through the modulator gate to a modulator amplifier 3, see FIG. 1. Modulator gate 2 provides some further amplification and buffering action to divorce the loading characteristics of modulating amplifier 3 from the voltage controlled tone generator 1. The current requirements of the modulating amplifier are higher than the capacity of the output section of the voltage controlled tone generator 1. Modulator gate 2 has an amplifying transistor 77 whose emitter circuit returns to ground through the collector of a gate control transistor 76. When transistor 76 is conducting, transistor 77 is also forced into conduction and therefore functions as a normal grounded-emitter amplifier. This combination furnishes the current and voltage requirements of modulating amplifier 3.

The gated output buffer performs two functions, namely: (1) it cleans up and amplifies the ultrasonic tone developed by the voltage controlled tone generator and (2) it provides a method of switching the output of the generator on and off.

In the "on" condition, no control signal is supplied, thus allowing all of the current flowing through resistor 75 to flow through the base-emitter junction of transistor 76. This forces transistor 76 into saturated conduction, bringing its collector and thus the emitter of transistor 77 to ground, allowing transistor 77 to function as a normal amplifier. Ultrasonic current pulses flowing through transistor 119 and load resistor 117 divide, a portion flowing through resistor 124 and the remainder through the base of transistor 77, causing transistor 77 to turn on and off at the ultrasonic rate determined by the voltage controlled generator. In the momentarily "on" condition, transistor 77 saturates causing its collector to assume a potential near ground while pulling a large current through load resistor 81. In the momentarily "off" condition, no current flows through transistor 119 or resistor 117 allowing resistor 124 to sweep away the accumulated charges on the base of transistor 77, thus cutting off the current through the collector of transistor 77 which in turn permits load resistor 81 to bring the collector of transistor 77 to a potential of -12 volts.

If a gating voltage of +12 volts is applied, resistors 83 and 75 will form a voltage divide which will apply a small but effective positive potential to the base of transistor 76 causing transistor 76 to become cut-off. This interrupts the current path for the emitter of transistor 77 which prevents current from flowing in either the base or collector circuits of transistor 77. Since, in this mode, transistor 77 is unable to respond to the ultrasonic wave form present at its base, its collector will assume a potential of -12 volts and remain at this potential until the gating signal is removed, thus allowing amplifier action to resume.

The low ohmic value of resistor 81 allows sizeable loads to be driven from output B since resistor 81 can deliver upwards of 40 milliampers to the load if necessary.

MODULATING AMPLIFIER, POWER OSCILLATOR AND TRANSDUCER

Modulating amplifier 3 is a high-level, direct-coupled operational amplifier designed to deliver nominally .+-.100 volts to the screens of tubes used in the power oscillator circuit 4. The latter here employs a pair of high sensitivity beam tetrodes operating in parallel. The power oscillator is capable of producing from 70 to 100 watts of radio frequency energy at a voltage of approximately 20,000 volts. This is achieved by the use of a shunt-fed tesla transformer. Oscillations are sustained by sampling a small fraction of the high potential so generated and introducing this sample upon the control grids of the oscillator tubes. Modulation is achieved by application of appropriate potentials to the screen grids of the oscillator tubes. Standard Colpitts oscillator design is appropriate for generating the radio frequency energy required to excite the ionization tip within the Klein cell here used as a transducer. The structure and functioning of this type of transducer is more fully disclosed in Klein U.S. Pat. No. 2,768,246. The present system using the Klein transducer will faithfully deliver a wide spectrum of ultrasonic sound which will, for present purposes, normally vary from about 20 to 50 kHz and at very high sound intensities in the order of 140 db. which is capable of rodent dispersal in areas of several thousand square feet.

Claims

I claim:

1. In a pest control apparatus having a tone generator providing an output frequency as a function of applied voltage and a transducer connected to the output of said generator, the improvement comprising:

generator means providing an interrupted slope sawtooth output connected to the input of said tone generator, the periodically repeating sawtooth ramps each comprising contiguous segments alternately of constant and sloping amplitudes and providing successive tone generator outputs of substantially constant and of continuously changing frequency respectively; and

gating means connected to the output of said tone generator and to said generator means and chopping said tone generator output in synchronism with said segments and selectively passing only tone generator outputs of changing frequency and providing interspersed tone generator output rest periods.

2. An apparatus as defined in claim 1, and a D.C. amplifier having input and output terminals connected to the output of said sawtooth generator means and the input of said tone generator respectively, said amplifier having offset and gain controls determining the center frequency and the frequency range respectively of the output of said tone generator.

3. An apparatus as defined in claim 2, said segments being of substantially equal duration.

4. An apparatus as defined in claim 2, said amplifier having a second input terminal and functioning to mix the signal therefrom with the signal impressed on said first named input terminal; and

a random noise generator connected to said second input terminal and providing a random vibrato signal to said transducer.

5. An apparatus as defined in claim 4, said last named generator comprising a random white noise source; and

a filter connected between said noise generator and said second input terminal for optimizing the coefficient of annoyance.

6. An apparatus as defined in claim 4, said filter passing signals having frequencies up to about 200 Hz.

7. An apparatus as defined in claim 6, and signal amplitude clipping means connected between said random noise generator and said second input terminal.