US3569923A

US3569923A - Adaptive acoustic detector apparatus

Info

Publication number: US3569923A
Application number: US679255A
Authority: US
Inventors: Henry Naubereit; David L Baddorf
Original assignee: US Department of Navy
Current assignee: US Department of Navy
Priority date: 1967-10-30
Filing date: 1967-10-30
Publication date: 1971-03-09
Anticipated expiration: 1988-03-09

Abstract

An adaptive acoustic detector provides a signal for switching on the power supply of approximately positioned transmitter upon the detection of desired information which may comprise either a continuous or an impulse acoustic signal, irrespective of the attendant background noise. A composite signal composed of both the desired information and background noise is separated by a background insensitive amplifier which selectively feeds back only the background information thereby allowing the unattenuated passage of the desired information. An appropriate detector, upon the receipt of the desired information, permits the passage of a signal indicative thereof to provide the above mentioned switching signal. The invention described herein 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.

Description

United States Patent [72] inventors Henry Naubereit Browns Mills, NJ.; David L. Baddorf, Hatboro, Pa. [21] Appl. No. 679,255 [22] Filed Oct. 30, 1967 [45] Patented Mar. 9, 1971 [73] Assignee the United States of America as represented by the Secretary of Navy [54] ADAPTIVE ACOUSTIC DETECTOR APPARATUS 11 Claims, 3 Drawing Figs.

[52] U.S. Cl. 340/16, 340/258, 340/261 [51] Int. Cl G081: 13/00 [50] Field ofSearch 340/3, 5,6, 15, 16, 16 (R), 16 (M), 258, 258 (D), 261

[56] References Cited UNITED STATES PATENTS 3,056,928 10/1962 Marks 340/16-R 3,111,657 11/1963 Bagno 340/258 3,147,467 9/1964 Laakmann 340/261 3,202,968 8/ 1965 Eady, Jr. et al. 340/5X 3,218,556 1l/1965 Chisholm 343/17.1X

3 ,238 ,502 3/1966 Babcock 340/1 6-R 3,240,930 3/1966 Porter et al. 340/l6-R 3,320,576 5/1967 Dixon et a1. 340/5 3,341,810 9/1967 Wallen 340/16 Primary Examiner-Richard A. Farley Attorneys-G. J. Rubens and Henry Hansen ABSTRACT: An adaptive acoustic detector provides a signal for switching on the power supply of approximately positioned transmitter upon the detection of desired information which may comprise either a continuous or an impulse acoustic signal, irrespective of the attendant background noise. A composite signal composed of both the desired information and background noise is separated by a background insensitive amplifier which selectively feeds back only the background in- 10 $52 r 'j I 43 F33 T0 42 DELAY CO/Vf/NUOUS XMITTER RESPONSIVE i I MODULATOR I DETECTOR L NETWORK I 22 l 3 l 31 Low 55 BACKGROUND 7 FILTER INSENslT/VE 13 BIASING DIFFERENCE BIASHNG I AMPLIFIER NETWORK AME uzrwonx 2, r J EMITOTEERR sou. w 1 is I so 51 14 I 7 7 DELAY l me I coulz RESPONSIVE 1 AMP. NETWORK 17 1 l T Low PASS l I J AGC. FILTER 9 IMPULSE OR I mpur Lever- DETECTOR CONTROLLER AMP. J l 277 l XMITTER ADAPTIVE ACOUSTEC DETECTOR APPARATUS BACKGROUND OF THE INVENTION The invention relates to a detecting apparatus for receiving acoustic signals and more particularly to a network of amplifiers, filters, and detectors which selectively extract desired information from a wide band of frequencies having associated therewith random or other background noise. These selective extractions are in effect target detections which may comprise, for example, vehicle movement and/or button bomb detonation which may be caused by enemy troop movement. Vehicle movement produces a continuous audible signal while bomb detonation produces an audible impulse. Both of these phenomena are detected by the apparatus. The attendant noise may be attributed to animal life, rustling of leaves or other foliage, or other random phenomena occurring in a jungle environment.

The present invention is contemplated for use in a jungle environment as an electronic spy to listen for and activate means for transmitting vehicle and/or troop movements. The invention may be secreted behind enemy lines and, once so secreted, may be unrecoverable. Consequently, it is paramount that transmission should be kept at a minimum to both conserve power and avoid detection by the enemy. It is also paramount that information, when transmitted, may reasonably be assumed to contain the desired vehicle and troop movement components rather than random noise alone. This assumption was not reasonable and in fact not possible in the prior art as acoustic-sensing apparatus were heretofore adjusted to indicate detection only when the ambient sound level exceeded a preselected threshold. Due to the wide variations in background conditions, however, no practical fixed setting was plausible. Too high a threshold resulted in the loss of targets while too low a threshold rendered the detector overly sensitive to random noise. As loss of a target could not be tolerated, prior art devices were biased at low threshold levels thereby to insure the receipt of all target information. As a result, many of the targets detected by these acoustic sensors were false. Much undesired information was obtained and an unacceptable number of false alarms occurred.

Moreover, the power supplies associated with the respective transmitters, being almost always on, were rapidly depleted thus rendering the acoustic-sensing apparatus useless.

it is thus apparent that an acoustic detector which rejects unwanted noise information, enhances the receipt of desired signal information, and provides a signal in response thereto for switching on a transmitter with a high degree of probability that desired information will be transmitted, is highly advantageous.

SUMMARY OF THE INVENTION Accordingly, it is the general purpose of this invention to provide an acoustic detector adapted for use in a jungle environment and capable of detecting vehicle movement and/or button bomb detonation with a minimum of false indications from environmental conditions.

in brief, an acoustic sensor receives a composite signal composed of both desired information and background noise. This composite signal is fed into a background insensitive amplifier which selectively feeds back only the background information, thereby allowing the unattenuated passage of the desired information. It is noted that this desired information may comprise either a continuous signal, as from vehicle movement, or an impulse signal, as from a button bomb detonation. If the former occurs, an appropriate detector permits the passage of a signal indicative thereof after a predetermined time delay to actuate the power supply of a transmitter. Similarly, if an impulse is received, it also is detected and a signal indicative thereof is passed, the power supply again being activated after a predetermined time delay. Suitable feedback is provided between the two detectbrs to insure that the continuous detector will operate only upon the receipt of a continuous signal LII and the impulse detector will operate only upon the receipt thereto of an acoustic impulse. Further controls are provided within both the continuous detector and the impulse detector to insure that the signals received are in fact indicative of either vehicle movement or impulse phenomena and not merely noise transients or the like. The time delay of the continuous detector is sufficiently long so as to insure that the device will not trigger on transient phenomena which it may receive. Similarly, the time delay in the impulse detector affords further protection from unwanted signals by assuring that an impulse of sufficient duration is present before an output pulse is generated.

It is noted that the word impulse" as used herein connotes a signal of relatively short duration and is not meant to comprise the classical electrical definition, namely a signal of infinite magnitude occurring in zero time.

BRIEF DESCRIPTION OF THE DRAWING F IG. 1 is a diagram of the apparatus showing the various components thereof in block diagram form;

FIG. 2 is an electrical representation of a portion of the continuous detector shown in FIG. 1; and

FIG. 3 is a graph of the general output of the background insensitive amplifier shown in F IG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawing, and more particularly to FIG. 1, there is shown an adaptive acoustic detector apparatus 10 having at its input an acoustic sensor 11, which may be a microphone. The acoustic sensor 111 is connected to a background insensitive amplifier shown generally at 12. This amplifier has four outputs, one of which goes to a transmitter modulator. Two outputs, l3 and 14, respectively, serve as input signals to a continuous detector 15 and an impulse detector 16. The output shown at 27 also serves as in input signal to continuous detector 15. Each of the: aforementioned detectors provides, respectively, signals l7 and 18 which serve as input signals to an OR gate or other suitable logic device 19. The output of the system 20 is taken at the output of this lastmcntioned logic device 19.

As mentioned heretofore, a signal is received by acoustic sensor 11. This signal may comprise desired and undesired information over a wide range of frequency distributions. The sensor 11 is preselected, however, to have a flat frequency response from 40 Hz. to 2500 Hz. Thus, any signal information occurring below or above this frequency band is severely attenuated within the sensor ll itself. The output signal of the sensor 11 is, however, still of the broadband type in that it is comprised of signal information over a wide frequency range, namely from 40 Hz. to 2500 Hz. The broadband signal thus derived is amplified by any suitable operational amplifier 2l. The output signal from amplifier 21 is. fed both to a low-pass filter 22 and to a transmitter modulator (not shown). The modulator will not operate upon the receipt of this signal as the transmitter power switch, associated therewith, is initially in the OFF position and will turn ON" only upon the receipt of a signal derived from the output 20 of the acoustic adaptive detector 10, in a manner to be hereinafter described.

The low-pass filter 22 passes all low frequencies up to and including 200 Hz. with minimum attenuation. However, this filter is 35 db down at 300 Hz. Thus, severe attenuation occurs above 200 Hz. The purpose of filter .22 is to reject the unwanted and troublesome signals occurring above 200 Hz. and thereby to restrict the information passed to amplifier 23. Two hundred Hz. was chosen as the cutoff frequency for filter 22 as prior testing and evaluation has shown that the desired vehicle movement and button bomb impulse detonation frequencies occur below this level. Further, it is noted that the major false alarms dictated by the prior art occurred above 200 Hz.

Amplifier 23 is of any desired type and amplifies those signals below 200 Hz. only, the signals occurring above 200 Hz. having been filtered out by low'pass filter 22. The output from amplifier 23 is fed to both an emitter follower 24 and a low-pass automatic gain control (AGC) filter 25. The AGC filter 25 output is fed to an AGC amplifier 26 and through line 27 is fed to one side of continuous detector 15. The AGC filter 25 and amplifier 26 provide feedback to the inputs of both amplifier 21 and amplifier 23. The purpose of this AGC feedback loop is to insure a constant amplitude signal output at point 28. This feedback, in effect, adjusts the gain of amplifier 23 with respect to the background or undesired information only, thereby preventing the desired information from affecting the system gain.

This result may be more readily understood by referring to FIG. 3. This figure is a graphical representation of the signal at the output of amplifier 23. The amplitude thereof is plotted on the ordinate axis, while the frequency is plotted on the abscissa. Referring now to this graph it is seen that vehicle movement and/or button bomb impulse detonation generally occur in the range between 80 Hz. and 200 Hz. Background noise, however, though present at all frequencies, is most predominate in the range from 40 Hz. to 80 Hz. Therefore,

. AGC filter 25 is designed to pass frequencies below 80 Hz. but

to reject those signals between range 80 Hz. to 200 Hz. Thus, that portion of the signaloccurring at the output of amplifier 23 which contains undesired noise is amplified by AGC amplifier 26 and fed back to the inputs of, respectively,

amplifiers

21 and 23 to reduce the background gain thereof without reducing the gain of the desired information.

Referring again to FIG. 1, it is seen that the output signal at point 28 from amplifier 23 is fed both to a continuous detector and an impulse detector 16 through an emitter follower 24, or the like, having respective outputs l3 and 14. The operation of these detectors will now be described.

If a continuous signal, indicative of vehicle movement, appears at the output 13 of emitter follower 24, it will be amplified by suitable amplification means 29 within continuous detector 15 and passed to biasing network 30. The frequency content of this signal may vary anywhere from 40 to 200 Hz. That is, the frequency content of this signal is the same as that appearing at the output 28 of amplifier 23. If vehicle movement is not present, the frequency content of the signal appearing at the input of biasing network 30 will vary only from 40 to 80 Hz. However, if vehicle movement is present, frequencies up to 200 Hz. will be generated.

As noted heretofore, one side of continuous detector 15 is fed by an output signal from AGC filter 25 through line 27. This signal feeds biasing network 31. It is noted, however, that the frequency content of this signal may vary only between 40 and 80 Hz. since the AGC filter 25 will not pass signals above this range. If vehicle movement is not present, biasing

networks

30 and 31 are balanced and the difference amplifier 32 is quiescent. However, if a continuous signal from vehicle movement is present, it will upset this normally balanced condition and the resultant imbalance between the two input signals will cause difference amplifier 32 to provide an output to both delay responsive network 33 through line 42 and, through line 34, to the input of an input level controller 35, in impulse detector 16. Line 34 is in fact a feedback line between the two detectors. The purpose of this feedback will be later described.

Referring to F IG. 2, the operation of the continuous detector will be described in more detail.

Biasing networks

30 and 31 is fact comprise a combination of resistors, capacitors, and diodes suitably connected to provide an effect known as voltage doubling. If a continuous signal from vehicle movement is not present, then a balanced condition appears at the two inputs 40 of difference amplifier 32. The difference amplifier 32 is shown, in part, as two transistors 52, each receiving an input signal at their respective bases 40 from biasing

networks

30 and 31. it is noted that the signal appearing at the output of amplifier 29 is in phase with the signal from the AGC filter 25 output signal. When these signals go positive" capacitors 36 charge up to a predetermined level. When the signals go negative capacitors 36 generate a current and discharge through diodes 37 and charge capacitors 38. Diodes 53 prevent capacitors 36 from discharging to ground. The capacities of capacitors 38 are greater than those of capacitors 36. Therefore, capacitors 38 are not fully charged by the first discharges of capacitors 36. Rather, they continue to charge until further positive going signals are applied thereto in the manner above described. At this point, when the signals again go negative, capacitors 38 discharge through respective resistors 39, thereby applying equal bias to each of the bases 40 of the two transistors 52. As long as this condition persists, ON/OFF switch 41, which may itself be a transistor, a plurality thereof, or other active element or elements, remains in the OFF" condition and no signal appears at the output of difference amplifier 32.

If, on the other hand, a signal appears which is between Hz. and 200 Hz., then the balance created within the biasing

networks

30 and 31 is no longer present. For, while the noise from 40 Hz. to 80 Hz. is also present with the greater than 80 Hz. signal, and is balanced out by the

respective biasing networks

30 and 31, there is nothing to balance out the greater than 80 Hz. signal. Therefore, the transistors will no longer be in balance and ON/OFF switch 41 will be turned ON" thereby causing a signal to appear at the output of difference amplifier 32.

This output signal, fed by line 42 to delay responsive network 33 and by line 34 to the input of input level controller 35, will in general be indicative of vehicle movement in the field. However, and though unlikely, it is possible that a random transient or other noise may have produced such an imbalance in difference amplifier 32. To protect against this possibility, the delay responsive network 33 is provided.

Prior testing has shown that a random transient, though it may occur between 80 Hz. and 200 Hz., will exist for a much shorter duration than the signal produced by a continuously moving vehicle. Thus, the delay responsive network 33 is provided to insure that a signal appearing at the output of dif-. ference amplifier 32 must last for at least 4 seconds before a signal is passed by the delay responsive network 33 to the OR gate 19.

The signal from difi'erence amplifier 32 enters the delay responsive network 33 via line 42 at point 43. The signal then passes through resistor 44 and begins to charge capacitor 45. This RC network is the heart of the time delay portion of delay responsive network 33. Element 46 is a four-layer siliconecontrolled switch which must receive a certain prescribed minimum voltage or trigger level before it will conduct. Capacitor 45 continues to charge through resistor 44 as long as the signal is applied at point 43. If the signal is due to transient or other phenomena, it will decay long before the capacitor 45 has charged to the point where controlled device 46 breaks down and conducts. However, if a continuous signal from vehicle movement is present, capacitor 45 will continue to charge and, at the trigger level of element 46, will discharge therethrough through resistor 47.

Diode 48 and capacitor 49 serve as a pulse stretcher to operate on the resultant pulse discharge from capacitor 45 such that it will be of sufficient duration to enable OR gate 19. It is noted that resistor 47 is chosen to have an ohmic value very much less than that of resistance 44 such that the capacitor 45 will discharge through the resistance 47 at a much more rapid rate than it charged through resistor 44. This will produce a heavy burst of current sufficiently capable of enabling OR gate 19 and thereby turning on the transmitter power supply switch.

Referring again to FIG. 1, the impulse detector will now be described in detail. If an impulse signal appears at point 28 it will be transmitted by the emitter follower 24 and line 14 to the input of controlled amplifier 50 where it will be amplified and fed to both delay responsive network 51 and input level controller 35. The input level controller 35 is, in effect, an amplifier connected in the negative feedback mode with the controlled amplifier 50. Controller 35 operates to reduce the output level of controlled amplifier 50 at the rate of 50 db per second or 2.5 db in 50 milliseconds. Moreover, the delay sponsive network 511 will emit a signal indicative of button bomb detonation only if a signal having a magnitude of at least 3 db persists for at least 50 milliseconds. it can be seen therefore, that an impulse of at least 5.5 db, which does not decay below 3 db in 50 milliseconds, must be present at the input of controlled amplifier 5% before delay responsive network 51 emits a pulse. Tests have shown that background impulse noise occurring in the range from 40 Hz. to 200 l-iz., while amplified by controlled amplifier 5t), is reduced substantially by controller amplifier 35 such that the amplitude and duration thereof is insufficient to cause delay responsive network 51 to emit a pulse.

Delay responsive network 51 is identical in schematic structure to that of delay responsive network 33, the values of the components, however, being difierent. Network 51 is designed to provide a 50 millisecond delay before it emits a signal. As noted above, prior testing has shown that impulses due to background noise are far shorter than this time interval and are therefore not transmitted.

When a button bomb goes off, however, the amplitude thereof far exceeds that of the background level and hence controlled amplifier 50 is driven into a high gain condition, a condition which input level controller 35 is not capable of reducing. if this signal lasts for at least 50 milliseconds without dropping below 3 db, and a button bomb detonation will so last, delay responsive network 51 emits a signal representative of the button bomb impulse through line 18 to OR gate 19 and hence to the transmitter power supply switch, thereby turning the transmitter on.

it is noted that the transmitted information from the transmitter modulator is coded such that upon receipt thereof it can be determined whether the desired information is produced by continuous or impulse phenomena. it was therefore necessary to provide feedback line 3d between difference amplifier 32 and input level controller 35 which serves to provide a means for inhibiting the controlled amplifier 50 of impulse detector l6 upon the receipt of a continuous signal by continuous detector l5. Thus line 34 provides a feedback path for inhibiting the controlled amplifier 50 thereby, in effect, electrically disengaging the impulse detector 16 from the apparatus and preventing it from operating upon the continuous signal received by continuous detector 15. It is noted that the circuitry of continuous detector is not susceptible to impulse phenomena.

it is contemplated that many of the above-described elements may be fabricated as integrated circuits. For example, amplifier 2i might readily comprise a 10 terminal integrated circuit. Qne such representative type is SN-12l9 manufactured by Texas instruments. Also, while the above-described frequency cutoff points and periods of time delay have been enumerated with some degree of particularity, it is to be understood that these values may be varied as desired by one skilled in the art to provide optimum results. Moreover, it is contemplated that the invention may be used to listen for, and activate means for transmitting, sounds other than those produced by continuous vehicles or troop movements. For example, the invention could readily be utilized to listen for enemy aircraft or missiles or the like that approach a position below friendly radar.

Accordingly, the above-described arrangements are illustrative of the application of the principles of the invention only of a preferred embodiment for the practicing thereof. it will of course be recognized that numerous modifications and alterations may be made in the above-described apparatus without departing from the spirit or scope of the invention as set forth in the appended claims.

We claim:

ll. An apparatus for detecting vehicle movement or troop movement in a jungle environment, comprising:

acoustic sensing means for providing an electrical signal within a predetermined range upon the receipt thereto of sound signals;

a background insensitive amplifier connected with and responsive to said acoustic sensor means for selectively providing a first signal between a first set of frequency limits and a second signal between a second set of frequency limits;

first detector means connected to said amplifier and receiving said first signal and said second signal for providing a third signal indicative of said vehicle movement;

second detector means connected to said amplifier and receiving only said first signal for providing a fourth signal indicative of said troop movement; and

a logic gate connected to said first and said second detector means for providing an output signal upon receiving either said third signal or said fourth signal.

2. The invention as defined in claim I wherein:

said logic gate output signal activates the power supply switch of a proximately positioned transmitter thereby to effect transmission of the electrical signal provided by said acoustic sensing means.

3. The invention as defined in claim 1 wherein said background insensitive amplifier comprises:

first filter means operatively connected to said sensing means for passing a first preselected portion of said electrical signal thereby providing said first signal; and

second filter means operatively connected to receive said first signal for feeding back a predetermined portion thereof to said first filter means and for passing a second predetermined portion, said second portion being said second signal.

4. The invention as defined in claim 3 wherein said first detector means comprises:

first means responsive to said first signal passed by said first filter means;

second means responsive to said second signal passed by said second filter means;

difference amplifier means interposed between and responsive to said first means and said second means for providing a signal when said first means receives a signal different from that received by said second means; and

delay means responsive to the signal produced by said difference amplifier means for providing said third signal to said logic gate after a predetermined time delay.

5. The invention as defined in claim 3 wherein said second detector comprises:

first amplifier means responsive to said first signal passed by said first filter means;

second amplifier means connected to said first amplifier means for reducing the output level thereof at a predetermined rate; and

delay responsive means connected to said first amplifier means and said second amplifier means for providing said fourth signal to said logic gate upon the expiration of a predetermined time delay.

6. The invention as defined in claim 3 wherein said background insensitive amplifier further comprises:

first amplifying means connected between said sensing means and said first filter means for amplifying said electrical signal;

second amplifying means connected to the output of said first filter means for amplifying said first signal passed thereby;

automatic gain control means connected to the inputs of said first amplifying means and said second amplifying means for maintaining the gain of said second amplifying means at a constant level; and

an emitter follower connected to said second amplifier means for providing signals to said first and said second detector means.

7. The invention as defined in claim wherein said delay means comprises:

a first resistor;

a capacitor connected at one end thereof to said first resistor;

a second resistor; and

a silicone-controlled switch interposed between said connection and said second resistor whereby said capacitor stores electrical energy for a predetermined time as governed by said first resistor and discharges said energy through said second resistor after having charged to a predetermined trigger level of said silicone-controlled switch.

8. Acoustic detecting apparatus, comprising:

acoustic sensing means receiving an acoustic signal;

first filter means operatively connected to said sensing 10 means for passing a preselected portion of said signal;

second filter means operatively connected to receive the signal passed by said first filter means for feeding back thereto a predetennined portion thereof and for passing a second predetermined portion;

' first detector 'means connected to receive only said signal passed by said first filter means for providing a first output signal upon the detection of a first desired signal;

second detector means connected to receive said signal passed by said first and said second filter means for providing a second output signal upon the detection of a second desired signal; and

a logic gate connected to said first and said second detector means for providing a third output signal upon receiving either said first or said second output signal. 9. The invention as defined in claim 8 wherein said second detector means comprises:

first means responsive to said signal passed by said first filter means; second means responsive to said signal passed by said second filter means; difference amplifier means interposed between and responsive to first means and said second means for providing a signal when said first means receives a signal different from that received by said second means; and

delay means responsive to the signal produced by said difference amplifier means for providing said second output signal to said logic gate after a predetermined time delay.

10. The invention as defined in claim 8 wherein said first detector means comprises:

first amplifier means responsive to the signal passed by said first filter means;

delay responsive means connected to said first amplifier means and said second amplifier means for providing said first output signal to said logic gate upon the expiration of a predetermined time delay.

11. The invention as defined in claim 9 wherein said delay responsive means comprises:

a first resistor;

a capacitor connected at one end thereof to said first resistor;

a second resistor; and

Claims

1. An apparatus for detecting vehicle movement or troop movement in a jungle environment, comprising: acoustic sensing means for providing an electrical signal within a predetermined range upon the receipt thereto of sound signals; a background insensitive amplifier connected with and responsive to said acoustic sensor means for selectively providing a first signal between a first set of frequency limits and a second signal between a second set of frequency limits; first detector means connected to said amplifier and receiving said first signal and said second signal for providing a third signal indicative of said vehicle movement; second detector means connected to said amplifier and receiving only said first signal for providing a fourth signal indicative of said troop movement; and a logic gate connected to said first and said second detector means for providing an output signal upon receiving either said third signal or said fourth signal.

2. The invention as defined in claim 1 wherein: said logic gate output signal activates the power supply switch of a proximately positioned transmitter thereby to effect transmission of the electrical signal provided by said acoustic sensing means.

3. The invention as defined in claim 1 wherein said background insensitive amplifier comprises: first filter means operatively connected to said sensing means for passing a first preselected portion of said electrical signal thereby providing said first signal; and second filter means operatively connected to receive said first signal for feeding back a predetermined portion thereof to said first filter means and for passing a second predetermined portion, said second portion being said second signal.

4. The invention as defined in claim 3 wherein said first detector means comprises: first means responsive to said first signal passed by said first filter means; second means responsive to said second signal passed by said second filter means; difference amplifier means interposed between and responsive to said first means and said second means for providing a signal when said first means receives a signal different from that received by said second means; and delay means responsive to the signal produced by said difference amplifier means for providing said third signal to said logic gate after a predetermined time delay.

5. The invention as defined in claim 3 wherein said second detector comprises: first amplifier means responsive to said first signal passed by said first filter means; second amplifier means connected to said first amplifier means for reducing the output level thereof at a predetermined rate; and delay responsive means connected to said first amplifier means and said second amplifier means for providing said fourth signal to said logic gate upon the expiration of a predetermined time delay.

6. The invention as defined in claim 3 wherein said background insensitive amplifier further comprises: first amplifying means connected between said sensing means and said first filter means for amplifying said electrical signal; second amplifying means connected to the output of said first filter means for amplifying said first signal passed thereby; automatic gain control means connected to the inputs of said first amplifying means and said second amplifying means for maintaining the gain of said second amplifying means at a constant level; and an emitter follower connected to said second amplifier means for providing signals to said first and said second detector means.

7. The invention as defined in claim 4 wherein said delay means comprises: a first resistor; a capacitor connected at one end thereof to said first resistor; a second resistor; and a silicone-controlled switch interposed between said connection and said second resistor whereby said capacitor stores electrical energy for a predetermined time as governed by said first resistor and discharges said energy through said second resistor after having charged to a predetermined trigger level of said silicone-controlled switch.

8. Acoustic detecting apparatus, comprising: acoustic sensing means receiving an acoustic signal; first filter means operatively connected to said sensing means for passing a preselected portion of said signal; second filter means operatively connected to receive the signal passed by said first filter means for feeding back thereto a predetermined portion thereof and for passing a second predetermined portion; first detector means connected to receive only said signal passed by said first filter means for providing a first output signal upon the detection of a first desired signal; second detector means connected to receive said signal passed by said first and said second filter means for providing a second output signal upon the detection of a second desired signal; and a logic gate connected to said first and said second detector means for providing a third output signal upon receiving either said first or said second output signal.

9. The invention as defined in claim 8 wherein said second detector means comprises: first means responsive to said signaL passed by said first filter means; second means responsive to said signal passed by said second filter means; difference amplifier means interposed between and responsive to first means and said second means for providing a signal when said first means receives a signal different from that received by said second means; and delay means responsive to the signal produced by said difference amplifier means for providing said second output signal to said logic gate after a predetermined time delay.

10. The invention as defined in claim 8 wherein said first detector means comprises: first amplifier means responsive to the signal passed by said first filter means; second amplifier means connected to said first amplifier means for reducing the output level thereof at a predetermined rate; and delay responsive means connected to said first amplifier means and said second amplifier means for providing said first output signal to said logic gate upon the expiration of a predetermined time delay.

11. The invention as defined in claim 9 wherein said delay responsive means comprises: a first resistor; a capacitor connected at one end thereof to said first resistor; a second resistor; and a silicone-controlled switch interposed between said connection and said second resistor whereby said capacitor stores electrical energy for a predetermined time as governed by said first resistor and discharges said energy through said second resistor after having charged to a predetermined trigger level of said silicone-controlled switch.