US 3626365 A
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United States Patent 1 72] inventors Elliott H. Press 555 Bonnie Brae Ave., Rochester, N.Y. 14618; Hillel Pitlik, 18 Hollow Ridge Road, Needham, Mass. 02194  Appl. No. 882,024  Filed Dec. 4, 1969  Patented Dec. 7, I971  WARNING-DETECTING MEANS WITI-I DIRECTIONAL INDICATION 10 Claims, 10 Drawing Figs.
 U.S. Cl 340/34, 340/16 R, 179/1 VE [51 1 Int. Cl 601s 3/80  Field of Search 181/26; 340/16 R, 34; 179/1 V8  References Cited UNITED STATES PATENTS 2,962,697 1 1/1960 Rockwell 340/16 2,787,736 4/1957 Ellison etal. l8l/26X MICROPHONES 3 milifli 1 5/ DETECTORS 6/ T ME DEL I uun' were: 8/
Primary ExaminerWilliam C. Cooper A!!0rneysCharles Shepard and Stonebraker & Shepard ABSTRACT: A vehicle is equipped with a plurality of directionally responsive sound detectors, such as microphones, each oriented or baffled to receive sound from a different direction. Warning sounds originating in the vicinity, such as an ordinary automobile horn, or a warning siren of an emergency vehicle, are picked up by the directionally sensitive receivers and are translated through electronic circuits into light signals on a panel visible to the driver of the vehicle, so that by observing the lights on the panel the driver can tell from which direction the warning sound comes, even though he may not hear the sound either because he is hard of hearing or because the windows of the vehicle are closed or the external warning sound is drowned by a high level of noise within the vehicle. In a modification of the invention, a receiver on the vehicle may be responsive to inaudible energy, such as radio waves, emitted by mechanism on an emergency vehicle such as an ambulance or a fire truck. The electronic circuits will light the visible panel to indicate that emergency signals are present.
GATES LIGHT DRIVERS FLASHER PATENTED DEC 7 197i SHEET 1 of 2 31 25; 3 5
MICROPHONES ?3/ AMPLIFIERS 7 AND FILTERS 52 53 DETECTORS F62 63 Uafi T-s HI 2 ,3
STORAGE AND DRIVER UNITS 83 GATES g K g Ill 1/2 //3 //4 usm umvERs 22 /23 I24 LAMPS 13/ I34 I33 $04 J /43 FLASHER I Fig. I 7
PATENTEU DEC 7 I97! SHEET 2 BF 2 Fig. 6.
22's as Fig. 9.
WARNING-DETECTING MEANS WITH DIRECTIONAL INDICATION BACKGROUND OF THE INVENTION Under modern traffic conditions, with relatively high speeds of moving vehicles, it has become increasingly important for drivers of vehicles to be able to identify instantly the direction from which a warning sound comes. This applies not only to what one might call normal warning sounds, such as the horn of an overtaking vehicle or the horn of a vehicle on an intersecting course from one side or the other, but especially to what may be called emergency warning sounds; that is, the horns or sirens of emergency vehicles such as police vehicles, fire vehicles, ambulances, and utility company (gas and electric) emergency vehicles. It is important, of course, to be able to hear the warning sounds, but it is much more important to be able to identify quickly the direction from which the sound comes.
Various attempts have been made to ensure that the driver of a vehicle be alerted to the existence of the warning sound, notwithstanding that the driver may be hard of hearing or perhaps totally deaf, or notwithstanding that the sound might not be heard by a driver of normal hearing, either because the windows of the vehicle he is driving are closed, or because the warning is drowned out by sounds originating within the vehicle such as caused by a radio playing or by loud conversation, even if the windows are open, or drowned out by unusually loud traffic noise (truck exhaust, etc.) in the vicinity. Prior attempts to deal with this problem are represented, for example by U.S. Pat. No. 2,963,693, granted Dec. 6, [960; US. Pat. No. 3,014,199, granted Dec. I9, 1961; and U.S. Pat. No. 3,430,195, granted Feb. 25, I969. The arrangements disclosed in these patents, however, serve merely to alert the driver to the existence of a warning signal, without indicating the direction from which the signal comes.
A directional indication of an incoming warning sound is proposed in U.S. Pat. No. 3,158,835, granted Nov. 24, I964, but in this arrangement the directional indication is received by the driver entirely aurally, so that the arrangement is of no value or limited value to drivers having defective hearing, and its value to drivers of normal hearing may be greatly reduced by presence of a high level of noise within the vehicle, as a result of loud conversation or playing a radio, or as a result of unusually loud external trafiic noises.
An object of the present invention is the provision of an improved and more satisfactory warning-detecting means in which not only the existence but also the direction of a warning signal is indicated to the driver visually rather than exclusively orally, thus being of maximum value to the driver in spite of hearing defects and in spite ofloud noise in the vicinity.
Another object is the provision of means for accomplishing the object just mentioned, by relatively simple and inexpensive devices, which are sturdy in use and not likely to get out of order.
A further object is the provision of a directionally sensitive warning system which is responsive to special inaudible warning signals, such as radio signals, emitted by an emergency vehicle, in place of or in addition to conventional audible warning signals.
BRIEF DESCRIPTION OF THE DRAWINGS The arrangements here disclosed are intended as illustrative examples of the invention, rather than as limitations on the invention, and are subject to modification within the skill of the art. In the accompanying drawings, which are largely schematic or diagrammatic:
FIG. I is a top plan view of a fragment of the roof ofa vehicle equipped with the present invention, illustrating one manner of placing the sound-detecting units;
FIG. 2 is a plan of a fragment of a dashboard of an automobile equipped with the present invention, showing one form of visual indicator panel for indicating the direction from which a warning signal comes;
FIG. 3 is a wiring diagram of a preferred form of circuitry for activating the directional indications on the visual panel of FIG. 2 from the output received from the sensing devices of FIG. 1;
FIGS. 4-6 are views similar to FIG. 1, showing alternative arrangements of the directionally sensitive pickup devices on the vehicle;
FIGS. 7-9 are views similar to FIG. 2, showing alternative arrangements of the directional indicating panel observable by the driver of the vehicle; and
FIG. 10 is a schematic view of another form of the invention, comprising an emergency vehicle equipped with means for emitting a radio frequency warning signal in addition to the audible signal emitted by a siren or the like and another vehicle equipped with a receiver responsive to the emitted radio frequency warning of the emergency vehicle, either in addition to or as a substitute for the directionally sensitive receiver of audible warning signals.
DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1 there is indicated at 21 a portion of the roof of any vehicle to which the invention is to be applied. A plurality of directionally sensitive devices for picking up audible signals are mounted on the roof 21, this being the most convenient place to mount them, although it is within the scope of the invention to mount the signal pickup devices in any other suitable location on or in the vehicle where they will be exposed to the sound coming from the desired direction. Most conveniently, the pickup devices are in the form of microphones with hornlike baffles to limit the direction from which sound will be received.
In the embodiment of the invention shown in FIG. I, there are three such pickup devices, indicated respectively at 31, 32, and 33, and each pickup device is baffled so as to pick up sound mainly from a horizontal are or sector of approximately I20", as indicated schematically by the angular lines apart in connection with each pickup device. The first pickup device 31 is faced forwardly; that is, the horn or baffling structure is faced directly forwardly, along the centerline of the vehicle, so as to pick up mainly those sounds arriving through the sector or are of 120, 60, at each side of the longitudinal centerline of the vehicle. The other two pickup microphones 32 and 33 face generally rearwardly, with the horns or baffles so oriented that the device 32 picks up sound mainly in the sector 120 clockwise from straight behind, and the device 33 picks up sound mainly in the sector 120 counterclockwise from straight behind. Of course the three microphones can be clustered together near the center of the roof of the vehicle, if desired, or may be separated to any desired extent.
Through suitable electronic circuits such as those described below in connection with FIG. 3, the signals picked up by the directionally sensitive pickup devices 31, 32, and 33 cause the illumination of conspicuous visual signaling means within the easy range of view of the driver of the vehicle equipped with this invention. The visual indicator, as shown schematically in FIG. 2, is preferably in the form of a small panel 37 mounted near the steering wheel on the dashboard of the vehicle, a fragment of the dashboard being shown at 39. The panel 37 may lie flat on the dashboard, flush with an approximately horizontal surface thereof, but preferably it stands up at some convenient angle of 30 or 45 or more to the horizontal, to be conspicuous and more easily seen within the range of marginal vision of the driver of the vehicle, without distracting his main vision from close attention to surrounding traffic conditions.
One embodiment of the panel has various translucent portions selectively illuminated by small signal lamps beneath such translucent portions. The arrangement of such translucent portions is subject to considerable variation. In the form illustrated in FIG. 2, there are four translucent portions approximately at the four corners of the panel or unit 37, each of these signal portions or areas forming a right-angled corner somewhat like the letter L. They are respectively designated by the numerals 41 (forward left corner), 42 (forward right corner), 43 (rear right corner), and 44 (rear left corner. In this particular arrangement, when a warning sound coming from the front of the vehicle is detected, that is, when the pickup device 31 is actuated and the others are not actuated, both of the warning areas 41 and 42 will be illuminated, indicating to the observing driver that a warning signal is coming from in front of his vehicle. if only the detector 32 is activated, indicating sound coming from the left rear quarter, the indicator 44 alone will be lit. if only the detector 32 is activated, indicating sound coming from the left rear quarter, the indicator 44 alone will be lit. If only the detector 33 is actuated, indicating a warning sound coming from the right rear quarter, only the indicator 43 will be actuated. If both detectors 32 and 33 are simultaneously actuated, as a result of a warning sound coming from the rear, such as a horn or siren of an overtaking vehicle directly behind, then both signal areas 43 and 44 will be illuminated, indicating a warning from behind, just as the simultaneous illumination of indicating areas 41 and 42 indicates a warning from in front.
Various other combinations are possible and desirable. Thus for example simultaneous actuation of detectors 31 and 32 would indicate a warning from the front left quarter, and the warning indicator 41 alone would be illuminated. Similarly, simultaneous actuation of detectors 31 and 33 would indicate a warning from the front right quarter, and the indicator 42 alone would be activated. it must be realized that the directional responsiveness of the detectors 31, 32, and 33 is not absolute or perfect. The horns or baffles in connection with the microphones give a reasonable degree of directional selectivity, but a warning sound coming from the right front quarter, for example, would be picked up somewhat by the detectors 31 and 33, even though neither of these detectors is actually beamed to or intended primarily for the front right quarter. The arrangement of detectors as shown in FIG. 1 is preferably when it is desired to use only three detectors each bafiled for primarily a 120 input, because such an arrangement takes care of most of the incoming warning signals occurring in actual practice. But as above indicated, other arrangements are possible, and as mentioned below, it is possible to use four detectors each baffled for a 90 input.
According to the present invention, the sound receivers or signal receivers 31, 32, and 33 are selective not only with respect to the direction from which the signal comes, but also with respect to the loudness, the duration, and the frequency of the incoming signal. This is important because they should, so far as reasonably possible, be insensitive to normal ambient sound naturally occurring in the vicinity of traffic, but should be able reasonable well to pick up the signals which are likely to be ofa warning nature, such as the sound ofa motor vehicle horn, or of a siren such as used on emergency vehicles. This selectivity is accomplished as follows:
Referring to the upper part of FIG. 3, each of the sensing devices schematically shown at 31, 32, and 33, respectively, has its output connected first to a selective amplifier which is also a filter, indicated schematically at 51, 52, and 53, respectively. Each of these units 51, 52, and 53 will amplify detected signals in the desired frequency and will filter out signals of other frequencies, not desired. Since the warning signals which are to be detected are usually in the range of about 1,200 to 8,000 cycles per second, or have strong harmonics in this range, and since much of the extraneous traffic noise normally encountered on streets has a frequency below about 1,200 cycles per second, the units 51, 52, and 53 are constructed to pass and amplify signals in the band from about 1,200 to 8,000 c.p.s. and to filter out signals below about l,200 c.p.s.
The construction of such selective amplifiers is well understood by those skilled in the electronics field, and as the details are not important for purposes of the present invention, the details of such units are not further described.
The outputs of the respective units 51, 52, and 53 go preferable to audio detector circuit units indicated respectively at 61, 62, and 63 where the signal from the respective amplifier is detected and a suitable output signal is produced. The output signal from each of these units 61, 62, and 63 then goes to a respective time delay unit 71, 72, and 73, respectively, from which the signal is transmitted forwardly (that is, to an output) if the incoming signal has a duration of more than a preselected time. Usually this preselected time delay is about 0.5 second, but it may be more or less than this, and time delay may be eliminated entirely, if desired. It is assumed that, in general, if a sound lasts less than one-half of a second, it is not intended as a warning signal, and that any warning signal will have a duration greater than one-half second. Hence these units 71, 72, and 73 are used to block from the subsequent part of the circuit, any signal with a duration less than this, but to transmit signals having a greater duration.
Suitable audio detector units such as 61, etc., and time delay units, such as 71, etc., are well understood by those skilled in the electronics field. The detailed construction of such units is not important for purposes of the present invention, only the result being important, and this result, as above stated, is within the skill of the art once the desire for this result is known and appreciated. Hence the detailed construction of the units 61, etc., and 71, etc., is not further described.
Next beyond each of the respective time delay units 71, 72, and 73 is an output storage and river 81, 82, and 83, respectively, of a detailed construction known per se and unimportant for purposes of the present invention, which stores and produces an output of suitable form when it receives a suitable input from the preceding unit 71, etc. Each of these storage and output drivers has an output conductor which is branched or forked to provide a main branch 91, 92, and 93, respectively, and a secondary branch 95, 96, and 97, respectively, the latter branches going to respective inverting units 101, 102, and 103, respectively so arranged in conventional manner that with each of these units, when there is no input signal received, there will be an output signal, but when an input signal is received, there will be no output signal.
Gate units 11, 112, 113, and 114 are provided, one for each lamp which is to illuminate the various parts of the visible signal panel. In the panel construction illustrated in FIG. 2, there are four portions to be separately illuminated, so there are four signal lamps and four of these gates 111, etc. Each gate has the characteristic, well understood in the electronics field, particularly in the field oflogic" circuits, that when two inputs are received, there will be an output, but when only one input is received, or no input at all, there will be no output.
it will be seen from the wiring diagram, which will be well understood by those familiar with electronics and particularly with logic" circuits, that the main branch 91 coming (through the intermediate units) from the detector 31 goes to one input side of each of the two gates 111 and 112. The main branch 92 coming from the detector 32 goes to one input side of the gate 114, and the main branch 93 from the detector 33 goes to one input side of the gate 113. Likewise, it will be seen from the diagram that the second branch 95 from the detector 31 goes, through the reversing unit 101, to one input of each of the gates 113 and 114. The secondary branch 96 from the detector 32 goes, through the reverser 102, to one input side of the gate 112, and the secondary branch 97 from the detector 33 goes, through the reverser 103, to one input of the gate 111.
With this arrangement, those familiar with logic circuits will see that there will be an output from the gate 111 if the driver 81 is activated, either alone or in conjunction with the driver 82, provided the driver 83 is not activated. But if the driver 83 is activated, there will be no output from the gate 111 even though the driver 81 is also activated. Activation of the driver 81 alone, without any other driver being activated, will also result in an output from the gate 112, in addition to the output from the gate 111. Activation of the drivers 81 and 83 together will produce an output from the gate 112 but not from any other gate. Outputs from both drivers 82 and 83 together, with no output from the driver 81, will produce outputs from the gates 113 and 114, with no output from the other gates 111 and 112. The results of various other combinations of activating drivers will be readily apparent from a study of the wiring diagram in FIG. 3.
The outputs of these gates 111, etc., are delivered to light drivers 121, 122, 123, and 124, respectively, which can be in the form of relays completing a circuit to the battery of the vehicle, or of any other suitable known form. These drivers 121, etc., are in turn connected in series with the lamps 131, 132, 133, and 134, respectively, which serve to illuminate the indicator panels or translucent portions 41, 42, 43, and 44, respectively. The other side of each lamp is connected to one side of a suitable source of current such as the battery 136, the opposite side of which may be, as usual, grounded at 137 to the metallic frame of the vehicle.
At times it may be desired to have the lamps 131, etc., remain lit steadily so long as the incoming warning signal remains in effect, and at other times it may be desired to have the warning lamps flash, in order to attract more attention. For this reason, it is preferred to provide a conventional flasher unit 141, such as commonly used for flashing the right and left turn indicators of a motor vehicle, or any other known form of flasher device. A switch 142 is inserted in the circuit between the battery 136 and the flasher 141. When the switch is in one position, shown in solid lines in FIG. 3, the circuit goes through the flasher 141 and thence to the lamps 131, etc. When the switch 142 is in the other position, shown in dotted lines in FIG. 3, the circuit bypasses the flasher 141 and goes directly from the battery 136 to the lamps 131, etc., so that they will remain lit steadily (so long as there is the proper incoming signal) instead of flashing.
The storage and output drivers 81, 82, and 83 preferably have delay features so arranged that once an input signal has been received, the output signal will be continued for a minimum time sure to be observed by the driver of the vehicle equipped with this invention. A suitable minimum time is about 1.5 or 2.0 seconds. It has already been mentioned that the delay units 71, 72, and 73 will not pass a signal unless it has a minimum length of at least about 0.5 seconds. The use of this in conjunction with the driver units 81, 82, and 83 which continue the signal for at least 1.5 or 2.0 seconds, means that if the incoming warning signal has a length of, say, 0.7 seconds, it will pass the units 71, etc., and activate the units 81, etc., which will then produce an output throughout a minimum time of about 1.5 or 2.0 seconds, so the respective lamps 131, etc., will remain lit long enough for the driver to have a reasonable chance of observing them. A signal lasting for only 0.7 second might not be readily observed by the driver. Of course if the incoming warning signal is of longer duration, it will continue to show on the visual warning panel. If desired, each of the drivers 81, 82, and 83 may be provided with an adjusting knob so that the driver may suit the minimum length of signal to his own desires, keeping each lamp lit (once it has become lit) for a minimum time of 3 or 4 seconds, if desired, even though meanwhile the incoming signal has stopped. However, a minimum signal time of 1.5 or 2.0 seconds seems to be about right for average drivers, alerting them sufficiently to relatively short warning signals so long as these signals are sufficiently long (for example, 0.5 second or longer) to pass the time delay units 71,72, and 73.
With this arrangement as above described, it is seen that a warning signal from the front (detected by the detector 31), if of the right frequency and loudness, and continuing for a sufficient interval, will cause the two signaling indicators 41 and 42 to be illuminated. A signal from the back will cause the two portions 43 and 44 to be illuminated. A signal from the right side, detected by the detector 33, will cause the portion 43 of the panel to be illuminated, and a signal from the left, detected by the detector 32, will cause the portion 44 to be illuminated.
Other arrangements of detectors and of visual signal panel parts can be used, within the broad concept of the invention. For example, FIG. 4 shows a fragment of a vehicle roof equipped with four detectors 151, 152, 153, and 154, each of these microphones having a horn or baffle covering a sector of 90, so that the four together cover the entire circumference of 360.
In FIG. 5, two pickup devices or microphones are used, as shown at 161 and 162. Each of them is baffled to cover I", one faced directly forwardly and the other faced directly rearwardly.
In FIG. 6, there is an arrangement of three detectors, shown at 171, 172, and 173, each covering a sector of One of these (173) has its axis pointed straight rearwardly, while the other two (171 and 172) have their axes directed transversely to the center line of the vehicle, at right angles thereto.
Still another modification, not specifically illustrated, is to arrange the detectors as shown in FIG. 6, but have each of them cover a sector of 120 instead of only 90.
Of course appropriate changes are needed in the circuitry illustrated in FIG. 3, if more than three detectors are used, or if they are arranged substantially differently than in FIG. 1. However, in view of the explanations already given, those skilled in the an will now be able to rearrange the circuit as may be necessary depending upon the number of detectors and the various segments which they are intended to cover.
Some of the possible variations in the visual indicating panel are shown in FIGS. 7-9. In place of the angular indicating portions arranged at the corners of a square, as shown at 41, 42, 43, and 44 in FIG. 2, there may be indicating portions 201, 202, 203, and 204 (FIG. 7) arranged midway of the sides of a square rather than at the corners, these indicating portions preferably being triangular shaped to point, like arrowheads, in the direction from which the detected sound comes. Of course they may be not merely triangular arrowheads, but may have stems or shanks connected to the arrowheads if desired.
Another arrangement, in FIG. 8, again has indicating portions 211, 212, 213, and 214 arranged on the sides ofa square rather than at the corners, but these signaling portions are rectangular rather than triangular as in FIG. 7.
Another possible arrangement is shown in FIG. 9, where the four indicating or visual signal portions 221, 22, 223, and 224 are arranged as segments of a circle.
These illustrate some of the many possible variations. In the broader aspect of the present invention, the exact arrangement of the visual signaling panel observable by' the driver is not critical, so long as it is an arrangement reasonably calculated to inform the driver easily and quickly as to the direction from which the detected warning signal comes.
In view of the foregoing explanation as to the circuitry, given in connection with FIG. 3, those skilled in the electronics art and particularly in the use of logic circuits, will be able to adapt the circuit to the various possible arrangements of detectors or receivers, in order to produce the necessary indications according to the various possible arrangements of the visual indicating panel.
A further modification of the invention is schematically illustrated in FIG. 10, in which an emergency vehicle (fire truck, ambulance, police car, etc.) is shown schematically at 251, and another vehicle is shown schematically at 253. The emergency vehicle 251 is equipped with the usual audible warning device, such as the siren 255. In addition, it is also equipped with a low power radio transmitter 257 producing a warning radio frequency signal through an antenna 259 on a predetermined frequency assigned for this purpose and sufficiently different from the normal communication frequency of emergency vehicles so as not to interfere with communication to or from the vehicle. This transmitter 257 is of relatively low power so as to transmit signals which can be received through a limited distance of, say, 50 yards or yards, but not much beyond some limited distance such as this. The radio transmitter 257 is wired in parallel with the audible warning siren 255, so that when the siren is turned on, the special transmitter 257 is simultaneously turned on, and transmits its special low-power warning signal whenever the siren is operating.
The other vehicle 253, which may be an ordinary passenger car or a truck, is equipped with a radio receiving antenna 261. The signals picked up by this antenna feed into electronic circuitry indicated schematically by the block 271, the circuitry being basically a radio frequency detector, and it may be arranged to illuminate some or all portions of the main indicating panel to alert the driver to the approach of an emergency vehicle, or to illuminate a special portion of such main panel, or to illuminate a special supplementary panel or warning light.
In addition to the output of the receiver 271 for illuminating the visual warning signal, the receiver may also have an output converted to audio frequency and fed to a loudspeaker in the vehicle. Thus the driver is doubly alerted, both visually and audibly, to the presence of an emergency vehicle in the vicinity.
This warning system activated by radio transmission from an emergency vehicle may be as a substitute for, or in addition to, the previously described arrangements for receiving and indicating the direction of audible warning signals. In other words, the vehicle 253 in FIG. 10 may be fully equipped, in the manner previously described, for receiving audible warning signals and indicating the direction thereof, and it may also be equipped with the antenna system for receiving inaudible radio frequency warning signals.
Assuming that the radio frequency warning signal system is adapted by the authorities in a particular area, the radio frequency warning receiver could thereafter be required in all new automobiles sold within that area. This would have certain advantages over using the audible receiver system alone, since the warning signals produced by emergency vehicles are usually more important than those produced by ordinary automobile horns. With the dual system of both a receiver for audible warnings and a receiver for radio frequency warnings, there would be greater reliability in receiving and correctly identifying the important warnings produced by emergency vehicles, while the audible warning receiver system would operate for warnings from the horns of nonemergency vehicles, as well as being responsive of course, to the audible warnings produced by emergency vehicles.
What is claimed is:
1. A warning signal indicator for visually indicating the existence and direction of an audible airborne warning signal in the vicinity ofa vehicle, comprising a plurality of microphones mounted on said vehicle, each microphone being oriented and baffled to be selectively responsive mainly to sound waves approaching from a given direction within a limited lateral angle on each side of an axis of orientation of that microphone, the axis of orientation of each microphone being at a material horizontal angle to the axis of orientation of each other microphone, a plurality of visual signal areas in position to be readily observed by the driver of said vehicle while engaged in driving the same, electric lamps for selectively illuminating certain of said signal areas, and electric circuit means operatively interposed between said microphones and said electric lamps for illuminating said lamps selectively in response to sound waves continuing for at least a predetermined minimum time picked up by the different microphones and independently of differences in the time of arrival of incoming sound waves at different microphones.
2. A construction as defined in claim 1, in which the number of visual signal areas is greater than the number of microphones, and in which said electric circuit means includes a logic circuit for illuminating said lamps to illuminate said signal areas in various combinations depending upon which microphone or microphones are activated by incoming sound waves.
3. A warning signal indicator for visually indicating the existence and direction of an audible warning signal in the vicinity of a vehicle, comprising a plurality of microphones mounted on said vehicle, each microphone being oriented and baffled to be selectively responsive mainly to sound waves approaching from a given direction within a limited lateral angle on each side of an axis of orientation of that microphone, the axis of orientation of each microphone being at a material horizontal angle to the axis of orientation of each other microphone, a plurality of visual signal areas in position to be readily observed by the driver of said vehicle while engaged in driving the same, electric lamps for selectively illuminating certain of said signal areas, and electric circuit means operatively interposed between said microphones and said electric lamps for illuminating said lamps selectively in response to sound waves picked up by the different microphones, said electric circuit means including, with respect to each microphone, a filter passing signals having a frequency above a predetermined frequency and blocking signals having a frequency below said predetermined frequency, and a time delay unit passing signals continuing for longer than a predetermined duration and rejecting signals of less than said predetermined duration, and an output storage and driver unit. v
4. A construction as defined in claim 3, in which said predetermined duration is approximately one-half second, and said predetermined frequency is about 1,200 cycles per second.
5. A construction as defined in claim 3, in which said output storage and driver unit for each microphone feeds to at least one gate unit and to an inverter feeding to a different gate unit, and in which each gate unit controls illumination of one of said electric lamps.
6. A construction as defined in claim 3, in which there are three microphones having their axes of orientation spaced at an angle of approximately 120 from each other.
7. A construction as defined in claim 3, in which there are four microphones having their axes of orientation spaced at an angle of approximately from each other.
8. A construction as defined in claim 3, in which at least one microphone is oriented to be responsive to sound waves approaching from the front of the vehicle, another microphone is oriented to be responsive to sound waves approaching from the rear and the right-hand side, and another microphone is oriented to be responsive to sound waves approaching from the rear and the left-hand side.
9. A construction as defined in claim 3 in which said electric circuit means includes means for keeping an illuminated lamp still lit for a predetermined minimum time notwithstanding that an incoming signal which caused illumination of such lamp has meanwhile ceased.
10. The method of visually indicating to the driver of a vehicle the existence and direction of an audible airborne warning signal in the vicinity of the vehicle, which comprises the steps of disposing on the vehicle a plurality of microphones each oriented and baffled to be selectively responsive mainly to sound waves approaching from a given direction within a limited lateral angle on each side of an axis of orientation of the microphone, the axis of orientation of each microphone being in a direction materially different from that of each other microphone, disposing a plurality of visual signal areas in the vehicle in position to be readily observed by the driver of the vehicle while he is driving it, and causing outputs of the respective microphones to illuminate selectively the visual signal areas to indicate to the observing driver the direction from which a signal comes independently of differences in the time of arrival of incoming sound waves at different microphones.