Subaudible frequency detection system

Claim 2

A detection system responsive to subaudible frequency vibrations, comprising: transducer means responsive to subaudible frequency vibrations for developing a subaudible frequency signal corresponding to said vibrations; first modulator means responsive to the subaudible frequency signal and to a supplied first reference signal for developing a first modulated signal having a first set of frequency components corresponding to the sum of the subaudible frequency signal and said first reference signal frequency and a second set corresponding to the difference therebetween; second modulator means responsive to the subaudible frequency signal and to a supplied second reference signal for developing a second modulated signal having a first set of frequencies corresponding to the sum of the subaudible frequency signal and said second reference signal frequency and a second set corresponding to the difference therebetween; mixer means responsive to said first and second modulated signals for providing an output signal having a single set of audio frequency components each frequency component in the subaudible frequency signal being represented by a single audio frequency component in said set; and means responsive to the output signal for providing an aural presentation of said output signal wherein the audio frequency components therein representatiVe of the subaudible frequency vibrations caused by an object are aurally distinguishable for permitting detection of said object.

Claim 3

A detection system as described in claim 1, wherein the first and second reference signals are both of substantially the same audio frequency but differ in phase from each other by a prescribed angle phi .

Claim 4

A detection system as described in claim 2, wherein the first and second reference signals differ in phase from each other by approximately 90*; said first modulating means is a first balanced modulator; said second modulating means includes phase shifter means for shifting the phase of said subaudible frequency signal by an angle Alpha and further includes a second balanced modulator responsive to the phase shifted subaudible frequency signal and to the second reference signal for providing said second modulated signal, the first component set being substantially equal in amplitude to the first component set in the first modulated signal but differs in phase therefrom by a prescribed angle Beta , and the second component set being substantially equal in amplitude and phase to the second component set in said first modulated signal; and wherein said mixing means provides the additive sum of the first and second modulated signals.

Claim 5

A detection system responsive to subaudible frequency vibrations, comprising: transducer means responsive to subaudible frequency vibrations for developing a subaudible frequency signal corresponding to said vibrations; a first balanced modulator responsive to the subaudible frequency signal and to a first audio frequency reference signal for developing a first modulated signal having a first set of audio frequency components corresponding to the sum of the subaudible frequency signal and the first reference signal frequency and a second set corresponding to the difference thereof; phase shifter means for shifting the phase of the subaudible frequency signal by a phase angle of approximately 90* ; a second balanced modulator responsive to the phase-shifted subaudible frequency signal and to a second supplied audio frequency reference signal corresponding to said first reference signal shifted in phase by an angle of approximately 90* for developing a second modulated signal having a first set and a second set of audio frequency components, the first set being substantially equal in amplitude to the first component set in said first modulated signal but differs in phase therefrom by approximately 180*, and the second component set being substantially equal in amplitude and phase to the second component set in said first modulated signal; mixer means responsive to the first and second modulated signals for adding the modulated signals to develop an output signal having a single set of audio frequency components whose amplitude is substantially equal to the additive sum of the modulated signal second component sets, each frequency component in the subaudible frequency signal being represented by a corresponding single audio frequency component in the output signal set; and sound reproduction means responsive to the output signal for providing an aural presentation of said output signal wherein audio frequency components therein representative of the subaudible frequency vibrations caused by an object are aurally distinguishable for permitting detection of said object.

Claim 6

A detection system responsive to subaudible frequency vibrations, comprising: transducer means responsive to subaudible frequency vibrations for developing a subaudible frequency signal corresponding to said vibrations; a first balanced modulator responsive to the subaudible frequency signal and to a first audio frequency reference signal for developing a first modulated signal having a first set of audio frequency components corresponding to the sum of the subaudible frequency signal and the first reference signal frequency and a second set corresponding to the difference thereof; phase shifter means for shifting the phase of the subaudible frequency signal by a phase angle of approximately 90* ; a second balanced modulator responsive to the phase-shifted subaudible frequency signal and to a second supplied audio frequency reference signal corresponding to said first reference signal shifted in phase by an angle of approximately 90* for developing a second modulated signal having a first set and a second set of audio frequency components, the first set being substantially equal in amplitude to the first component set in said first modulated signal but differs in phase therefrom by approximately 180*, and the second component set being substantially equal in amplitude and phase to the second component set in said first modulated signal; mixer means responsive to the first and second modulated signals for adding the modulated signals to develop an output signal having a single set of audio frequency components whose amplitude is substantially equal to the additive sum of the modulated signal second component sets, each frequency component in the subaudible frequency signal being represented by a corresponding single audio frequency component in the output signal set; and sound reproduction means responsive to the output signal for providing an aural presentation of said output signal wherein audio frequency components therein representative of the subaudible frequency vibrations caused by an object are aurally distinguishable for permitting detection of said object.

Claim 7

A detection system as described in claim 4, wherein said transducer means is a seismic transducer means responsive to subaudible frequency vibrations in the earth for developing the subaudible frequency signal.

Claim 8

A detection system as described in claim 4, wherein said transducer means is an electro-acoustic transducer means responsive to subaudible frequency acoustic vibrations for developing the subaudible frequency signal.

Claim 9

A detection system responsive to subaudible frequency vibrations, comprising: transducer means responsive to subaudible frequency vibrations for developing a subaudible frequency signal corresponding to said vibrations; means for modifying the amplitude of the subaudible frequency signal to develop an output signal having the fundamental frequency components of said subaudible frequency signal and audio frequency harmonic components thereof, all said components being representative of the subaudible frequency vibrations; and means responsive to the output signal for providing an aural presentation of said output signal wherein audio frequency harmonic components therein representative of the subaudible frequency vibrations caused by an object are aurally distinguishable for permitting detection of said object.

Claim 10

A detection system responsive to subaudible frequency vibrations, comprising: transducer means responsive to subaudible frequency vibrations for developing a subaudible frequency signal corresponding to said vibrations; means for modifying the amplitude of the subaudible frequency signal to develop an output signal having the fundamental frequency components of said subaudible frequency signal and audio frequency harmonic components thereof, all said components being representative of the subaudible frequency vibrations; and means responsive to the output signal for providing an aural presentation of said output signal wherein audio frequency harmonic components therein representative of the subaudible frequency vibrations caused by an object are aurally distinguishable for permitting detection of said object.

Claim 11

A detection system as described in claim 7, wherein said modifying means is a limiter circuit for clipping the upper and lower amplitude peaks of the subaudible frequency signal to provide a square wave-like output signal which includes the fundamental frequency components of said subaudible frequency signal together with audio frequency harmonic components thereof.

Claim 12

A detection system as described in claim 7, wherein said transducer means is a seismic transducer means responsive to subaudible frequency vibrations in the earth for developing the subaudible frequency signal.

Claim 13

A detection system as described in claim 7, wherein said transducer means is an electro-acoustic transducer means responsive to subaudible frequency acoustic vibrations for developing the subaudible frequency signal.