Acoustic presence detection method and apparatus

• US 4,083,031 A
• Filed: 07/23/1970
• Issued: 04/04/1978
• Est. Priority Date: 07/23/1970
• Status: Expired due to Term

• Current Assignee: the united states of america as represented by the secretary of the navy
• Original Assignee: the united states of america as represented by the secretary of the navy
• Inventors: Pharo, Lawrence C. Jr.
• Primary Examiner(s): Farley, Richard A.
• Application Number:
  US05/064,910
• Time in Patent Office: 2,812 Days
• Field of Search: 340/5 R, 340/6 R, 340/16 R, 340/15, 340/258 B, 340/258 D, 181/.5
• US Class Current: 367/135
• CPC Class Codes: G01H 3/00 Measuring characteristics of vibrations by using a detector in a fluid G01H7/00, G01H9/00, G01H11/00 take precedence
• CPC Class Codes: G01V 1/001 Acoustic presence detection
• CPC Class Codes: Y10S 367/901 Noise or unwanted signal reduction in nonseismic receiving system

Abstract

A method and means for passively detecting presence of noise radiating dees in a noise environment by determining the number of times per time sample that a received noise signal upwardly crosses a preselected threshold level, comparing this number with the number of upward crossings caused by a standard noise signal or background noise signal, and applying a predetermined ratio criteria. The noise sensors in the detection system may comprise a directional acoustic transducer in combination with an omnidirectional acoustic transducer, and acoustic transducer in combination with an artificially generated noise signal, or a single acoustic transducer.

Claims

1. A passive acoustic detection system for detecting noise radiating devices in a noise environment comprising:

a first acoustic transducer responsive to acoustic signals in its environment;

a source of acoustic signals producing signals representative of noise in the environment of said first transducer;

a first octave-band frequency filter connected to said first transducer for permitting passage of only one band of frequencies;

a second octave-band frequency filter connected to said source of acoustic signals for permitting passage of only one band of frequencies;

a first automatic level control circuit responsive to the output signal from said first band filter to provide a constant root mean square level for the output signal from said first band filter;

a second automatic level control circuit responsive to the output signal from said second band filter to provide a constant root mean square level for the output signal from said second band filter;

a first pulse-forming clamp circuit, having a selectably variable threshold level, responsive to the output of said first level control circuit for providing a pulse whenever said threshold level is exceeded;

a first means responsive to the output of said first pulse-forming clamp circuit to provide a constant amplitude pulse signal every time said first pulse-forming circuit generates a pulse;

a second pulse-forming clamp circuit, having a selectable variable threshold level, responsive to the output of said second level control circuit for providing a pulse whenever said threshold level is exceeded;

a second means responsive to the output of said second pulse-forming clamp circuit to provide a constant amplitude pulse signal every time said second pulse-forming circuit generates a pulse;

a ratio detector responsive to the output of said first and second means for measuring the ratio of the pulses from said first means to the pulses from said second means;

whereby a presence is detected if the ratio of pulses from said first means to said second means equals or exceeds a predetermined amount.

2. The detection system of claim 1 wherein said first acoustic transducer comprises a hydrophone.

3. The detection system of claim 1 wherein said first and second octave-band frequency filters selectively provide a plurality of optimized frequency pass bands as determined by the DUCA characteristics of the radiated noise signal.

4. The detection system of claim 3 wherein said first acoustic transducer comprises a hydrophone.

5. The detection system of claim 1 wherein said source of acoustic signals comprises a second acoustic transducer responsive to acoustic signals in its environment.

6. The detection system of claim 5 wherein said first acoustic transducer comprises a directional acoustic transducer and said second acoustic transducer comprises an omnidirectional transducer.

7. The detection system of claim 6 wherein said first and second automatic level control circuits are disposed so as to be switchably inserted and removed from in between the respective said octave-band frequency filters and said pulse forming clamp circuits.

8. The detection system of claim 7 wherein said directional acoustic transducer comprises a directional hydrophone and said omnidirectional transducer comprises an omnidirectional hydrophone.

9. The detection system of claim 7 wherein said first and second octave-band frequency filters selectively provide a plurality of frequency pass bands.

10. The detection system of claim 9 wherein said directional acoustic transducer comprises a directional hydrophone and said omnidirectional transducer comprises an omnidirectional hydrophone.

11. A method for detecting the presence of noise generating devices in a noise environment comprising the steps of:

monitoring the environment for acoustic signals by a transducer having directional sensitivity and a transducer having omnidirectional sensitivity;

transducing the directionally monitored acoustic signals into electrical signals in a first channel;

transducing the omnidirectionally monitored acoustic signals into electrical signals in a second channel;

filtering the electrical signals in the first channel by a first adjustable band pass filter;

filtering the electrical signals in the second channel by a second adjustable band pass filter;

generating a pulse signal every time the amplitude of the transduced electrical signal in the first channel exceeds a predetermined threshold value;

generating a pulse every time the amplitude of the transduced electrical signal in the second channel exceeds the same predetermined threshold value;

counting the number of times a pulse signal is generated in the first channel in a predetermined time period;

counting the number of times a pulse signal is generated in the second channel in the same predetermined time period.

comparing the count in the first channel with the count in the second channel;

indicating detection of a noise generation device if the count in the first channel exceeds the count in the second channel by a predetermined ratio.

12. The method of claim 11 wherein the filtered electrical signals in the first channel are kept at a constant predetermined root mean square level before the step of generating a pulse signal in the first channel;

and

the filtered electrical signals in the second channel are kept at a constant predetermined root mean square level before the step of generating a pulse signal in the second channel.