Time reversal acoustics method and apparatus for detecting changes in environments

1. A method for detecting a change in an environment comprising:

forming, via one or more transducers, a test acoustic signal based on time-reversal of a baseline acoustic signal after the baseline acoustic signal has traversed the environment while the environment is in a baseline condition;

detecting, via the one or more transducers, the test acoustic signal after the test acoustic signal has traversed the environment while the environment is in an unknown condition;

determining, via a processor, a correlation of the baseline acoustic signal with the detected test acoustic signal after the test acoustic signal has traversed the environment while the environment is in the unknown condition;

determining, via the processor, that there has been a change in the environment by comparing the determined correlation with a correlation of the baseline acoustic signal with the detected test acoustic signal after the test acoustic signal has traversed the environment while the environment is in the baseline condition; and

outputting, via the processor, an indication that there has been a change in the environment upon determining that there has been a change in the environment.

2. The method of claim 1 , wherein:

the baseline acoustic signal is transmitted by a first one of the one or more transducers; and

the test acoustic signal is detected by the one or more transducers after the test acoustic signal has traversed the environment while the environment is in the unknown condition.

3. The method of claim 2 , wherein:

the test acoustic signal is transmitted by a second transducer of the one or more transducers; and

the baseline acoustic signal is detected by the second transducer after the baseline acoustic signal has traversed the environment while the environment is in a baseline condition.

4. The method of claim 1 , wherein the determining that there has been a change in the environment comprises comparing the detected test acoustic signal after the test acoustic signal has traversed the environment while the environment is in the unknown condition with a detected test acoustic signal after the test acoustic signal has traversed the environment while the environment is in the baseline condition.

5. The method of claim 1 , further comprising determining that there has been a change in the environment when the determined correlation is lower than the correlation of the baseline acoustic signal with a detected test acoustic signal after the test acoustic signal has traversed the environment while the environment is in the baseline condition.

6. The method of claim 5 , further comprising determining that there has been a change in the environment when the determined correlation is lower than half of the correlation of the baseline acoustic signal with a detected test acoustic signal after the test acoustic signal has traversed the environment while the environment is in the baseline condition.

7. The method of claim 1 , further comprising forming the test acoustic signal, wherein forming the test acoustic signal comprises:

transmitting the baseline acoustic signal;

detecting the transmitted baseline acoustic signal after the baseline acoustic signal has traversed the environment while the environment is in the baseline condition; and

time-reversing the detected baseline acoustic signal to form the test acoustic signal.

8. The method of claim 7 , wherein:

the baseline acoustic signal is transmitted by a first transducer of the one or more transducers;

the test acoustic signal is detected by the first transducer after the test acoustic signal has traversed the environment while the environment is in an unknown condition;

the test acoustic signal is transmitted by a second transducer of the one or more transducers; and

the baseline acoustic signal is detected by the second transducer after the baseline acoustic signal has traversed the environment while the environment is in a baseline condition.

9. The method of claim 1 further comprising:

generating multiple test acoustic signals with at least four transducers of the one or more transducers; and

wherein determining that there has been a change in the environment comprises determining correlations of the baseline acoustic signal with the multiple test acoustic signals.

10. The method of claim 9 , wherein determining correlations of the baseline acoustic signal with the multiple test acoustic signals improves a resolution of determining that there has been a change in the environment over determining correlations of the baseline acoustic signal with a single test acoustic signal.

11. The method of claim 9 , further comprising determining a location where a change in the environment has occurred.

12. The method of claim 1 , wherein the environment comprises at least one of a room, a shipping container, a vault, a nuclear fuel storage facility, a weapons storage facility, a storage facility for high value items, an area storing sensitive documents, a circuit board, or a computer.

13. The method of claim 1 , wherein the change in the environment comprises at least one of a security breach, tampering with equipment, removal of an object from the environment, placement of an object in the environment, placement of a listening device in the environment, and unauthorized retrieval of information stored in the environment.

14. The method of claim 1 , wherein a frequency of the baseline acoustic signal is 40 kHz or more.

15. The method of claim 14 , wherein the frequency of the baseline acoustic signal is different than a frequency of ambient noise in the environment.

16. The method of claim 1 , further comprising detecting changes in the environment at a size scale that is less than a wavelength of the baseline acoustic signal.

17. A method for detecting a change in an environment comprising:

forming, via one or more transducers, multiple test acoustic signals based on time-reversal of a baseline acoustic signal after the baseline acoustic signal has traversed the environment while the environment is in a baseline condition;

detecting, via the one or more transducers, the multiple test acoustic signals after the multiple test acoustic signals have traversed the environment while the environment is in an unknown condition;

determining, via a processor, a correlation of the baseline acoustic signal with the detected multiple test acoustic signals after the multiple test acoustic signals have traversed the environment while the environment is in the unknown condition;

determining, via the processor, that there has been a change in the environment by comparing the determined correlation with a correlation of the baseline acoustic signal with the detected multiple test acoustic signals after the multiple test acoustic signals have traversed the environment while the environment is in the baseline condition; and

outputting, via the processor, an indication that there has been a change in the environment in response to determining that there has been a change in the environment.

18. A system comprising:

at least two transducers;

at least one processor associated with the at least two transducers;

a storage medium associated with the at least one processor, the storage medium storing instructions executable by the processor to:

form, via the at least two transducers, a test acoustic signal based on time-reversal of a baseline acoustic signal after the baseline acoustic signal has traversed the environment while the environment is in a baseline condition;

detect, via the at least two transducers, the test acoustic signal after the test acoustic signal has traversed the environment while the environment is in an unknown condition;

determine a correlation of the baseline acoustic signal with the detected test acoustic signal after the test acoustic signal has traversed the environment while the environment is in the unknown condition;

determine that there has been a change in the environment by comparing the determined correlation with a correlation of the baseline acoustic signal with the detected test acoustic signal after the test acoustic signal has traversed the environment while the environment is in the baseline condition; and

output an indication that there has been a change in the environment upon determining that there has been a change in the environment.