Time Reversal Acoustic Approach for Non-Lethal
Swimmer Deterrent
Alexander Sutin - Alexander.Sutin@stevens.edu
Stevens Institute of Technology
Hoboken, New Jersey, USA
Yegor Sinelnikov
Sound Interventions, Inc.
Stony Brook, New York, USA
Popular version of paper 2pBBa6
Presented Tuesday afternoon, November 16, 2010
2nd Pan-American/Iberian Meeting on Acoustics, Cancun, Mexico
In a volatile time of worldwide spread of stealth technology the risk to
the homeland port security from underwater threats increases significantly.
Explosives can be brought to domestic harbors by undetected scuba divers or attached to the vessel
hulls of ships entering a harbor. Although underwater detection
technologies exist, hostile diver identification and deterrent remains a
challenge. Since the terrorist attacks of September 11, 2001, it became
imperative to deploy effective systems to monitor both the surface and
underwater potential threats. While detection of scuba divers can be conducted
with the help of active and passive sonars, the major
underwater defense still constitutes the use of explosive charges that can kill
divers and all marine life around.
Application of lethal weapons is acceptable in the case of determined
attack that can not be stopped otherwise. Less
harmful, non-lethal methods of diver deterrent are based on application of the
intensive sound. Air-guns become a part of integrated anti-swimmer port
protection program. Typically, intensive underwater sound systems introduced
for diver deterrent generate low frequency underwater sound that interferes
with breathing, induces vestibular disorientation, panic, uncontrolled ascent
to surface and subsequent decompression sickness. It has been shown that strong
sound with acoustic pressure above 170 - 180 dB re 1Pa
causes lung and liver damage.. Yet, the major deficiency of all deployed
interdiction systems is that the sound is radiated in all directions and can negatively
affect marine life as well stun friendly divers. To avoid collateral damage to
marine life, the focusing of acoustic energy in a tight focal zone around the
target becomes most critical. Today’s power of signal processing and
thorough knowledge of underwater acoustical physics provide an excellent viable
platform and technology to construct an intelligent, safe and effective
underwater sound focusing system.
This paper discusses possible application of the Time Reversal Acoustic
(TRA) principles to enable precision targeting of a hostile diver with intense
focused sound and minimal impact to the marine life. The acoustic noise
radiated by a diver is used to focus the acoustic energy back to the diver. The
diver acts as an active self-disclosing acoustic beacon.
Time Reversal Acoustics is one of the hottest topics in acoustics. The
significance of TRA is in its ability to efficiently focus an acoustic signal
regardless of the position of the initial source and regardless of the
heterogeneity of the propagation medium. The TRA systems have a range of
multi-discipline applications, including destruction of tumors and kidney
stones in medicine, detection of defects in construction, and long-distance
communication in the ocean. Impressive results were shown for kidney stone
destruction, land mine detection and treatment of atrial
fibrillation. Recently, the experiments in shallow sea demonstrated superior
ability of the TRA system to focus the underwater sound across distances of up
to 30 km
We explored the feasibility of deterring the diver away from critical
port infrastructure by an application of the TRA principles. The major
advantage of proposed approach is that the TRA system for non-lethal swimmer
neutralization focuses all radiated, refracted, reflected and scattered
underwater sound back to a hostile swimmer. The TRA focusing technique further
takes an unprecedented advantage of spatial heterogeneities and multiple
reflections present in shallow harbors that decrease the efficiency of other
active insonation methods. The use of passive
recordings of sound radiated by a diver enables low cost and high efficiency
integrated detection and deterrent approach. Substantial research by the
Stevens Institute of Technology pawed the way to TRA deterrent by demonstrating
confident reception of the acoustic noise generated by scuba diver regulators
at distances up to 700 m.
The concept of the TRA system for non-lethal swimmer neutralization is presented in Figure 1. The recorded diver sound is time reversed, amplified and focused back to the diver location in real-time. In this process the diver acts as an acoustic beacon allowing precise spatial focusing of underwater sound back to a moving diver. The suggested interdiction TRA system consists of several receiver- emitter units that pick up diver sound and radiate it back.
Figure 1. Concept of the TRA system for non lethal swimmer neutralization
We developed a model to establish the feasibility of TRA focusing system and estimate an effective zone of the diver deterrence in a shallow sea model. Short sound bursts were numerically traced trough a generalized sea model with inclined seefloor that acted as a waveguide, where emitters were located at the shallow region. Two-dimensional maps of sound pressure level (SPL) were calculated at various distances, 50 m to 200 m from emitters to diver. Figure 2 shows the SPL in the vicinity of a diver for the TRA system with 20 underwater emitters, with acoustic power of 100W each, randomly distributed within approximately two meters from each other. Good focusing of sound waves in a direction of propagation (x) and in water depth (z) was achieved, while focusing in transverse direction (y) was less pronounced. Comparative calculations also demonstrated that noise burst of higher frequency focused better, while the lower frequency produced larger focal zone with comparable peak temporal averaged intensity of underwater sound.
Figure 2. Spatial distribution of the TRA focused acoustic pressure (dB re 1Pa) produced by the TRA system with 20 underwater emitters with acoustic power of 100W each in the frequency bands 0.5-2.5 kHz (top) and 2-4 kHz (bottom). Distance to target is 50 m, water column depth is changed from about 7 to 5 m.
Model calculations demonstrate that focused underwater SPL can
reach 190 dB re 1Pa,
which can be practical for non-lethal diver deterrent. According to the
publication of the Applied Research Laboratories of the University of Texas at
Austin, this is sufficient to disorient and divert any bareheaded swimmer due
to vestibular effects (dizziness) and overall loudness. Another important
implication is that the size of water column where high intensity sound is
focused is confined within a relatively narrow volume of several cubic meters,
while underwater sound intensities outside that region are substantially less and
it will not be harmful for marine life. The focused underwater sound intensity
decays with distance, as it is shown in Figure 3. At 100 m the sound pressure
level can reach 181 dB that is still in the range of the level required for
diver deterrent.
Figure 3. SPL at the focus calculated as a function of number of transmitters and distance. It is seen that level 180 dB can be reached for the system with 5 transmitters - at a distance of 60 m, and for system with 10 transmitters - at a distance of more than 100 m.
Based on our modeling the application TRA focusing to deter
and neutralize hostile divers is feasible. Major advantages of non-lethal TRA
interdiction approach constitute the precise focusing ability in arbitrary
heterogeneous environment, the ability to use passive detection for aiming the
impact, reduced harm to marine life, low cost and relatively small deployment
size. The objective of further research will be a construction of real TRA
non-lethal interdiction acoustic focusing system and verification of its
efficacy in marine environment.