Concentric fluid acoustic transponder

An acoustic transponder method and apparatus provides a high degree of frency and target strength selectivity by utilizing two or more concentrically disposed, hollow spheres each containing acoustically refractive fluids. The invention provides the possibility of using spheres of a variety of diameters and as well a number of different refractive fluids, enabling the production of an acoustic transponder having a wide variety of selective frequency responses as well as effective target diameters.

BACKGROUND OF THE INVENTION 
This invention relates generally to an underwater sound reflector and more 
particularly to a passive underwater sound transponder having a high 
degree of frequency and target strength selectivity. 
Undersea navigation, surveying, salvage, relocation and sonar system 
calibration often require that a platform located on the surface or 
underwater be able to identify specific, uniquely identifiable, locations 
on the ocean bottom or in the water column. Underwater location markers 
might also need to be uniquely identifiable from all other markers in an 
immediate area. 
To satisfy these requirements, active acoustic transponders are often used. 
These active devices require an internal battery power source and also 
contain an acoustic receiver, a transmitter and decoding circuitry. The 
navigating platform generates a waterborne acoustic interrogation pulse 
that is received by the transponder. The transponder replies by 
transmitting its own acoustic pulse at a unique and known frequency. The 
platform receives and decodes this unique reply. It can then be determined 
which specific transponder replied, and its approximate location. The 
transponder could also be used simply as a location or homing marker. 
Additionally, if the transponder's position is accurately known, this 
interrogation/reply process can be used for general navigation purposes. 
If several transponders are deployed, they can be used together in a 
network to provide precise navigation capabilities. 
The basic limitation of the active transponder is that it is a powered, 
active electronic device. These devices are expensive, and usually require 
careful handling during at-sea deployments. They are also subject to the 
normal circuit failures of any electronic device. Their operating life is 
relatively short due to the limited energy available from the internal 
battery power source. 
In addition to the active transponders, passive acoustic transponders of 
various types, i.e. transponders requiring no internal power source, are 
sometimes used for simple location marking and for sonar calibration 
purposes. These generally have an inherent, broadband frequency response 
with no frequency selectivity. It is this ability to work over a wide 
frequency range that makes passive transponders desirable for some 
applications. For instance, a single broadband, non-frequency selective 
target could be used for calibrating sonars operating on many different 
frequencies. 
A need exists for an acoustic transponder that does not have the drawbacks 
of battery and circuit failures while at the same time has the capability 
of a high degree of frequency and target strength selectivity. 
SUMMARY OF THE INVENTION 
The invention provides these needs as well as other advantages and features 
that will become apparent from the ensuing description. The concentric 
fluid acoustic transponder of the invention is a passive transponder 
capable of a high degree of frequency response selectivity. In a preferred 
embodiment, the transponder is constructed from two, concentrically 
disposed, hollow spheres. The "inner" and "outer" spheres are preferably 
of thin-walled, stainless steel construction. The spheres are held 
concentric with respect to each other by structural members that in one 
embodiment include two, small diameter, radially located positioning tubes 
attached between the spheres. In this embodiment, these tubes can be used 
for filling the inner and outer spheres with acoustically refractive 
fluids. The diameter of the inner sphere defines what is described herein 
as an inner, fluid core, region while the gap between the inner and outer 
spheres defines what is described as an outer, concentric fluid, region or 
layer. The invention provides the possibility of using spheres of a 
variety of diameters and as well a number of different refractive fluids, 
enabling the production of an acoustic transponder having a wide variety 
of selective frequency responses, target strengths and diameters. 
OBJECTS OF THE INVENTION 
An object of this invention to provide a passive acoustic transponder that 
can easily be tuned to provide a selected frequency response. 
A further object of this invention is to provide a passive acoustic 
transponder that can be tuned to provide a selected target strength. 
Still another object of the invention is to provide a passive acoustic 
transponder that is expendable. 
Still another object of the invention is to provide a passive acoustic 
transponder that is easily deployable. 
Still a further object of the invention is to provide a passive acoustic 
transponder that can be used for maritime and underwater navigation 
purposes. 
Yet another object of this invention is to provide a passive acoustic 
transponder that can be used for maritime and underwater surveying. 
Still another object of this invention is to provide a passive acoustic 
transponder that can be used as an underwater location marker. 
Other objects, advantages and new features of this invention will become 
apparent from the following detailed description of the invention when 
considered in conjunction with the accompanied drawings.

DESCRIPTION OF THE INVENTION 
The United States Navy has developed several passive, underwater acoustic 
target systems for use with sonar systems. One such traditional acoustic 
target uses a single fluid, spherical acoustic lens as an acoustic 
reflector. 
Such a spherical acoustic lens is, generally, omnidirectional, with a 
relatively broadband frequency response. Illumination of such a lens with 
sonic energy causes the incident acoustic energy to be focused inside the 
lens and redirected back along the same path from which it originated. 
These spherical acoustic lens are typically of thin-walled steel 
construction and are filled with a refractive fluid. Design parameters for 
such an acoustic lens include the following variables: 
1. Frequency of the incident acoustic energy 
2. Pulsewidth of the incident acoustic energy 
3. Sphere diameter 
4. Sphere-material physical/acoustic properties 
5. Sphere wall-thickness 
6. Refractive fluid properties 
These parameters are optimized to insure that the incident energy is 
sufficiently refracted (i.e. bent) and focused on the back-surface of the 
sphere. Focusing the incident acoustic energy in this way maximizes the 
target strength, or returned energy, for a particular lens configuration. 
Different target strengths are attained by using various 
sphere-diameter/refractive-fluid combinations. At ultrasonic frequencies 
above 100 kHz with pulsewidths less than 1 msec, these targets can be very 
compact (e.g., diameters of 10-15 cm/4-6") and efficient (e.g., -5 to -20 
dB target strengths). 
At these high frequencies and very short wavelengths, it was observed that 
the wall-thickness of the spherical steel sphere had a substantial effect 
on the frequency response of the target. As the wall-thickness approached 
a tenth-to-quarter wavelength of the acoustic energy involved, severe 
variations in the target strength of the lens can occur. To lessen this 
effect, very thin-walled spheres have been used (e.g., 0.75-1.30 
mm/0.03-0.05"; 20-18 gauge). It was found that if the ratio of the 
sphere's inner diameter to its outer diameter is no less than 0.98-0.99, 
these variations in target return strength are minimized. 
It was also learned that the sound velocity in the wall-material had a 
profound effect on the target strength of the lens. Any incident acoustic 
energy would be refracted somewhat while passing through the wall 
material. Ideally, the sphere's wall would be fabricated from a material 
thin enough to have little or no effect on the impinging acoustic energy. 
This would allow the properties of the internal refractive fluid to 
completely control the degree of sound raybending and focusing. 
Unfortunately, such a wall material is either non-existent or impractical 
because the wall must be constructed of a material able to maintain the 
target's expected form factor (i.e., a sphere), and be able to withstand 
the rigors of the ocean environment. This limits most wall material 
choices to either thin, high-density, materials with high sound 
velocities, or thick, low-density, materials with lower sound velocities. 
In either case, if the refraction within the wall-material occurs over a 
relatively large thickness, the preliminary raybending caused thereby can 
drastically alter the refractive (or focusing) effects of the fluid inside 
the sphere. As the focusing efficiency decreases, the number of multiple 
reflections within the sphere increases. This situation further degrades 
the target strength and frequency response performance of the target. 
While the effects described above are considered negative constraints when 
designing a traditional broadband, high target strength, acoustic lens, it 
was thought that these limiting factors could be used to purposely alter 
the frequency response of an acoustic lens. An empirical approach to 
characterizing lens performance was thus undertaken. In-water, 
laboratory-quality, acoustic measurements were used to investigate the 
effects of changing various lens parameters. These included wall-material, 
wall-thickness, sphere diameter and refractive fluid. Prefabricated 
spheres were used and were selected of materials considered to be suitable 
for full scale production. Several refractive fluids were selected based 
on past performance and experience. Table 1 lists the refractive fluids 
and sphere materials considered. 
TABLE 1 
______________________________________ 
NOMINAL SOUND SPEED OF TARGET-RELATED MATERIALS 
Nominal Sound Speed 
Wavelength 
Material (rn/sec @ 25-26 deg C.) 
@ 20 kHz (Cm/in) 
______________________________________ 
Fluids: 
Fresh Water 1466 (4810 ft/sec)* 
7.3 (2.9") 
Fluorolube FS-5 
883 (2896 ft/sec)* 
4.4 (1.7") 
(Chlorotrifluoroethylene 
Polymer) 
Freon 113 706 (2316 ft/sec)* 
3.5 (1.4") 
(Trichlorotrifluoroethane) 
Methyl Alcohol 
1160 (3805 ft/Sec)* 
5.8 (2.3") 
Perfluoro 619 (2031 ft/sec)* 
3.1 (1.2") 
(Methylcyclohexane) 
Spheres: 
Acetal Fibre 
Thermoplastic 
2314 (7590 ft/sec)* 
11.6 (4.6") 
Stainless Steel (SS) 
5000 (16400 ft/sec) 
25.0 (9.8") 
Syntactic Foam 
2073 (6800 tt/sec) 
10.4 (4.1") 
______________________________________ 
NOTE:* = The sound speed was determined from a direct measurement of 
acoustic travel time using acoustic reflectrometry techniques. 
The investigation began by considering a traditional acoustic lens 
configuration composed of a single, spherical, thin-walled shell 
containing a refractive fluid in its enclosed volume. This configuration 
will be identified herein as the "single sphere", or "single fluid", 
configuration. Table 2 lists the various sphere/fluid combinations tested 
during this initial test sequence. 
TABLE 2 
______________________________________ 
SINGLE SPHERE TEST CONFIGURATIONS 
Wall 
Sphere O.D. Thick. Refractive 
Freq Range (kHz) 
Material (in.) (in.) Fluid 10 60 50 250 
______________________________________ 
Type 304 SS 
5 0.0375 Freon x 
Type 304 SS 
6 0.0375 Fluorolube 
x x 
Perfluoro 
x 
Type 304 SS 
8 0.050 Freon x x 
Thermoplastic 
8 0.50 Fluorolube 
x x 
Freon x x 
Perfluoro 
x x 
Acetal Fibre 
8 0.25 Freon x x 
Syntactic Foam 
8 0.50 Fresh Water x 
Fluorolube 
x x 
______________________________________ 
Results of this test phase, to be further described, showed that the 
available sphere materials and wall-thicknesses did not provide 
satisfactory responses at lower frequencies and longer wavelengths of 
interest. 
This led to creation of the present invention, that of surrounding an inner 
core (i.e., solid sphere) of fluid with a concentric fluid layer (i.e., a 
spherical annulus). Such a construction would produce a target having 
"inner" and "outer" fluid regions. By using this method, a "fluid" 
wall-thickness of several inches, if necessary, could be provided. There 
are fluids available with a wide range of sound velocities and densities 
both lower and higher than seawater. These fluids could be used to 
simulate, or replace, the acoustic properties of the materials that would 
otherwise be needed to construct a spherical, thick-walled acoustic lens. 
A factor in using two, or more, fluids is one of maintaining a desired 
spatial relationship between them (e.g., concentric layers). A type of 
constraining, or containment, technique would be required. A practical 
target device would also allow for packaging, deployment and use in a 
hostile ocean environment. These problems were solved by placing a 
smaller, thin-walled spherical shell inside a larger, thin-walled 
spherical shell. Steel was chosen as a preferred material for the shells. 
Referring now to FIG. 1, a general configuration of a two-fluid acoustic 
transponder according to an exemplary embodiment of the invention is 
shown. In this figure, it can be seen that acoustic transponder 10 
includes a first fluid container 12 and a second fluid container 14 that 
surrounds fluid container 12. The term "fluid" as used herein is meant to 
be either a liquid or a gas, as one skilled in the art will realize that 
either of these forms of fluid may be used with the invention. In a 
preferred embodiment of the invention, fluid containers 12 and 14 are 
spherical, though one skilled in the art will realize that other shapes 
may also provide desired acoustic properties. Spheres 12 and 14 are 
preferably made of thin gauge stainless steel (18 to 20 gauge) and are 
preferably held concentric to each other by any of a variety of structural 
members such as by small diameter tubes 16 and 18. Of course, a variety of 
materials may be utilized as shell materials provided that these satisfy 
the operational and environmental requirements of the user. Similarly, the 
positioning of one shell concentrically within a second shell is 
considered to be a preferred arrangement of the shells, though it can be 
envisioned that other, non-concentric arrangements are possible and may in 
some cases be advantageous. 
For test purposes, a method was needed to fill, empty, and refill fluid 
core region 20 and fluid concentric region 22 with various liquid test 
fluids. Tubes 16 and 18, located in the space between shells 12 and 14, 
were used for these purposes. These tubes each have one or more ports, for 
example, tube 16 has ports 24 and tube 18 has port 26. Caps 28 and 30 
would be removed to permit filling and draining of regions 22 and 20, 
respectively. 
A production model of the transponder of the invention would not 
necessarily require the filler tubes illustrated in FIG. 1, since the 
particular fluid to be used would be known. In this case the inner sphere 
could be filled and sealed before final assembly. Smaller diameter, solid 
rods, could then be used to maintain the necessary interior spacing. 
In FIG. 2, an exemplary method of using the invention is shown. Referring 
now to this figure, it can be seen that concentric fluid acoustic 
transponder 100 of the invention can be employed within water column 102 
by suspending transponder 100 between a mooring 104 disposed on bottom 106 
of water column 102 and a float 108 located either on the surface of water 
column 102 or located subsurface, as shown. A cable 110, chain or other 
connector can be used to link transponder 100 to mooring 104 and float 
108. Cable 110 can be attached to transponder 100 by way of attachment 
loops, netting or other suitable attachment mechanism. In use, transponder 
100 will be insonified with acoustic energy 112, so as to absorb and 
reflect a portion of this energy resulting in the transponder having a 
unique acoustic signature detectable by surface and subsurface platforms. 
Those skilled in the art will realize that a variety of other deployment 
or mounting techniques utilizing the invention are feasible. For example, 
the transponder could be positioned in very close proximity to the ocean 
floor using a mechanical support or fixture of various designs. 
A technique was developed to fabricate the concentric fluid acoustic 
transponder of the invention in stages, using thin-walled hemispheres. For 
test purposes, two concentric-fluid targets were made. The first unit had 
a 7.6 cm (3") diameter inner shell, with a 12.7 cm (5") diameter outer 
shell. The second unit had a 12.7 cm (5") diameter inner shell, with a 
17.8 cm (7") diameter outer shell. In both cases, this resulted in a 
nominal 2.5 cm (1") wide gap between the two concentric, spherical shells. 
Equivalently, this gap produces a 2.5 cm (1") thick, outer concentric 
fluid-layer completely surrounding the inner fluid-core. All sphere shells 
tested had a wall-thickness of 0.0375" (20 gauge) stainless steel. Table 3 
lists the various concentric-fluid/sphere-diameter combinations tested. 
TABLE 3 
______________________________________ 
CONCENTRIC-FLUID TEST CONFIGURATIONS 
ID/OD Refractive Fluids 
Freg Range (kHz) 
(in.) (inner/outer) 10 60 50 250 
______________________________________ 
3/5 Freon/Freon x 
Freon/Perfluoro x 
Perfluoro/Fluorolube 
x 
5/7 Fluorolube/Perfluoro 
x x 
Fluorolube/Freon x 
Fluorolube/Methyl Alcohol 
x 
Freon/Perfluoro x 
Freon/ Fluorolube 
x 
Freon/Methyl Alcohol 
x 
Perfluoro/Freon x 
Perfluoro/Fluorolube 
x 
Perfluoro/Methyl Alcohol 
x 
Methyl Alcohol/Perfluoro 
x 
______________________________________ 
A computer program was developed to compute and plot the primary, 
nonreflected raypaths within the various transponders. This program 
permitted input of the various physical and acoustic characteristics of 
single, or multiple, sphere targets. The plots created through use of the 
program were used to determine the approximate focusing efficiency of the 
different target configurations. A sample program output is presented as 
FIG. 3. This particular diagram shows the poor focusing efficiency of a 
sphere containing a half inch thick outer thermoplastic layer with freon 
as the inner refractive fluid. The focal point is occurring before the 
back surface of the sphere. The predicted focusing efficiency is 
consistent with this transponder's relatively low target strength, as 
shown in FIG. 9. 
In FIGS. 4-24, the target strengths of various combinations of spheres and 
fluids are shown. These target strengths were measured at the United 
States Navy's Transducer Evaluation Center (TRANSDEC) using pulsed 
measurement techniques. The TRANSDEC facility is an anechoic pool 
outfitted for precise, direct-pulse, sonar-type measurements. The TRANSDEC 
Sampling Digital Waveform Recorder System (SDWRS) was used to obtain 
target strength data. The test-setup and processing time needed to conduct 
each individual test required that some constraints be placed on the test 
methodology. Measurements were thus limited to every 2.5 Khz in the 10-60 
Khz range, and every 5.0 Khz in the 50-250 Khz range. The test geometry 
and waveform sampling-time requirements limited the maximum usable 
pulsewidth to 5 msec. Appropriate sampling windows were selected for each 
pulsewidth and frequency range of interest. Calculations of the true rms 
energy content of the waveforms were made, the target strengths computed, 
and the results output in graphical form. Each data point presented on the 
accompanying graphs is the result of a calculation involving the average 
of four independent samples of each waveform at that frequency. Graph 
scaling was kept constant for all tests to make visual comparisons easier. 
Emphasis was given to obtaining measurements in the 10-60 Khz frequency 
range using pulsewidths of 1 msec and 5 msec. Often, additional 
measurements were made in the 50-250 Khz range using various pulsewidths. 
This allowed for some comparison of the current test results with past 
measurement data from the previous single sphere acoustic lens work. A 
small, battery operated pump, with appropriate miniature nozzles, was used 
to expedite the refractive fluid filling process. 
Regarding FIGS. 4-11, the annotation on a "single sphere" graph identifies 
the transponder's or target's sphere material, outer diameter, wall 
thickness and refractive fluid. The annotation on the "concentric-fluid" 
graphs, FIGS. 12-24, identifies the target's inner/outer shell diameters, 
and inner/outer fluids. The "fluid" wall-thickness of all concentric-fluid 
targets was selected to be 2.5 cm (1"). For these tests, the concentric 
fluid transponders were constructed of 20 gauge stainless steel shells. 
The results of selected "single sphere" tests are shown in FIGS. 4 through 
11. This data shows that the target strength of an individual target can 
vary widely with pulsewidth. The 1 msec and 5 msec curves may track each 
other in a general way, but considerable variations at a specific 
frequency can occur. Some variations (i.e., peaks and valleys) in the 
frequency response of these targets can be observed, but nothing 
substantial enough to be considered "tuned," or exhibiting frequency 
selectivity. 
In contrast, the results of the "concentric-fluid" tests are presented in 
FIGS. 12 through 24. The frequency response behavior for some 
concentric-sphere combinations is similar to that observed for the single 
sphere cases. In other cases, the 1 msec and 5 msec curves are more 
consistent in tracking each other. This suggests that certain 
concentric-fluid target configurations are less sensitive to pulsewidth 
variations. This could allow longer pulsewidths (e.g., 10 msec) to be used 
effectively with this type of target. 
It was observed that several concentric-fluid combinations exhibited 
dramatic variations in frequency response. The fluid combinations 
represented by FIGS. 12, 16, 18, 20, 21 and 22 show regions of frequency 
selectivity with slopes as great as 20 dB/octave. 
Several single and concentric sphere targets display a periodic, 
oscillatory response characteristic over a relatively broad frequency 
range, as evidenced in FIGS. 10, 13, and 14. The. periodic, oscillatory 
response characteristic offers the possibility of producing a passive 
target that could be "coded" to respond at a predefined series of 
frequencies. These targets, when insonified with the proper signal, would 
be uniquely identifiable. The insonifying signal could be discrete tonals, 
swept CW or white noise. 
Further examination also reveals some targets displaying various degrees of 
lowpass, highpass, notch or bandpass frequency response. These can be seen 
in FIGS. 9, 15, 17, 20, 21, 23 and 24. These characteristics could be 
used, for example, in applications requiring uniquely identifiable 
targets. 
These and other test results have shown that the outer fluid concentric 
region or layer behaves much like a "mechanical filter." This outer 
"filtering" layer can be configured to provide the target or transponder 
with varying degrees of frequency selectivity. The degree of selectivity 
(i.e., frequency bandwidth) will be determined by the various physical 
properties of the fluid, and by the outer fluid thickness relative to the 
wavelength of an impinging acoustic pulse. In this sense, it is possible 
to "groom" a particular transponder configuration to enhance frequency 
response characteristics. These include deep notches, and/or predefined 
periodic peaks, in the frequency response. Consequently, the outer fluid 
layer can be configured to provide a maximum acoustic response centered at 
a frequency, or wavelength, of interest. 
The inner sphere of fluid of the acoustic transponder of the invention 
forms an acoustic lens that focuses any acoustic energy that it receives 
from the outer fluid layer. Any acoustic energy entering the inner fluid 
is refracted (i.e., raybending) by some amount. The degree of refraction 
will be determined by the refractive properties of the fluid. With the 
proper degree of refraction, this acoustic energy will be focused on, and 
reflected from, the back surface of the inner sphere. If this focusing 
effect is optimized correctly, the energy will be reflected back along a 
path in the direction from which it came. The reflected energy will exit 
the inner sphere along this path, and be transmitted back through the 
outer fluid layer. The efficiency of the inner sphere's focusing process, 
to a large extent, determines the overall strength of the reflected 
acoustic pulse (i.e., the "target strength"). 
For operational use, a target would be "tuned" to operate with a particular 
sonar system's transmit frequency and pulsewidth. After deployment at a 
worksite or other desired location, the target would be "illuminated" by 
the sonar's acoustic transmit pulse. The pulse would be accepted by the 
"tuned" target, focused, and reflected back toward the sonar platform. The 
reflected signal would be received and processed as usual. The relatively 
high target strength at the sonar's specific operating frequency would 
allow for reliable identification of the target. Other targets in the area 
"tuned" to a different, non-overlapping frequency range would not return 
any usable amount of energy. A sonar system transmitting broadband noise 
could also be used to acoustically illuminate a target, or targets. Each 
target would return substantially more energy in that portion of the noise 
bandwidth for which it has been "tuned" to accept. A swept-frequency, or 
frequency spectrum analysis, type sonar receiver would then be able to 
identify where a particular target was located by identifying its unique 
"frequency coded" acoustic reflection. A network of targets, each "tuned" 
to a different center frequency, could be simultaneously identified and 
located using this method. 
The concentric-fluid transponder design of the invention allows for the 
construction of useful, thick-walled acoustic targets. This invention 
offers several potential advantages over the traditional, single-sphere, 
single-fluid, acoustic lens. Thick-walled targets can be fabricated by 
using various diameter, thin-walled shells to produce a desired 
outer-layer width, or thickness. Multiple combinations of refractive 
fluids offer more choices for adjusting frequency response as well as 
focusing, or target strength, performance. 
The concentric-fluid transponder has proven to be rugged and convenient to 
use. This transponder is totally passive (i.e., requires no power source), 
has relatively high target strength, requires no maintenance, and has 
essentially an omnidirectional response characteristic in all planes. This 
allows for long-term use under adverse conditions. It can be used at 
almost any relative bearing between the navigating platform and the 
target. These transponders should not be difficult to package for 
deployment and use in the ocean. 
It may be possible to increase the frequency selectivity, and/or the target 
strength, of this device by adding more fluid layers with different 
chemical and physical properties. This would require the development of a 
containment vessel with numerous containers that can be filled 
individually with fluid. Other target shapes besides spheres (e.g., 
cylinders) may also prove beneficial. 
Obviously, many modifications and variations of the invention are possible 
in light of the above teachings. It is therefore to be understood that 
within the scope of the appended claims, the invention may be practiced 
otherwise than as has been specifically described.