Communications – electrical: acoustic wave systems and devices – Signal transducers – Underwater type
Reexamination Certificate
2002-09-12
2004-02-10
Pihulic, Daniel T. (Department: 3662)
Communications, electrical: acoustic wave systems and devices
Signal transducers
Underwater type
Reexamination Certificate
active
06690620
ABSTRACT:
STATEMENT OF GOVERNMENT INTEREST
The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.
BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention is directed to a sonar transducer with a tuning plate and a tuning fluid having increased maximum power which may be radiated from a given transducer by increasing the radiation resistance seen by the transducer. The present invention also is directed to providing a method for setting the mechanical resonant frequency of a sonar transducer above or below the value which is characteristic of the transducer without the tuning system.
(2) Description of the Prior Art
In general, the power which may be radiated in a fluid medium by a transducer is proportional to the product of the area of the radiating surface, the square of the surface velocity, and the characteristic impedance (sound speed times fluid density) of the medium. Thus to increase the radiated power, either the surface area, or the velocity, or both, must be increased. However, the radiating surface area of a sonar transducer is usually limited by the requirement to produce a specific directional characteristic (beam pattern). In addition, the maximum velocity which may be achieved is limited by physical constraints on the transduction material, such as maximum electrical field or maximum mechanical strain.
In a sonar transducer in which body stress in the active transduction material (such as, for example, electrostrictive or magnetostrictive materials) is used to convert electrical energy into mechanical energy, the optimum design with respect to power output and bandwidth is achieved when the characteristic mechanical impedance (density times sound speed times cross sectional area) of the transduction material is matched to (i.e. equals) that of the acoustical load. Since the characteristic impedance (density times sound speed) of the transduction material and the fluid medium are inherent material properties, an attempt to achieve the desired match of the characteristic mechanical impedances may take the form of a mechanical area transformation. In the design of a longitudinal vibrator or tonpilz such an impedance match is often approximated by employing a piston whose area is much greater than the area of the active transduction material. In particular, the characteristic impedance of typical piezoelectric ceramic materials (such as lead zirconate titanate) is in the order of 34,000,000 MKS Rayls, whereas the characteristic impedance of sea water is about 1,500,000 MKS Rayls. This would indicate an area ratio of about 23 to 1 to achieve the optimum match of the characteristic mechanical impedance. The dimensions of the radiating piston are generally dictated by directivity considerations, leaving only the area of the ceramic as a design variable. However, use of an area ratio of 23 to 1 would result in a very fragile ceramic configuration. Other considerations, such as withstanding hydrostatic pressure and explosive shock, dictate that the maximum practical area ratio is in the order of 6 to 1. This results in about a 4 to 1 mismatch between the sonar transducer and the fluid medium. For these reasons, it has never been possible to achieve a perfect match.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to provide a method for designing a transducer so that the radiation resistance seen by the transducer is increased to facilitate higher power output.
It is a further object of the present invention to provide a method for designing a sonar transducer having a mechanical resonant frequency which may be decreased or increased.
It is still a further object of the present invention to provide a smaller, lighter sonar transducer.
The foregoing objects are achieved by the method and the sonar transducer of the present invention.
In accordance with the present invention, a method for maximizing the radiated power of a transducer broadly comprises the steps of providing a transducer system comprising a transducer operating at a frequency f and having a face, a tuning fluid medium having a density of &rgr;
1
and a speed of sound c
1
, a tuning plate having a density &rgr;
p
and a thickness t spaced a distance s from the transducer face, and an external fluid medium having a density &rgr;
2
and a speed of sound c
2
; and tuning the transducer by choosing &rgr;
p
, t, and s so that
tan
(
2
π
f
s
c
1
)
=
ρ
1
c
1
2
π
f
ρ
p
t
(
1
)
Further, in accordance with the present invention, a method for changing the resonance frequency of a transducer is provided. The method comprises the steps of providing a transducer system having a transducer with a face and an operating frequency f, a tuning plate spaced from the transducer face by a distance s, and a fluid medium between the transducer face and the tuning plate having a density &rgr;
1
and a speed of sound c
1
; and decreasing the resonance frequency if the expression below is positive or increasing the resonance frequency if it is negative.
ρ
1
c
1
cot
(
2
π
f
s
c
1
)
(
2
)
The present invention also relates to a transducer system for use on a vehicle traveling through an external fluid medium having a density &rgr;
2
and a speed of sound c
2
. The transducer system broadly comprises a transducer having a radiating face and an operating frequency f, rigid wall means for positioning the transducer relative to an exterior wall of the vehicle, a tuning plate for separating the transducer from the external medium, which tuning plate has a density &rgr;
p
and a thickness t and being spaced from the radiating face by a distance s, and a tuning fluid positioned intermediate the tuning plate and the radiating face, which tuning fluid has a density &rgr;
1
and a speed of sound c
1
. The system has a complex specific acoustical impedance at the radiating face in accordance with the equation:
Z
(
s
,
f
)
=
[
(
2
π
f
ρ
p
t
)
2
ρ
2
c
2
ρ
1
c
1
+
ρ
1
c
1
ρ
2
c
2
]
ρ
1
c
1
+
i
ρ
1
c
1
cot
(
2
π
f
s
c
1
)
(
3
)
REFERENCES:
patent: 2906993 (1959-09-01), Steinberger
patent: 4364117 (1982-12-01), Snow
patent: 4694440 (1987-09-01), Ogura et al.
Kasischke James M.
Nasser Jean-Paul A.
Oglo Michael F.
Pihulic Daniel T.
The United States of America as represented by the Secretary of
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