Multilayer acoustic impedance converter for ultrasonic transducers

An ultrasonic transducer comprising:

• a piezoelectric element having a characteristic acoustic impedance;
• a front acoustic impedance converter coupled to said piezoelectric element, said front acoustic impedance converter comprising:
• a front polymer layer having a thickness tp1 coupled to said piezoelectric element;
• and
• a front metal layer for transmitting acoustic energy between said front polymer layer and a propagation medium having a characteristic acoustic impedance, said front metal layer having a thickness tm1 and being coupled to said front polymer layer,
• wherein said characteristic acoustic impedance of said propagation medium is lower than said characteristic acoustic impedance of said piezoelectric element,
• wherein said front acoustic impedance converter has an effective characteristic acoustic impedance ZC between said piezoelectric element and said propagation medium characteristic acoustic impedances, and
• wherein said thicknesses tm1 and tp1 are substantially:
• tm1=ZC/(ρm2πfo);
• and
• tp1=Vp2ρp/(2πfoZC),
• wherein, Zc is said effective characteristic acoustic impedance of said front acoustic impedance converter,
• ρm is the density of the front metal layer,
• fo is the predetermined center resonant frequency,
• Vp is the velocity of sound in the front polymer layer, and
• ρp is the density of the front polymer layer.

An ultrasonic transducer comprising:

• a piezoelectric element having a characteristic acoustic impedance;
• a front acoustic impedance converter coupled to said piezoelectric element, said front acoustic impedance converter comprising:
• a front polymer layer having a thickness tp1 coupled to said piezoelectric element;
• and
• a front metal layer for transmitting acoustic energy between said front polymer layer and a propagation medium having a characteristic acoustic impedance, said front metal layer having a thickness tm1 and being coupled to said front polymer layer,
• a backing absorber coupled to said piezoelectric element, said backing absorber having a characteristic acoustic impedance, and
• a back acoustic impedance converter interposed between said backing absorber and said piezoelectric element,
• wherein said characteristic acoustic impedance of said propagation medium is lower than said characteristic acoustic impedance of said piezoelectric element, and
• wherein said front acoustic impedance converter has an effective characteristic acoustic impedance ZC between said piezoelectric element and said propagation medium characteristic acoustic impedances.

An ultrasonic transducer comprising:

• a piezoelectric body having a characteristic acoustic impedance and having first and second surfaces;
• and
• an acoustic impedance converter coupled to said first surface of said piezoelectric body, said acoustic impedance converter comprising:
• a polymer layer having a generally uniform thickness tp1 and having first and second surfaces, said first surface of said polymer layer being coupled to said first surface of said piezoelectric body;
• and
• a metal layer for being in contact with a propagation medium having a characteristic acoustic impedance, said metal layer having a thickness tm1 and having first and second surfaces, said first surface of said metal layer being coupled to said second surface of said polymer layer,
• wherein said characteristic acoustic impedance of said piezoelectric body is higher than said characteristic acoustic impedance of said propagation medium,
• wherein said acoustic impedance converter has an effective characteristic acoustic impedance ZC between said piezoelectric body and said propagation medium characteristic acoustic impedances, and
• wherein said thicknesses tm1 and tp1 are selected based on the densities of said metal layer and said polymer layer, said effective characteristic acoustic impedance ZC, a center resonant frequency of said ultrasonic transducer and the velocity of sound in said polymer layer.

An ultrasonic transducer comprising:

• a piezoelectric body having a characteristic acoustic impedance and having first and second surfaces;
• and
• an acoustic impedance converter coupled to said first surface of said piezoelectric body, said acoustic impedance converter comprising:
• a polymer layer having a thickness tp1 and having first and second surfaces, said first surface of said polymer layer being coupled to said first surface of said piezoelectric body;
• and
• a metal layer for being in contact with a propagation medium having a characteristic acoustic impedance, said metal layer having a thickness tm1 and having first and second surfaces, said first surface of said metal layer being coupled to said second surface of said polymer layer,
• wherein said characteristic acoustic impedance of said piezoelectric body is higher than said characteristic acoustic impedance of said propagation medium,
• wherein said acoustic impedance converter has an effective characteristic acoustic impedance ZC between said piezoelectric body and said propagation medium characteristic acoustic impedances,
• wherein said thicknesses tm1 and tp1 are selected based on the densities of said metal layer and said polymer layer, said effective characteristic acoustic impedance ZC, a center resonant frequency of said ultrasonic transducer and the velocity of sound in said polymer layer, and
• wherein said thicknesses tm1 and tp1 are substantially:
• tm1=ZC/(ρm2πfo);
• and
• tp1=Vp2ρp/(2πfoZC),
• wherein, ZC is said effective characteristic acoustic impedance of the acoustic impedance converter,
• ρm is the density of the metal layer,
• fo is the center resonant frequency,
• Vp is the velocity of sound in the polymer layer, and
• ρp is the density of the polymer layer.

A method for forming an ultrasonic transducer comprising the steps of:

• providing a piezoelectric body having a characteristic acoustic impedance;
• providing a backing absorber coupled to said piezoelectric body, said backing absorber comprising a low characteristic acoustic impedance material having a characteristic acoustic impedance lower than said characteristic acoustic impedance of said piezoelectric body;
• providing a polymer layer having a thickness tp1 coupled to the piezoelectric body;
• providing a metal layer for transmitting ultrasonic energy between a propagation medium having a characteristic acoustic impedance and said piezoelectric body through said polymer layer and having a thickness tm1 coupled to said polymer layer,
• wherein said polymer layer is arranged intermediate said piezoelectric body and said metal layer such that there is no contact between said piezoelectric body and said metal layer,
• wherein said characteristic acoustic impedance of said piezoelectric body is higher than said characteristic acoustic impedance of said propagation medium, and
• wherein said polymer layer and said metal layer define an impedance converter having a predetermined effective characteristic acoustic impedance intermediate that of said piezoelectric body and propagation medium characteristic acoustic impedances.

An ultrasonic transducer comprising:

• a piezoelectric element having a characteristic acoustic impedance;
• a front acoustic impedance converter coupled to said piezoelectric element, said front acoustic impedance converter comprising:
• a front polymer layer having a thickness tp1 coupled to said piezoelectric element;
• and
• a front metal layer for transmitting acoustic energy between said front polymer layer and a propagation medium having a characteristic acoustic impedance, said front metal layer having a thickness tm1 and being coupled to said front polymer layer, and
• a backing absorber coupled to said piezoelectric element by means of a back polymer layer substrate having a first planar surface on which is disposed an inner metal layer adapted as a signal trace for feeding current to the piezoelectric layer, and a second planar surface opposite the first planar surface and on which is disposed an outer metal shielding layer that is coupled to said backing layer,
• wherein said back polymer layer substrate and said outer metal shielding layer define a back impedance converter interposed between said backing absorber and said piezoelectric element,
• wherein said characteristic acoustic impedance of said propagation medium is lower than said characteristic acoustic impedance of said piezoelectric element, and
• wherein said front acoustic impedance converter has an effective characteristic acoustic impedance ZC between said piezoelectric element and said propagation medium characteristic acoustic impedances.