46 Electroacoustic Devices 46.1 Introduction 46.2 Transduction Mechanisms ctricity· Magnetostriction· Electrodynamic Electrostatic· Magnetic. Hydraulic· Fiber Optic· Parametric Transducers. Carbon Microphones 46.3 Sensitivity and Source Level 46.5 Canonical Equations and Electroacoustic Coupling Peter H. Rogers 46.6 Radiation Impedance Georgia Institute of Technology 16.7 Directivity 46.1 Introduction Electroacoustics is concerned with the transduction of acoustical to electrical energy and vice versa. Devices which convert acoustical signals into electrical signals are referred to as"microphones"or"hydrophones depending on whether the acoustic medium is air or water. Devices which convert electrical signals into acoustical waves are referred to as"loudspeakers"(or earphones) in air and"projectors"in water. 46.2 Transduction mechanisms lezoelectricity Certain crystals produce charge on their surfaces when strained or conversely become strained when placed an electric field. Important piezoelectric crystals include quartz, ADP, lithium sulphate, rochelle salt, and tourmaline. Lithium sulphate and tourmaline are"volume expanders, that is, their volume changes when objected to an electric field in the proper direction. Such crystals can detect hydrostatic pressure directly. Crystals which are not volume expanders must have one or more surfaces shielded from the pressure field in order to convert the pressure to a uniaxial strain which can be detected. Tourmaline is relatively insensitive and used primarily in blast gauges, while quartz is used principally in high Q ultrasonic transducers Certain ceramics such as lead zirconate titanate(pZt), barium titanate, and lead metaniobate become piezoelectric when polarized. They exhibit relatively high electromechanical coupling, are capable of producing very large forces, and are used extensively as sources and receivers for underwater sound. PZT and barium titanate have only a small volume sensitivity; hence they must have one or more surfaces shielded in order to detect sound efficiently. Piezoelectric ceramics have extraordinarily high dielectric coefficients and hence hig capacitance, and they are thus capable of driving long cables without preamplifier This chapter is adapted from R. M. Besancon, Encyclopedia of Physics, 3rd ed, New York: Chapman Hall, 1985 pp 337-341. With permission. c 2000 by CRC Press LLC© 2000 by CRC Press LLC 46 Electroacoustic Devices1 46.1 Introduction 46.2 Transduction Mechanisms Piezoelectricity • Magnetostriction • Electrodynamic • Electrostatic • Magnetic • Hydraulic • Fiber Optic • Parametric Transducers • Carbon Microphones 46.3 Sensitivity and Source Level 46.4 Reciprocity 46.5 Canonical Equations and Electroacoustic Coupling 46.6 Radiation Impedance 46.7 Directivity 46.1 Introduction Electroacoustics is concerned with the transduction of acoustical to electrical energy and vice versa. Devices which convert acoustical signals into electrical signals are referred to as “microphones” or “hydrophones” depending on whether the acoustic medium is air or water. Devices which convert electrical signals into acoustical waves are referred to as “loudspeakers” (or earphones) in air and “projectors” in water. 46.2 Transduction Mechanisms Piezoelectricity Certain crystals produce charge on their surfaces when strained or conversely become strained when placed in an electric field. Important piezoelectric crystals include quartz, ADP, lithium sulphate, rochelle salt, and tourmaline. Lithium sulphate and tourmaline are “volume expanders,” that is, their volume changes when subjected to an electric field in the proper direction. Such crystals can detect hydrostatic pressure directly. Crystals which are not volume expanders must have one or more surfaces shielded from the pressure field in order to convert the pressure to a uniaxial strain which can be detected. Tourmaline is relatively insensitive and used primarily in blast gauges, while quartz is used principally in high Q ultrasonic transducers. Certain ceramics such as lead zirconate titanate (PZT), barium titanate, and lead metaniobate become piezoelectric when polarized. They exhibit relatively high electromechanical coupling, are capable of producing very large forces, and are used extensively as sources and receivers for underwater sound. PZT and barium titanate have only a small volume sensitivity; hence they must have one or more surfaces shielded in order to detect sound efficiently. Piezoelectric ceramics have extraordinarily high dielectric coefficients and hence high capacitance, and they are thus capable of driving long cables without preamplifiers. 1 This chapter is adapted from R. M. Besançon, Encyclopedia of Physics, 3rd ed., New York: Chapman & Hall, 1985, pp. 337–341. With permission. Peter H. Rogers Georgia Institute of Technology