ecently, it has been discovered that certain polymers, notably polyvinylidene fluoride, are piezoelectric when stretched. Such piezoelectric polymers are finding use in directional microphones and ultrasonic hydrophones. Magnetostriction Some ferromagnetic materials become strained when subjected to a magnetic field. The effect is quadratic in the field, so a bias field or dc current is required for linear operation. Important magnetostrictive metals and alloys include nickel and permendur. At one time, magnetostrictive transducers were used extensively in active sonars but have now been largely replaced by ceramic transducers. Magnetostrictive transducers are rugged and reliable but inefficient and configurationally awkward. Recently, it has been discovered that certain rare earth iron alloys such as terbium-dysprosium-iron possess extremely large magnetostriction(as much as 100 times that of nickel). They have relatively low eddy current losses but require large bias fields, are fragile, and have yet to find significant applications. Metallic glasses have also recently been considered for magneto- strictive transducers electrodynamic ectrodynamic transducers exploit the forces produced on a current-carrying conductor in a magnetic field and, conversely, the currents produced by a conductor moving in a magnetic field. Direct radiation moving coil transducers dominate the loudspeaker field. Prototypes of high-power underwater projectors have been constructed using superconducting magnets. Electrodynamic microphones, particularly the directional ribbon microphones, are also common Electrostatic Electrostatic sources utilize the force of attraction between charged capacitor plates. The force is independent of the sign of the voltage, so a bias voltage is necessary for linear operation. Because the forces are relatively weak, a large area is needed to obtain significant acoustic output. The effect is reciprocal, with the change in the separation of the plates(i.e, the capacitance) produced by an incident acoustic pressure generating a voltage The impedance of a condenser microphone, however, is high, so a preamplifier located close to the sensor is required Condenser microphones are very flat and extremely sensitive. The change in capacitance induced by an acoustic field can also be detected by making the capacitor a part of a bridge circuit or, alternatively, a part of an oscillator circuit. The acoustic signal will then appear as either an amplitude or frequency modulation of some ac carrier. The charge storage properties of electrets have been exploited to produce electrostatic microphones which do not require a bias voltage Magnetic Magnetic transducers utilize the force of attraction between magnetic poles and, reciprocally, the voltages produced when the reluctance of a magnetic circuit is changed. Magnetic speakers are used extensively in telephone receivers. Hydraulic Nonreversible, low-frequency, high-power underwater projectors can be constructed utilizing hydraulic forces acting to move large pistons. Electroacoustic transduction is achieved by modulating the hydraulic pressure with a spool valve actuated by an electrostrictive(PZt)stack. Fiber Optic An acoustic field acting on an optical fiber will change the optical path length by changing the length and index of refraction of the fiber. Extremely sensitive hydrophones and microphones can be made by using a fiber exposed to an acoustic field as one leg of an optical interferometer. Path length changes of the order of 10-6 optical wavelengths can be detected. The principal advantages of such sensors are their configurational flexibility, c 2000 by CRC Press LLC© 2000 by CRC Press LLC Recently, it has been discovered that certain polymers, notably polyvinylidene fluoride, are piezoelectric when stretched. Such piezoelectric polymers are finding use in directional microphones and ultrasonic hydrophones. Magnetostriction Some ferromagnetic materials become strained when subjected to a magnetic field. The effect is quadratic in the field, so a bias field or dc current is required for linear operation. Important magnetostrictive metals and alloys include nickel and permendur. At one time, magnetostrictive transducers were used extensively in active sonars but have now been largely replaced by ceramic transducers. Magnetostrictive transducers are rugged and reliable but inefficient and configurationally awkward. Recently, it has been discovered that certain rare earth iron alloys such as terbium-dysprosium-iron possess extremely large magnetostrictions (as much as 100 times that of nickel). They have relatively low eddy current losses but require large bias fields, are fragile, and have yet to find significant applications. Metallic glasses have also recently been considered for magneto￾strictive transducers. Electrodynamic Electrodynamic transducers exploit the forces produced on a current-carrying conductor in a magnetic field and, conversely, the currents produced by a conductor moving in a magnetic field. Direct radiation moving coil transducers dominate the loudspeaker field. Prototypes of high-power underwater projectors have been constructed using superconducting magnets. Electrodynamic microphones, particularly the directional ribbon microphones, are also common. Electrostatic Electrostatic sources utilize the force of attraction between charged capacitor plates. The force is independent of the sign of the voltage, so a bias voltage is necessary for linear operation. Because the forces are relatively weak, a large area is needed to obtain significant acoustic output. The effect is reciprocal, with the change in the separation of the plates (i.e., the capacitance) produced by an incident acoustic pressure generating a voltage. The impedance of a condenser microphone, however, is high, so a preamplifier located close to the sensor is required. Condenser microphones are very flat and extremely sensitive. The change in capacitance induced by an acoustic field can also be detected by making the capacitor a part of a bridge circuit or, alternatively, a part of an oscillator circuit. The acoustic signal will then appear as either an amplitude or frequency modulation of some ac carrier. The charge storage properties of electrets have been exploited to produce electrostatic microphones which do not require a bias voltage. Magnetic Magnetic transducers utilize the force of attraction between magnetic poles and, reciprocally, the voltages produced when the reluctance of a magnetic circuit is changed. Magnetic speakers are used extensively in telephone receivers. Hydraulic Nonreversible, low-frequency, high-power underwater projectors can be constructed utilizing hydraulic forces acting to move large pistons. Electroacoustic transduction is achieved by modulating the hydraulic pressure with a spool valve actuated by an electrostrictive (PZT) stack. Fiber Optic An acoustic field acting on an optical fiber will change the optical path length by changing the length and index of refraction of the fiber. Extremely sensitive hydrophones and microphones can be made by using a fiber exposed to an acoustic field as one leg of an optical interferometer. Path length changes of the order of 10–6 optical wavelengths can be detected. The principal advantages of such sensors are their configurational flexibility