Electroacoustic Transducer

a device that converts electric energy into acoustic energy (the energy of elastic vibrations in a medium) and vice versa. Electroacoustic transducers are called radiators or receivers depending on the direction of the conversion. They are used extensively in communications technology and sound reproduction to radiate and receive sound, in ultrasonic technology to measure and receive elastic vibrations, in sonar, and in acoustoelectronics.

The most common electroacoustic transducers are linear transducers, which satisfy the requirement for the undistorted transmission of signals, and reversible transducers, which can function both as a radiator and as a receiver and which satisfy the reciprocity principle. In most electroacoustic transducers a double energy conversion takes place (Figure 1): an electrical-mechanical conversion, as a result of which a portion of the electric energy supplied to the transducer is converted to the energy of vibrations in some mechanical system, and a mechanical-acoustic conversion, in which an acoustic field is produced in a medium as the result of the vibrations in a mechanical system.

Figure 1. Block diagram of an electroacoustic transducer: (1) electrical side, (2) mechanical oscillatory system, (3) acoustic field. Solid arrows indicate an electrical-mechanical (or mechanical-electrical) conversion; broken arrows indicate a mechanical-acoustical (or acoustical-mechanical) conversion.

Some electroacoustic transducers have no mechanical oscillatory system and produce vibrations directly in a medium, for example, electric-spark radiators, which generate intense acoustic vibrations as the result of an electric discharge in a fluid. Radiators of this type function on the principle of electrostriction of fluids. Such radiators are not reversible and are seldom used. A special class of electroacoustic transducers includes acoustic receivers (not reversible) whose operation is based on the variation of electrical resistance in a sensing element in response to sound pressure, for example, a carbon microphone or a semiconductor receiver that makes use of the elastoresistance effect (the dependence of a semiconductor’s resistance on mechanical stresses). When an electroacoustic transducer is used as a radiator, a voltage U and a current i are fed to the radiator input; they determine the sound pressure p and the vibration velocity v in the radiator’s field. When a transducer is used as a receiver, a pressure p or a vibration velocity v acts on the receiver input to produce a voltage V and a current I at the output (on the electrical side). The theoretical design of an electroacoustic transducer provides a relationship between input and output parameters.

The mechanical oscillatory systems of electroacoustic transducers include rods, plates, shells of various shapes (hollow cylinders or spheres that vibrate in various ways), and mechanical systems of more complex configuration. The vibration velocities and strains that arise in a system as the result of forces distributed throughout the volume can in turn have a fairly complex distribution. In a number of cases, however, a mechanical system may have elements whose vibrations are characterized to an adequate approximation only by kinetic and potential energies or by the energy of mechanical losses. Such elements have a characteristic corresponding to mass M, elasticity 1/C, and mechanical resistance r (a system with lumped parameters). Often a real system can be artificially reduced to an equivalent system (from the standpoint of an energy balance) with lumped parameters after determination of the equivalent mass M_eq, elasticity 1/C_eq, and frictional resistance r_m. The characteristics of a mechanical system having lumped parameters can then be calculated using the method of electromechanical analogies.

In the majority of cases of electromechanical conversion, the energy of an electric or magnetic field is converted into mechanical energy (and vice versa). Thus, the corresponding reversible electroacoustic transducers can be divided into the following groups: electrodynamic transducers, whose operating principle is based on the reaction between the current in one or more movable coils and the current in one or more fixed coils (radiators) and electromagnetic induction (receivers), such as loudspeakers and microphones; electrostatic transducers, whose operation is based on the variation in the force of attraction between plates when a voltage varies and on the variation in the charge or voltage during relative motion of the plates of a capacitor (loudspeakers and microphones); piezoelectric transducers, whose operation is based on the direct and converse piezoelectric effects (seePIEZOELECTRICITY); electromagnetic transducers, whose operation is based on the vibrations of a ferromagnetic armature in a variable magnetic field and on the variation of magnetic flux during motion of the armature; and magnetostriction transducers, which use the direct and inverse phenomena of magnetostriction.

The properties of an electroacoustic transducer functioning as a receiver are described by its sensitivity in the open-circuit mode γ_xx = V/p and by its internal impedance Z_E. Depending on the frequency dependence of V/p, a distinction is made between broadband and resonant receivers. The operation of an electroacoustic transducer functioning as a radiator is described by the sensitivity, which is equal to the ratio of p at some distance from the radiator along the axis of the directivity pattern to U or i; by the internal impedance, which constitutes the load for the source of electric energy; and by the acoustic-electric efficiency η_A/E = W_A/W_E, where W_A is the active acoustic power on the load, W_E is the active electric power input, and W_A – Z₁v₀² (v₀ is the vibration velocity of the central driving point on the radiating surface and Z₁, is the impedance of the acoustic load, which is equal to the radiation impedance Z_s when the transducer is in contact with a continuous medium). These parameters are frequency-dependent. The magnitudes of p and η_A/E attain maximum values at the frequencies of mechanical resonance; thus powerful radiators usually have resonances. The design of an electroacoustic transducer depends on its function and application and consequently exhibits great diversity.