Electroacoustics is an engineering science combining acoustics and electrotechnology. Its object is the study, conception, and realization of technical devices concerning the production, transmission, recording, reproduction, measurement, and industrial applications of sound, which generally involve one or more transformations from acoustical to electrical energy and vice versa.
A transducer is a device that receives energy from one or several systems or media and supplies a corresponding energy, of another nature, to one or several other devices or media. Our interest here is focused on electroacoustic transducers, which can be electromechanical (conversion from electrical to mechanical energy and vice versa) and electromagnetic (conversion from electrical to magnetic energy and vice versa).
An electromechanical transducer develops a process of energy conversion, also called electromechanical coupling. A reversible coupling provides a conversion in either direction. Such a transducer is employed in driving mode when it converts electrical energy into mechanical energy and in generator mode when operating in the opposite direction.
A transducer is passive when all the energy it supplies comes exclusively from the energy it receives. When part or all the energy comes from an auxiliary source, the transducer is active.
The principal sound transmitters are:
- loudspeakers, designed to radiate acoustic power in air in the audio-frequency range;
- earphones or headphones, designed to be closely coupled acoustically to the ear.
The principal sound receivers are:
- microphones, meant for airborne sounds in the audio-frequency and ultrasound range up to approx. 80kHz.
A real world application
The Compact Disc (CD), a product made by the electroacoustic industry for the general public, is a well-known recording medium for sound. It involves the setting up of a cascade of processes: the recording of a master medium, the manufacture of the glass master, the moulding of the CDs, the diffusion by a distribution and sales network, and eventually the reproduction.
For the recording, the chain typically comprises:
- a studio, where the musicians, soloists, chorus, etc, gather;
- one or several microphones;
- one or several analog to digital converters;
- a control desk or mixing console;
- a recorder, which is very often a multitrack recorder;
- a digital master recorder
At the glass mastering, there is:
- a digital master player;
- a Laser Beam Recording (LBR) machine
The reproduction chain, comprises:
- a CD player with internal or external digital to analog converter;
- an amplifier with different equalizers and adjustments;
- 2 or more loudspeakers;
- a place where the listeners are
The quality of the sound obtained when playing back a CD depends on all the devices and components in the different chains, as well as the recording and listening conditions. Consequently, the specifications of each device, apparatus, or transducer are above all a result of the stipulated requirements. These requirements are themselves fixed, taking into account the effect of the device on the overall quality.
Technical, technological, and, finally, commercial demands will also intervene, linked to the accepted price and the type of market chosen. The first problem to solve when designing an electroacoustic device is to draw up its specifications. It is often a difficult problem that requires the definition of quality and performance criteria, which, in turn, depend on the anticipated use of the device.
For example, the specifications of a loudspeaker are the results of the examination of the anticipated listening conditions, technological constraints, and economic considerations. Drawing up such specifications is a matter of answering questions such as: what is the lowest frequency the loudspeaker must be able to reproduce, taking into account its technological feasibility, its use, its effect on other characteristics, and its cost?