A loudspeaker, speaker, or speaker system is an electromechanical transducer that converts an electrical signal to sound. The term loudspeaker can refer to individual transducer devices or drivers, or to complete systems consisting of an enclosure incorporating one or more drivers and electrical filter components.

A loudspeaker is a device for converting variations of electric energy into corresponding variations of acoustic energy, i.e., sound. Its functioning is therefore similar to that of a telephone receiver, except that the sound produced is much louder. In fact, the early loudspeakers were designed like large telephone receivers (Fig.1):

Alexander Graham Bell patented the first electrical loudspeaker as part of his telephone in 1876, which was followed in 1878 by an improved version from Ernst Siemens. Nikola Tesla reportedly created a similar device in 1881, but was not issued a patent. In 1898, Horace Short patented a design for a loudspeaker driven by compressed air, then sold the rights to Charles Parsons, who was issued several additional British patents before 1910. A few companies, including Victor Talking Machine Company and Pathe, produced record players using compressed-air loudspeakers. However, these designs were significantly limited by their poor sound quality and their inability to reproduce sound at low volume. The modern design of moving-coil drivers was established by Oliver Lodge in (1898). The moving coil principle was patented in 1924 by Chester W. Rice and Edward W. Kellogg.

The further development of the loudspeaker therefore had to aim at achieving, as far as possible, unrestrained vibration of the diaphragm. The first loudspeakers in which this principle was applied were constructed as shown in Fig.2: the diaphgram is a resiliently mounted paper cone which is set in motion by the armature which is energized by the speaker current which here, too, can vibrate freely in the field of a permanent magnet.

The next advance stage is represented by the dynamic loudspeakers (also known as moving-coil loudspeakers). In such speakers the armature which vibrates in the magnetic field consists of a coil attached to the conical diaphragm. In the electro-dynamic speaker (Fig.3) the moving coil oscillates inside an electromagnet which is energized with direct current, while in the permanent-magnet moving-coil speaker (Fig.4) the coil oscillates in an annular cavity of a specially-shaped permanent magnet.

All the loudspeakers described above use the electrodynamic principle for the conversion of electrical oscillations into mechanical vibrations which in turn produce sound waves in the air. Crystal loudspeakers (Fig.5) and electrostatic loudspeakers (Fig.6) are based on different principles. The crystal loudspeaker utilizes the piezo-electric effect, i.e., the phenomenon that certain crystals such as quartz, seignette salt develop and electric charge or potential difference when subjected to mechanical pressure and conversely undergo changes in thickness and thus produce mechanical forces when they are electrically charged by the application of a potential difference.

The condenser plates are perforated, so that the sound waves can emerge through them. The two last-mentioned types of loudspeaker are more particularly suitable for the reproduction of high frequencies. In high-fidelity (hi-fi) systems these speakers are used in combination with electrodynamics speakers to obtain sound-reproduction with a very high degree of accuracy.