Public address system

A public address system (PA system) is an electronic sound amplification and distribution system with a microphone, amplifier and loudspeakers, used to allow a person to address a large public, for example for announcements of movements at large and noisy air and rail terminals.

The term is also used for systems which may additionally have a mixing console, and amplifiers and loudspeakers suitable for music as well as speech, used to reinforce a sound source, such as recorded music or a person giving a speech or distributing the sound throughout a venue or building.

Simple PA systems are often used in small venues such as school auditoriums, churches, and small bars. PA systems with many speakers are widely used to make announcements in public, institutional and commercial buildings and locations. Intercom systems, installed in many buildings, have microphones in many rooms allowing the occupants to respond to announcements.

Sound reinforcement systems and PA systems may use some similar components, but with differing application, although the distinction between the two is not clear-cut. Sound reinforcement systems are for live music or performance, whereas PA systems are primarily for reproduction of speech.In Britain any PA system is sometimes colloquially referred to as a Tannoy, after the company of that name now owned by TC Electronic Group, which supplied a great many of the PA systems used in the past.

Early systems

Early public-address system from around 1920 using a Magnavox speaker. The microphone (then called a "transmitter") had a metal reflector which concentrated the sound waves, allowing the speaker to stand back so it wouldn't obscure his face. The early vacuum tubes couldn't produce much gain, and even with six tubes the amplifier had an output power of only about 10 watts. To produce enough volume, the system used a horn loudspeaker. The cylindrical driver unit under the horn contained the diaphragm which was vibrated by the voice coil, and the sound waves produced were conducted to the open air through the flaring horn. The function of the horn was to couple the diaphragm more efficiently with the air, so horn speakers produced far more sound power from a given amplifier than a cone speaker. Horns were used in virtually all early PA systems, and in most systems today.
Edwin Jensen and Peter Pridham of Magnavox began experimenting with sound reproduction in the 1910s; working from a laboratory in Napa, California, they filed the first patent for a moving coil loudspeaker in 1911.[3] Four years later, in 1915, they built a dynamic loudspeaker with a 1-inch (2.5 cm) voice coil, a 3-inch (7.6 cm) corrugated diaphragm and a horn measuring 34 inches (86 cm) with a 22-inch (56 cm) aperture. The electromagnet created a flux field of approximately 11,000 G.[3]