A loudspeaker is a device that converts electrical signals into audible sound waves. Before electricity, amplification was obtained by using an acoustical horn of the type seen on gramophones. The invention of the speaker came with the development of transducers, devices that convert mechanical energy into electrical energy. The first loudspeakers were patented in Germany in 1877 by Ernest Wermer and in 1898 by Oliver Lodge in England. Over the next twenty-seven years there was little advancement in loudspeaker technology.
During the early days of sending messages by radio, the transmitted signal was very weak. It was necessary to wear earphones and have absolute quiet in order to hear the feeble Morse code signals. Thanks to the work of Lee de Forest and Edwin Howard Armstrong in 1912, it became possible to amplify the weak signal considerably. (De Forest and Armstrong spent twenty years suing each other over the patent rights.) With a more powerful signal available, it was possible to construct an electric loudspeaker.
In 1924 C. W. Rice and E. W. Kelley invented the first "modern" loudspeaker in which the electric signal is sent to a coil surrounded by a magnet which is attached to a circular cone (or diaphragm) suspended from a frame. The signal, moving through the magnetic field, causes the cone to move. The movement matches the variations in the electric signal, and produces sound waves in the air. Increasing the volume is accomplished by increasing the signal passing to the coil.
As technology improved, more complicated loudspeakers were devised. In 1929 Harold Stephen Black invented a way to remove the distortion that had been amplified along with the signal. Different speaker cones, corresponding to a particular frequency range, were combined to produce better fidelity than the original single cone could provide. For example, a bass unit covers the 30 to 500 hertz range, a mid-range speaker goes from 500 hertz to 4 kilohertz, and a small high-frequency tweeter is used for the treble range. Electronic crossover circuits divide the audio signal into the appropriate frequency for each speaker.
In the 1950s other types of loudspeakers were invented. The ribbon loudspeaker uses a metal ribbon in a magnetic field. An electrostatic speaker uses a taut metal plate adjacent to a fixed plate. Very high voltage sets up a mechanical force between the plates and sound radiates through perforations. In the 1990s, a very unusual looking loudspeaker was introduced. A company called NXT invented the distributed mode loudspeaker (DML) comprised of little more than a perfectly flat panel. The NXT loudspeaker is simply a stiff rectangular sheet with either a transducer attached to its back with glue or a magnet and a coil clamped to it. The rippling of the panel transmits the sound. Its developers claim that this loudspeaker is more in tune with human hearing. The future of the big-sounding NXT loudspeaker probably lies in public address systems and other large places, though this may change as the NXT loudspeaker develops. Another loudspeaker that shows promise in the near future is one that relies on ultrasonic phenomenon. Although audio technology continues to evolve, allowing for more accurate sound reproduction, the operation of most loudspeakers is not much different from those of the distant past.
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