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 Stephen C. Thompson
Graduate Program in Acoustics The Pennsylvania State University Room 201, Applied Science Building University Park, Pennsylvania 16802 USA
As We Enter the Second Century of Electroacoustics...
A look back at a century of electroacoustic transducers and systems and an attempt to look forward.
The term electroacoustics is generally understood to include the design, develop- ment, and use of devices that convert acoustical signals to electrical signals, and vice versa. To some, the term may be limited to the field of audio engineering, where it describes the use of microphones, loudspeakers, and audio recording and pro- duction techniques. This article, however, takes a broader view to include devices working in media other than air such as sonar systems, underwater acoustic com- munication, and medical ultrasound systems. It does not include biological systems, such as the human ear, that convert sound into electrical nerve impulses or electri- cal discharge systems such as lightening that produces thunder. It also limits the discussion to transducer and acoustic system design and does not include the more artistic endeavor of audio recording and production.
I believe that the modern subject of electroacoustics requires the existence of elec- tronic amplifiers to preserve at least reasonable signal fidelity. With this requirement, modern electroacoustics began with the introduction of vacuum tube electronics that could provide high-impedance preamplifiers and reasonably linear low-impedance driving amplifiers for the output devices. Vacuum tubes were invented in the first decade of the twentieth century (Fleming, 1905; De Forest, 1907), although it took another couple of decades until they achieved the aforementioned characteristics. With this definition, the history of modern electroacoustics began about a century ago. However, there was a significant prehistory of less capable devices that were ready for the improvements possible when electronic amplifiers became available.
The telegraph of the mid-1800s may actually be considered a primitive electroacous- tic device. The telegrapher’s key acts as a switch at the transmitting end to send pulses of electrical current through the circuit. At the receiving end, the current pulses pass through a coil. These current pulses magnetically actuate a mechanical device whose motion generates clicking sounds. Although this system is primarily electromechanical in operation, it is the clicking sound at the receiving end that conveys information via Morse code to the receiving operator. Thus, the telegraph may be considered an electroacoustic system, although that is a huge stretch of the modern understanding of electroacoustics.
There was an early audio recording industry before the advent of electronics. That industry and its methods and devices were covered previously in Acoustics Today by Brock-Nannestad (2016). The early telephone system was also developed with- out electronic amplifiers in the late nineteenth century. The telephone circuit was an electrical direct current loop that included a carbon microphone and a moving armature speaker in the earpiece.
©2019 Acoustical Society of America. All rights reserved. volume 15, issue 4 | Winter 2019 | Acoustics Today | 55

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