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Second Century of Electroacoustics
enabled the design of balanced armature and moving coil speakers without the static power dissipation of field coils.
This time period also saw the first use of ultrasound transmis- sion for commercial applications (see the article in this issue of Acoustics Today by Suslick on the history of ultrasonics). Firestone was awarded a patent (1942) for the concept of “flaw detection” in solids using ultrasound. This work would grow into the field now known as nondestructive testing (NDT) that searches for flaws or imperfections in solid parts and welds in a way that does not damage or affect the perfor- mance of the parts so that they can continue to be used. The first application of medical imaging using ultrasound was in 1956 in Glasgow, Scotland. Both of these topics remain active areas of research and continued product development.
This time period also greatly expanded the development of the analysis methods considered standard today. Analog circuit models of transducer structures have multiple advan- tages, at least for electrical engineers, in merging seamlessly with the electrical transmission lines and filters that were part of most electroacoustic systems and with the thought pro- cess and design intuition of system designers. By midcentury, these methods were included in textbooks (e.g., Olson, 1947, 1958; Beranek, 1954; Hunt, 1954). These analysis methods, of course, preceded the development of modern computers but enabled the design and development of recording and play- back devices of consistently improving quality and fidelity.
This was also a time period that saw the early development of undersea systems. World War I had seen the employ- ment of German U-boats with devastating consequences to shipping lanes. Some of the first hydrophones in that time period employed carbon microphones in watertight housings that coupled the acoustic pressure through flex- ible membranes. Like telephone microphones, these early hydrophones did not have electronic amplification but could be operated with the static current from a battery. During World War II, simple devices evolved into complete active and passive sonar systems, with transducers based on magnetostriction in nickel.
By the end of the 1950s, the world of acoustic transduction had entered a state that would be mostly recognizable, if viewed as somewhat quaint, by the students of today. Every- thing was quite large by current sensibility, and essentially nothing could be powered by batteries. However, recogniz- able predecessors of the devices used today were generally
available as seen in Figure 3. The Brüel & Kjær (B&K) series of condenser measurement microphones had just been intro- duced. Vacuum tube electronics had advanced considerably as broadcast television grew. Laboratory electronic equip- ment included the Hewlett-Packard audio oscillator and Tektronix oscilloscopes. Moving coil loudspeakers had the general appearance of more modern devices. Long-playing vinyl recordings (LPs) were no longer new, and stereophonic recordings had recently been released.
Underwater transduction also saw significant advances after the end of World War II. Activities during the war had identified the need for far greater capability in naval sonars. Piezoelectric material developments provided vast improvements in sensitivity and power handling capability for underwater transducers (Berlincourt et al., 1964). By the end of this period, the ceramic material lead zirconate tita- nate, usually called PZT, was taking over many applications in underwater transduction. PZT has higher performance than other the piezoelectric materials due to its relatively high electromechanical coupling coefficient. As a ceramic, it can also be manufactured in a wide range of sizes and shapes to be used in a wide variety of transducer configurations. The development of naval high-power sonar arrays was then just in its infancy, but many of the early developments quickly migrated to the use of PZT.
Perhaps the single event that had the greatest technological impact in the second half of the twentieth century was the invention of the transistor in 1947. The development of transis- tor electronics and the nascent use of computers for computer aided design set the stage for another wave of progress in acoustic transduction. Of course, advances in materials and materials processing also continued to be important.
Progress from the 1960s Through the 1980s
Throughout the middle of the twentieth century, telephone companies continued to use carbon microphones in their handsets because they were small, rugged, inexpensive, and good enough for the telephone system. Small condenser microphones might have provided better performance, but the need for a large external bias voltage precluded their use.
That limitation was removed when Sessler and West iden- tified a suitable material and a manufacturing process for making an electret that could retain its charge indefinitely (see West, 1988, for a discussion of this discovery). The word electret had been used for a long time to mean a material
58 | Acoustics Today | Winter 2019

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