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Second Century of Electroacoustics
The problems were acknowledged and methods to reduce the symptoms were developed (Carson, 1962). However, a full treatment of the analytical methods needed to understand these problems became available in the open literature only recently (Sherman and Butler, 2007).
Advances in ultrasonic transduction using the new piezoelec- tric materials enabled significant growth of field of NDT. Its use was common, for example, in inspecting the integrity of airframes, nuclear reactor cooling pipes, and the integrity of welds in the structure of pressure vessels. Continuing research and commercial development in NDT had now broadened beyond just finding cracks and flaws in a structural element. Under the name Structural Health Monitoring, it had grown to include the continuous or periodic assessment of a struc- ture to determine the need for service, repair and eventual replacement of parts in the structure.
The use of ultrasound for medical diagnosis and treatment had also continued to grow. Biomedical acoustics had become a major industry (O’Brien, 2018).
Also, through the 1960s, the electronic computer began to take on the complicated analyses that are needed in engi- neering design. This was a significant aid in the design of the transducer elements, and it was essential to deal with the complexity inherent in the large arrays. Generally, the rel- evant computer codes did not have public distribution. One exception was the SEADUCER (Steady State Evaluation and
Analysis of Transducers) code developed by the San Diego Navy Laboratory (Ding et al., 1973). This code could provide a frequency domain analysis of the electric, mechanical and acoustic performance of a piezoelectric transducer. This and contemporary competing analysis codes at other laborato- ries and industrial design activities were the first examples of computer-aided design specifically intended for acoustic transducers. Much of the work concerning sonar transducer element and array analysis from the last decades of the twen- tieth century has been preserved in a collection assembled by Benthien and Hobbs (2005).
The Last Decades of the Twentieth Century
The dual themes of increasing capability of computational resources and the development of improved materials and manufacturing methods continues. Preexisting custom computer codes for transducer and acoustic system analysis began to be replaced by more general-purpose codes whose
development costs could be supported by a wide range of product technologies.
An example is the SPICE (Simulation Program with Integrated Circuit Emphasis) code for the analysis of increasingly complex integrated circuits being used for analog circuit models of multidomain systems including transducers. SPICE was originally written at the University of California, Berkeley (e.g., Nagel, 1975). Another example is finite element analysis (FEA) codes. Initially, these were written only for structural mechanical analysis. Now they were being broadened to allow and encourage multidomain analysis (Decarpigny et al., 1985).
A significant material improvement was the development of high-strength rare-earth magnets, culminating with neodymium-iron-boron magnets with significantly greater magnetomotive force capability. This, in combination with FEA magnetic field design methods, has enabled the design of significantly greater power-handling ability in moving-coil speaker designs. The improvements in speaker performance that were evident at the turn of the century have continued to the time of this publication. To those of us who purchased our first stereo systems in the 1960s or 1970s, the available output power level and sound clarity available in current commercial home entertainment systems seems remarkable.
By the end of the century, small electret microphones had become nearly ubiquitous in telephones and nearly all con- sumer devices. The internal noise level of small electret microphones precluded use in professional audio applica- tions, in hearing aids, and as measurement microphones where accurate, long-term calibration is necessary. In those applications, electrically biased condenser microphones or dynamic microphones continued to be used. In fact, it was not the noise or stability of the electret that created this situ- ation. Rather, it was the low-cost materials and electronic components and low-cost manufacturing methods that caused the noise and stability concerns. Electret measurement microphones have been introduced in the present decade.
Having invented the electret microphone that was now dom- inant in the market, Sessler and West, among others, went on to invent the silicon microphone as a possible replacement technology (Holm and Sessler, 1983; Lindenberger et al., 1985). This is a technology that uses the integrated-circuit manufacturing processes to build a condenser microphone
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