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structure into the silicon wafer. It was conceived as a way to integrate the mechanical structure of the microphone and the electronic components of the buffer amplifier onto the same silicon chip. The silicon microphones built during this time period had low-sensitivity and/or low-manufacturing yield that kept them from being competitive as production products. It would take about two decades from the initial concept publication for anyone to develop a viable com- mercial product.
Entering the Twenty-First Century
Much of the progress in the last two decades has been to consolidate the advances that were evident at the close of the last century. On the computational front, FEA codes have continued to advance and to provide enhanced multidomain analysis features and to provide significantly enhanced graph- ical user interfaces that make it easier for the design experts to use the codes. Several FEA codes now included the abil- ity to simultaneously model several physical domains (e.g., mechanical, magnetic, and electric domains) and to include the relevant interdomain coupling equations.
Analyses that might previously have been done with analog circuits and SPICE are now done with greater flexibility using new analysis languages. This activity started with the devel- opment of the Modelica programming language specifically intended for modeling complex physical systems in several domains. The continued development of this language is managed by The Modelica Association, with information available at modelica.org. Commercial and freely available Modelica simulation environments are available (Modelica Association, 2019). A similar modeling environment called Simscape is available from MathWorks (2019).
Continuing improvements have already been mentioned for moving-coil speakers in performance venues and home theater applications. The performance available in many com- pact, battery operated Bluetooth-connected speakers is an example that may be familiar to current readers.
The audio quality delivered by smartphones has also made significant advances in the last decade. Much of the improve- ment likely comes from careful analysis of sound generation by the microspeakers employed and from equally careful analysis of the sound propagation through the small passages and orifices in the device. Similar developments have also been made in the design of earphones and in-ear monitors for stage musicians. Manufacturers of microspeakers have
responded to new requirements by developing devices with a wider range of performance parameters to better match the demands of manufacturers of smartphones and earphones.
Another important development is the commercial availabil- ity of single-crystal piezoelectric material with a very high electromechanical coupling coefficient. Although this mate- rial development was initially funded by the Office of Naval Research for naval applications, its first broad commercial application is in medical ultrasound transducers. The pri- mary advantages in these devices are smaller size and wider transducer bandwidth.
Biomedical applications of ultrasound have continued to expand and improve as Acoustics Today has occasionally reported. These improvements include the use of micro- bubbles to improve image contrast (Matula and Chen, 2013), therapeutic uses of acoustically driven microbubbles (Gray et al., 2019), higher resolution systems operating at higher ultrasonic frequencies (Kettering and Silverman, 2017), and the use of ultrasound to aid in the transport of therapeutic agents across the blood-brain barrier (Konofagou, 2017).
The silicon microphone was introduced as a commercial product in 2004. Current terminology places the silicon microphone in the category of microelectromechanical system (MEMS) devices. Initial sales volumes of the MEMS microphones increased rapidly as mobile phone manufac- turers quickly switched their production away from electret microphones. A primary initial advantage was the ability of automated soldering methods in the production to connect the microphones to the circuit board. Those methods are used for all other components in the device, but they could not be used with electret microphones that need to be soldered in place by a manual operation because automated soldering would damage or destroy the low-cost electret microphones.
MEMS microphones are also generally smaller than other microphone types, including the miniature electret micro- phones that had previously been used in hearing aids and earphones. Their small size makes them suitable for use arrays because they can be made into a compact spherical array of microphones that are useful in measuring the three- dimensional nature of the sound field.
The term ambisonics has been used for this type of measure- ment since the mid-1970s (Fellgett, 1973). Initially, Fellgett proposed using four microphones arranged in a tetragonal
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