The miniaturisation of microphones to fit into the housing of a phone was made possible by the advances in electronics and materials engineering over the decades. The following are the factors that contributed to this process:

1. The development of silicon MOSFETs (metal-oxide-semiconductor field-effect transistors) in the late 1950s and early 1960s, which enabled the fabrication of integrated circuits (ICs) with high transistor density, low power consumption, and fast performance12. This allowed the reduction of the size and complexity of the audio hardware and circuitry needed to process and amplify the microphone signals.
2. The invention of electret microphones in the early 1960s, which use a permanently charged material (electret) to generate an electric field that interacts with a thin metal diaphragm3. Electret microphones have several advantages over conventional dynamic microphones, such as higher sensitivity, lower noise, lower cost, and smaller size. They are widely used in phones and other devices that require compact and reliable microphones.
3. The improvement of micro-electromechanical systems (MEMS) technology in the late 1990s and early 2000s, which enabled the fabrication of microphones on silicon chips using micro-machining techniques. MEMS microphones have several advantages over electret microphones, such as higher integration, lower power consumption, better immunity to electromagnetic interference, and greater robustness. They are also compatible with IC fabrication processes, which allows for mass production and lower cost.
4. The innovation of piezoelectric microphones in the 2010s, which use a thin film of piezoelectric material (such as zinc oxide) to convert sound waves into electrical signals. Piezoelectric microphones have several advantages over MEMS microphones, such as higher sensitivity, wider frequency response, lower distortion, and lower noise. They are also flexible and can be fabricated on various substrates, such as plastic or paper.

In short, the miniaturisation of microphones to fit into the housing of a phone was achieved by the progress in electronics and materials engineering that enabled the development of different types of microphones with smaller size and better performance.
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