remaining hearing in a useful and efficient manner. The first hearing aids were mechanical resonating devices that included tubes and horns of various sizes. In the early 1900s, electrical instruments replaced mechanical hearing aids. The first truly wearable electrical hearing aid was manufactured in the 1930s; in the 1950s, transistors were introduced into hearing aids. Since the 1950s, hearing aids have become smaller, and battery cost has lessened. In the 1990s, programmable digital hearing aids were developed, which allow for clearer sound quality and precise fitting paradigms.
Batteries
The power supply to the hearing aid is derived from its battery. Hearing aid batteries used currently are of 2 main types, zinc-air and mercury, although most in use today are zinc-air cells.
The primary feature of the zinc-air cell is its longer shelf life compared to the mercury- or silver-based hearing aid batteries. Zinc-air cells are not activated until a tape seal is removed from the positive side of the battery. This side contains small holes through which air enters to initiate activation. In most situations, the zinc-air cells last longer than their mercury counterparts.
Hearing aid batteries have a relatively flat discharge rate, and the battery's capacity is rated in milliampere hours (mAh). If the current drain of a hearing aid is known, an estimate of the expected life of the battery can be calculated by dividing the battery's capacity by the current drain measured in milliamperes (mA).
Conclusion
The basic components of a contemporary hearing aid include a microphone, an amplifier, a receiver, and a power supply. A microphone is a transducer that converts the sound signal into electrical energy. The amplifier is a transformer that increases the amplitude of the electrical signal that is sent to the receiver. The receiver then changes the modified electrical signal back into sound energy that is directed into the ear. Hearing aid amplifiers are transformers primarily composed of transistors that are built into an integrated circuit. These transistors provide a current source and serve a variety of functions. In these transistors, the primary function of the amplifier is to increase the power of the electrical signal received from the microphone.

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