What material is commonly used in filters to reject unwanted signals?

The use of quartz crystals in filters is pivotal due to their unique piezoelectric properties, which enable them to resonate at specific frequencies. When incorporated into electronic filters, quartz crystals help to define the precise frequency at which the filter operates, allowing desired signals to pass while effectively rejecting unwanted signals. This selectivity is crucial in applications like communication systems where maintaining signal integrity and minimizing noise is essential.

In contrast, other materials listed do not possess the same characteristics for filtering signals. Copper is primarily used for conducting electric currents rather than signal filtering. Silicon is commonly used in transistors and diodes, serving different functions within electronic circuits. Plastic, while useful in many applications, does not have the necessary properties for effective signal rejection in the context of filters. Thus, quartz crystals stand out as the optimal choice for this purpose.