Passive filters are electronic circuits that allow certain frequencies of signals to pass while blocking others. They are made up of passive components like resistors, capacitors, and inductors, which do not require an external power source to operate. Common types of passive filters include low-pass, high-pass, band-pass, and band-stop filters, each designed to manage specific frequency ranges. These filters are widely used in various applications, such as audio systems and radio communications, to improve sound quality or reduce noise. By carefully selecting the values of the components, engineers can design passive filters to meet the needs of specific devices, ensuring optimal performance in systems like speakers and transmitters.