What is meant by "graduating density" in filter design?

"Graduating density" in filter design refers to the intentional arrangement of fibers with varying sizes within the filter media. This design consists of larger fibers on the air entering side of the filter and smaller fibers on the air exit side. This structure creates a gradient in the filter, allowing for efficient trapping of particles of different sizes.

Having larger fibers at the entrance side helps capture larger particles that would otherwise clog smaller fibers. As the air moves through the filter, smaller fibers positioned towards the exit side can effectively trap finer particles. This arrangement not only enhances the overall efficiency of the filter by providing multiple levels of filtration but also helps in maintaining airflow and pressure drop by preventing premature clogging.

Thus, the correct option captures the essence of graduating density by noting that it utilizes both larger and smaller fibers arranged in a strategic pattern, optimizing the filter's performance across a range of particle sizes. This approach is essential in ensuring that the filter operates efficiently for a longer duration and provides consistent air quality.