All the varieties of passive filters can also be found in active filters.

Another common piezoelectric material is quartz, which has also been used in mechanical filters.

This was the first development in sound filters.

It is unknown how long for certain the material was used in filters.

This approach did not find any immediate applications, and is not common in passive filters.

It was often used in mechanical filters before quartz.

I have that in filters, you know, in camera filters.

Simple gravel and other aggregates have been used successfully in filters.

Hence they are very popular in filters of many types.

What is activated charcoal and why is it used in filters?