filter vs filter bed

filter and filter bed both are nouns.

Word	Noun	Adjective	Verb	Adverb
filter	Yes	No	Yes	No
filter bed	Yes	No	No	No

As nouns, filter bed is a hyponym of filter; that is, filter bed is a word with a more specific, narrower meaning than filter:

filter: device that removes something from whatever passes through it
filter bed: filter consisting of a layer of sand or gravel for filtering water

filter (noun)	filter bed (noun)
device that removes something from whatever passes through it	filter consisting of a layer of sand or gravel for filtering water
an electrical device that alters the frequency spectrum of signals passing through it

filter (verb)	filter bed (verb)
remove by passing through a filter
pass through
run or flow slowly, as in drops or in an unsteady stream