Editing Guide to technical writing (section)

===Resonance frequency or resonant frequency?===
"Frequency" is a noun, "resonance" is a noun, and "resonant" is an adjective.  An adjective describes a noun. If we say "resonant frequency" then we are describing the frequency with an adjective. But "resonant" is the description of the circuit, not the frequency.  So the correct expression is "resonance frequency" which is a compound noun.   So the whole expression is the name of the type of frequency.   

Another example, to show what we have said makes sense is to consider the two nouns "fire" and "fly" (as in an insect).  Putting the two together is then the name of a type of fly called the "fire fly."   Notice we are not trying to describe the fly as being on fire, so we are not using any adjectives here. We are using two nouns and this is a compound noun that names the type of fly. This explains why "resonant frequency" is wrong, and "resonance frequency" is right.

Now I often forget myself, so a good trick to remember is to note that "resonance frequency" can be interchanged to "frequency of resonance," which means the same thing. But if you were to incorrectly say "resonant frequency," the interchange to "frequency of resonant" of course sounds completely wrong.