Reverberation (4)
'reverberation' is the definition.
(I know that reverberation can be written as echo)
This is the entire clue.
(Other definitions for echo that I've seen before include "Reminder" , "copy" , "Audio effect" , "Response" , "Mythical nymph" .)
(Other definitions for resonance that I've seen before include "Plangency" , "Sonority" , "Deep, clear, ringing quality" , "shaking" , "continuation of noise" .)