If a scientist says, "The sound I hear is caused by air vibrations", he is talking about an existent effect, namely a sound.
(a scientist; says; ) (The sound; hear; I) (The sound I hear; is caused; by air vibrations) (he; is talking; about an existent effect)
a scientist The sound The sound I hear he
says hear is caused is talking
I
by air vibrations
about an existent effect