Sentence:  I noticed ... that when a very cheesy synthesized violin sound plays in <WSD>counterpoint</WSD> with a real violin, it can quite convincingly seem as if two violins are playing. 
Meaning of the word marked with <WSD> tags:  A melody added to an existing one, especially one added to provide harmony whilst each retains its simultaneous identity; a composition consisting of such contrapuntal melodies.