Science of sound (9)
I believe the answer is:
acoustics
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">science of sound</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for acoustics that I've seen before include "Science of sound" , "So caustic about properties of sound" , "The study of the properties of sound" , "CIA scouts seeking properties of sound" .)