Sound

All sound is the invisible in the form of a piercer of envelopes. Whether it be bodies, rooms, apartments, castles, fortified cities. Immaterial, it breaks all barriers. . . . Hearing is not like seeing. What is seen can be abolished by the eyelids, can be stopped by partitions or curtains, can be rendered immediately inaccessible by walls. What is heard knows neither eyelids, nor partitions, neither curtains, nor walls. . . . Sound rushes in. It violates. 

- Pascal Quignard,  The Hatred of Music (1996) 

According to Alex Ross, sound is all the more potent because it is inescapable: it saturates a space and can pass through walls. This explains why reactions to undesirable sounds can be extreme. We are confronting faceless intruders; we are being touched by invisible hands.

It's all the more remarkable when we consider that sound is not a "thing" - it's just the movement of other things.  Sound waves pass through a wall, but no "thing" actually passes through the wall, just like when people do the wave in an arena. everyone stays in the same seat yet the wave moves the whole circle round.

Music is sound, and when music reaches your ears, there's nothing really there - it's just that your eardrums start vibrating to the same frequency as the air in the ear canal, which is vibrating at the same frequency as the air outside, which is vibrating at the same frequency as the air coming from the singers' throat, which is vibrating at the same frequency as the singer's larynx. Your ear and the musician's larynx, your body and theirs, have become entangled, have merged.  You have been violated.