What makes music sound "scary"?

With Halloween just around the corner, I've
been contemplating what makes music scary. Some of my younger
readers may not know that for a couple of decades, movies were silent.
In other words, the audience just read the dialogue at the bottom of the
screen, and a pianist sat to the side of the screen and literally
improvised whatever music seemed appropriate to what was happening on the
screen. This was quite an art and just anyone couldn't do it.
The musician had to be able to represent not only horror and fear but also romance, humor, religious feeling and tremendous joy.

Now that movies have soundtracks, the
music that has been composed for them will be among the classics of
tomorrow. The scary movies have some of the most famous themes.
Two that come to mind immediately are the themes from "Jaws" and "Psycho."
Alfred Hitchcock's Psycho is a definite classic horror film. It's
music, by Bernard Herrmann, truly evokes fear and panic. The famous shower
scene music (the screeching violins) is parodied and copied in media all
over the world.

Of course, most of this music is in a minor
key and incorporates sudden changes of dynamics (louds and softs).
You might also hear unusual instruments such as a digiridoo or perhaps a
sitar. The purpose is to create an atmosphere that is unfamiliar; a
soundscape that disorients and confuses. Have a fun Halloween and
pay attention to the music