What Makes a Horror Movie Scary?

This may seem a rather random and unexpected post, but I am genuinely wondering this. In fact, it was yesterday, when I was traipsing down the road to my house down the longest road to man kind (I truly believe it deserves a record) that I stumbled on this topic. I should probably explain now, these long walks I go on sometimes are where I get these blog ideas and things like that from, so blame my rambles on bus diversions (damn you Bozza). Anyway, for anyone who knows me well enough to have an idea about my music collection, which truthfully, isn't many people, they will know that I have a borderline addiction to soundtrack music. Currently on my iPod I have these sountracks:

Doctor Who (all of them from the reboot onwards so around seven albums)
Now You See Me
Two Steps from Heaven
Harry Potter
Glee (would we call Glee a soundtrack?)
Narnia
The Great Gatsby
The 2012 Olympics (no seriously)
Pitch Perfect

So, as you can see, I think I'm pretty qualified to talk about soundtracks. I may not have a degree in music, but I'm a full blown soundtrack junkie. It's almost embarrassing. Anyway, my big main question is this: what is it that makes a horror movie scary. Is it the acting (which nine times out of ten is more wooden than an oak tree)? Is it the special effects? Is it the fact that people completely and 100% buy in to the most ridiculous plots? Or, is it the music?

I like to believe it is the music, and I think I can prove this quite simply. Next time you're watching a horror movie, and it gets to the terrifying moment where the heroine is neatly/brutally hacked to death, and turn the volume down. Put on the subtitles, but see if the scene still scares you. I think you'd be surprised.....

Oh, and just to prove my point further, try listening to this little gem alone in your house without screaming and running out the room. Bet you can't.