How shall I phrase this? I have an idea for a story but it depends upon there being something, anything, that can protect against or distort Infrasound.
Basically in my story there’s going to be a ghost problem. A character is going to invent a device to stop people seeing the ghosts.
For some reason I decided to research possible causes of ghosts and discovered Infrasound. Apparently it produces a sensation of fear in people and at a frequency of 19 Hertz may cause hallucinations.
Being quite interested in audio over the years, I've read a bit about low frequencies (among other frequencies, of course). There are some interesting effects of tones at frequencies below the accepted range of human hearing - the effects aren't really on the ears so much as on the whole body, so even deaf people should be affected.
Perfect I thought. That’s how the ghosts operate. Now what blocks Infrasound?
There’s the problem. Nothing blocks Infrasound.
I’m ok with that.
Mostly true, but not quite - a sealed room (where air can't go in or out) with a closed, tight-fitting door stops infrasound, especially if each wall and the ceiling are made with several layers of drywall. That's how they isolate sound in recording studios. But that gets expensive, and that's one reason recording studios charge so much money - so they can record that sensitive flute or acoustic guitar music without also recording the dump truck or SUV with the 1200 watt bass speaker driving by outside.
A car with the windows rolled up is a fair approximation of a "sealed room.," but I guess that can't be part of your story. On the other hand, a car going down the road with one or both windows down CAN generate high-volume infrasonics - if you think of the car cab as being a huge whistle with the wind blowing by the window, that's exactly how the infrasound is generated.
But can any of you come up with a theoretical something which might plausibly ever interfere with Infrasound. And plausibly is the word here. It doesn’t have to work. It can be crazy Science. Just give me a little room to manoeuvre and I will be forever grateful. 
Noise-canceling headphones, tuned to 19 Hz.
They actually work very well for cancelling what goes into the ear canal, but you can still FEEL such a low frequency (if it's of significant volume) in your body. The "accepted" range of human hearing is 20Hz to 20kHz, though each end varies slightly between people (especially the high end drops off as one ages). And feeling it on/in one's body is really the problem.
Here's a couple of links
http://forum.sneflecteryuo.com/showthread.php?t=890
(I'd suggest disregarding the responses in that thread, which refer to loud sound well within the range of human hearing used for annoyance value - yes it can be annoying, but that's not what we're talking about here)
http://blogcritics.org/archives/2003/09/09/121851.php
I recall discussion that in some large churches, when the preacher reaches some critical points he wants to make, the organist plays one or more of the lowest pedals, activating pipes that output in the infrasonic range, causing the "willies" one of the above articles mentions, and thus making the preacher's words more effective as they "hit home."
Here's one more article discussing infrasound, and mentioning Tesla's vibrating platforms (a device you stand on, with motor and a cam arangement to make the platform move up and down at about nine times per second - 9 Hz):
http://www.geocities.com/Area51/Shadowlands/6583/project458.html
The platform DIRECTLY COUPLES the vibrations into the body through the feet, and likely could cause diarehha, as well as the other serious health problems indicated.
The "brown tone" (9 Hz, and named because of this alleged effect) in air probably cannot generate a high enough intensity in the body to cause this, and the idea that a tone at that frequency can directly cause diarehha seems to be an urban legend. But still, the other effects, the generation of feelings of "general unease" appear well documented.
Anyway ... you were asking if there were something to "protect against" this. I recall one of the above links mentions ghosts, and something about a mechanical device that changes the frequency of a naturally-occuring infrasonic device seemed to make them go away.
But also, I can imagine a large speaker, a woofer in a large cabinet and run by a large audio power amplifier, could be wired to a microphone that detects infrasound and then generates the same infrasound but of the opposite polarity (or something meaning something quite similar, but not quite the same thing, "out of phase"), would cancel out the effect near it (within a few feet). This device would just be a bigger version of noise-cancelling headphones. If you're in a large room, which I presume you are, the device wouldn't significantly change the infrasound unless you were near it. But if you really wanted to push your luck and still be quasi-scientific, you could say it cancelled the effect in half the room.
