how to record infra-sound
 

I am a composer, scoring a film now. One of the instruments I'll be using is the bass harmonica. It's lowest note has 2 reeds in , one at 40Hz and the other at 20Hz. the difference tones, which I want to catch will be in the 4 to 6 Hz range. any one know how I would record that low?
The lowest mic I've found starts at 10 Hz but this is not low enough, some help please David

Even harps like the Tombo Contra Bass only go to c, and that's 34Hz, but the sympathetic frequencies that go along with that are all you really hear. You can't hear 10Hz. The 69Hz overtone is what you really hear, with subtones filling in.
Needless to say, you're not going to be recording 10Hz.
Use a small diaphragm condenser in a large room, or a large diaphragm condenser in a tight room. I recorded Alan Glen for Linda Ronstadt's Los Cobres recordings and we used BK 4011's with John Hardy M1 preamps. Sounded smokin' good.
Joanie Madden uses AT 4055's in the studio on her bass harp. It sounds a tad muddy to me, which is expected from a large diaphragm in a loose room.

it's not just the sound
 

Thank you Doug for your reply. While I know that folk in a cinama or any where else are not going to be 'hearing' in the 10Hz range.

But the thing is that I am working with Charles Lucy, see lucytune.com, on psycho-acoustic trigering. This involves using the beats brought about in parts of the sound shape interacting, along with other things(wing of bat, eye of newt etc.)which I can't go into with out giving it all away. What starts low with difference tones, quickly goes up as the difference tones and partials all add up in the areas where one is hearing.

The more information we can make a sound carry the more accurate our evocation of an emotional state can be. It is a case of what you see isn't what you get at all. We can make a flute sound like a flute yet at the same time it will get the reaction that is got from scraping fingernails across blackboard. Charles Lucy takes the psycho active parts from one sound and induces it into another sound. The sound that you induce the forigen psycho active parts from, cloak those parts which elisiit the reaction. Hence the name "Lucy Sound Cloaks®". That the reaction is evoked, synthetic, dosen't seem to make it any less potent to the audence, in the limeted studies that we've done.
It is early days yet, and I don't think that we've invented "feelies" but you will be able to see/hear for your self how it's doing, as we hope to have it usable for Camera Shy's production Roslyn. Where, the director willing and it working well enough we will apply it in order to extract extra bang for your buck out of a little spine twisting exersize. Out on an edge again - David

I'm rather interested in this very topic but for differing reasons, just want to record the very low end of the spectrum as well as the rest of it. Of cousre just how you reproduce that is another issue entirely.
I worked on a system a few years ago that worked down to seconds per cycle for studying explosion propogation in coal mines however there we recorded air speed using 100s of pitot tubes connected to sensitive pressure transducers, hardly practical for what you're trying to do.
I suspect the LDC mics will work down to 0 Hz but they have built in filters in the electronics to prevent it getting through.
What I'd suggest you do is fire off an email to Rode, Freedman industries would probably love to work on this issue. A converntion data logger rather than an audio recorder can record at 0 Hz so that might be one answer for a recording device.
Another thought, some of the early condensor mics used RF bias with the condensor as part of a bridge, that meant you got a signal that was the absolute position of the plates, that avoids the roll off due to dV/dT at low frequencies.

Special purpose microphones are available that handle very low frequencies.

Schoeps can build you a microphone that goes down to 3 hz. It is mentioned on their web site. Other manufacturers can probably handle this frequency range also.

http://www.schoeps.de/E-2004/cmc.html

Please note that very low frequencies (infra-sonics) can cause a lot of problems. Please refer to the website listed above.