I produce a podcast with my company and we shoot in one of our offices with a simple set. We've got some good equipment (Sennheiser ew 100 G2 mics, an Edirol mixer) that picks up voices great but it also catches the sound of the building's AC. It can't be shut off in just one office, unfortunately. The noise it creates is this constant, low, wind-like sound that's sometimes accompanied by a high-pitched, somewhat squeal-like sound.

What I'm wondering is if there are any suggestions as to how to mask the sound either in the room (we've thought of foam padding but are not sure if we can get away with it in our building) or if the mics can be set better to help eliminate the sound or if its best to do so in post (I've used Soundtrack Pro and FCP using things like Hum Remover but that only goes so far).

Any help is greatly appreciated!

AC is a constant type noise. Record some room tone before you start the real shoot and then using one of your audio programs you should be able to pull a noise print of that noise then use either your NR program OR graphic EQ to find the frequency and drag it down to eliminate it without changing the tenor of the voices. I eliminate most AC noise by using a preset in my track EQ in Vegas and adjust accordingly. AC is a very low freq noise but it can be gotten rid of. The key is getting that noise print before hand.

I could be wrong but I don't think padding the room will help get rid of that noise although it might help the audio person when they bang their heads againt the wall while getting rid of the noise ;-)

First of all, the mics you're using are good, but they are omni-directional, meaning they pick up sounds from everywhere in the room, including the air handling ducts. Not much you could do with mic placement to help, I think. You said this is mostly a low rumble, and sometimes a high pitched squeal. Cutting the extreme low and high frequencies on the mixer may help. Optionally, you could do this more surgically in post.
The trick would be finding the offending frequencies (try the Voxengo SPAN plugin. KVR: Voxengo SPAN - Virtual Effect (http://www.kvraudio.com/get/1023.html) Its a free spectrum analysis software. You should be able to use that to see where the offensive frequencies are). Once you've found the source of the problem, you can dial in a parametric EQ to those frequencies in post and tone them down a bit. It will take a bit of playing around to mitigate the noise without making the voices sound unnatural.
One other option would be to place some heavy moving blankets over the air ducts if they are on the floor. You may have to weigh them down with something (sand bags, books, anything heavy). If the ducts are on the ceiling or wall, this might not be possible. Best of luck! Andy

Try recording some room tone (maybe at night but with aircon noise) lay it up adjacent to the length of the voice track and throw it 180d out of phase.

Raise and lower its level while examining what it does to your voice sound, if the aircon is low frequency enough it should be Ok for podcasting.

Cheers.

You need to move your mikes closer to the talents voice and reduce your gain. A mic will pick up a voice and room noise if it is placed mid chest level. By moving it up closer to the collar and having your talent speak up you compensate by lowering your levels and reducing your mic pick up of any ambient room noise.

Tighter (directional) type mics
Position closer to talent (source)
Use Low Cut filter aka: High Pass
Possibility of aiming mic at source which is in front of a non-reflective surface