In the future, it is possible to add one additional microphone in a room with the idea that the second microphone containes the room sound of the first microphone. It is helpful if that second microphone is within a foor of the main microphone. In production reversing the phase of the room mic and mixing it in judiciously can help to reduce room sound.

Did you ever notice that the grateful dead often appeared on stage with two microphones on every stand but they only sang into one of them? One microphone was in the reverse phase of the other. The two microphones when combined canceled stage sound in the mix.

Another technique that can work sometimes - not always. double the audio track. Mix the double in at reverse phase and delay it by several ms. Experiment all the way from 5 to 250 ms until you hear it do something funny with the room echo. Then reduce the volume of the second audio track so that ist effect is subtle but effective. When it works it cuts the room echo in half and doubles its distance from the origianl latency making the voice clearer.

Once room echo is on a single track it is a very stubborn thing. I have no idea if Sound Soap can handle this sort of thing but it would not surprise me if there are plugins out there that can sort of extrapolate backwards asking, "What sort of echo was added to produce this sound?" THey then determine what sort of echo was added and use a reverse phase method to reduce it.

The problem with trying to phase cancel Room is it's rarely the same thing with a simple delay. You have multiple reflections which also are not phase coherent, sound bounces around in the corners of the room etc.
I've used this trick DSE mentioned with vary success, in a large space it works remarkably well, when it's a video of say someone giving a speech I'd say it's quite acceptable, the visual cues give credance to the sound of their voice. However it wouldn't hold up so well with a dissembodied voice.
Mics such as the Sanken CSS 3 handle small room echo very well, using multiple elements to phase cancel the echo. The trick is the 3 mic elements are within a millimeter or so of being coplanar. Without having them extremely close you'll introduce cancellations within the wanted sound i.e. a comb filter.

Bob.

Earlier threads ...

Echo cancelling - Bobs tip

Is it possible to cancel echo?

While I think that phase cancelling is probably a very interesting approach, in this case I can say with conviction that there is only one good way to get good sound - and it's easy.
You MUST use an off camera mike and get it as close as possible to the talent. Generally about 1 to 2 feet will be fine with a short shotgun mike.
In addition, we use "C" stands setup to make a "T" configuration. We then clip simple furniture blankets to the "T" stands with spring clamps.
We put one on each side of the talent and one on the floor under him/her. Sometimes in an extremely boomy location we will also put one just below camera sight line behind the talent, and one in front of the talent.
This makes a teeny little location sound studio, no matter where you are.
I've done this on thousands of setups with the best sound engineers in the business. They all do it exactly as I've described.
And that's how you get "good" sound.
It really doesn't cost a lot of money or take too much time to do.

Amen on the Sanken CS-3e (the little "e" indicates a slightly improved model).

I really love mine, but the original reason I picked it was that doesn't change its frequency response much over about a 40-45 degree angle, which is important in many situations, and I don't know of any other shotgun mike that is even close in this respect.

Austin Storms on rec.arts.movies.production.sound said this:

The show I've been working on has had a lot of man on the street type

On no.... He gets cut off just when he is going to talk about downward expansion. The mp4 that DSE linked.