“Okay, thanks to your WFTD archives I have a grasp of what critical distance means, but I’m curious how to determine this distance in practice? What is the best way to lengthen the critical distance in a room?”
There are a number of methods that aren’t very scientific. The most common is to place a sound source in the room and move away from it until the volume stops dropping, which will usually be accompanied by a more reverberant quality to the sound. This method is not all that accurate for obvious reasons, but in a pinch you can learn a lot with such a simple test.
If you are willing to get a little more involved you can employ a noise source and an SPL meter (a cheap Radio Shack model will work fine). A noise source could be as simple as a radio tuned between stations. Turn it up to a reasonably loud level – ideally about 90 dB SPL on the meter at a short distance like one foot. Next, double the distance between the meter and the sound source (two feet). According to the inverse square law your SPL reading should drop by 6 dB, however, since you are presumably in a closed space you shouldn’t expect it to drop by much more than 4 or 5 dB. Double the distance again ( to four feet), and again observe the change in level. Each time you should lose four to six (at the very most) dB of level. At some point one of your readings will result in a relatively minimal change in level. Somewhere between this point and your last reading is the critical distance. You can zero in on it somewhat by moving back and forth with the meter.
Changing the critical distance of a space means changing the characteristics of the space. This can be accomplished through a variety of construction techniques, but the least invasive method is acoustic treatment. There are a variety of specially designed foam products on the market that do an excellent job of damping reverberation, which results in being able to be further from a source before reaching critical distance. If you are using microphones you can certainly use more directional mics to overcome this problem. For example, a mic with a cardioid pattern can be placed up to twice as far from a source as an omnidirectional mic with about the same results in terms of intelligibility relating to reverberant field problems. Your Sweetwater Sales Engineer can help you with more options.