Manoel,
I use two pieces of equipment that actually are using FFT to make acoustic measurements in situ, i.e. in the room of interest. By using FFT and sweeping the room you can easily see the effects on frequency response as a function of reflections or by adjusting the parameters you can close down the sample cycle so that you can actually sweep the room while you are talking without affecting the measurement. The instrument also allows a manipulation to take a look at phase vs. frequency. The instrument I use most is called TEF (Time Energy Frequency) and it was originally designed and built by Crown International. You can now see the product line at
http://www.gold-line.com/tef/tef.htm. This instrument allows us to quantify anomalies that we hear but could not measure. It has made great advances in acoustic design and treatment. We now know for instance that your ear shape has significant effect on how you hear. We also know that there is a delay after you hear a sound while you ear processes the data called the Hass Effect. By far the biggest effect in an acoustic environment revolves around reflections and the time it takes to arrive at your ears. In mixing rooms, for instance, reflections from the audio console face can cause severe notches in the frequency response. In electrical terms, two signals, in phase and equal in level, will only add to +6dB. The same two signals, 180 degrees out of phase, will produce a notch of infinite depth. What is more surprising is that there are rapid phase changes when sweeping across the notch. Rapidly changing phase plays havoc with human hearing as phase (one ear compared with the other ear) is what helps give us directional clues as to the source of the sound. Additionally, significant reflections will be in phase at some frequencies and out of phase at others, causing what is known as a comb filter. This type of filter will have frequency related notches in response throughout the audible spectrum.
In typical Catholic churches, particularly in the US, sound system designers have installed two speakers, one on each side of the altar. In this design, the only people who will hear without interference are those who are walking down the center of the center aisle. All others will experience some form of notched response due to the time misalignment of sound arriving at their ears at two different times.