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The room controls everything. If you can stand in the front of the room and talk very loudly and be clearly understood in the back of the room, you should be able to expect good results from the sound system. The room is said to have good intelligibility. Auditoriums, chapels, and sanctuaries can have three types of acoustical problems that interfere with intelligibility. The first two are related to sound reflected within the room. Reflections that occur within about 3/100 of a second of the original sound are perceived by the ear as the same acoustical event as the original sound. In other words, a reflection has to be delayed by more than 3/100 second to be perceived as an echo. Echoes that are delayed more than 3/100 second interfere with intelligibility. Echoes delayed from 5/100 to 15/100 seconds are the most troublesome. Echoes with longer delays are heard clearly, but do not interfere as much with intelligibility. If you can clap your hands at the front of your room and hear a clear echo, you should contact an acoustical consultant before working on the sound system.

Another effect of reflected sound is reverberation. This is the steady sort of rumble that occurs in a room after the original sound has stopped. It is different from echoes in that echoes are individual sound events. Rooms are usually characterized by the time required for a normal person's voice reverberations to become inaudible after the person has stopped talking. This is called the reverb time or RT60. A room's reverb time is controlled by the room's size and reflectiveness. Recording studios have extremely short reverb times; cathedrals have very long ones. In between are auditoriums for speech, auditoriums for music, most sanctuaries, etc. The ideal reverb time is always a compromise, and is affected by many variables. In general, contemporary music and speech are best heard in rooms with fairly short reverb times, while organ and choir music sound better with longer reverb times.

The final room problem is one that is often overlooked: ambient noise. Usually this is the noise of the heaters, ventilators, and air conditioners (HVAC). Basically there are two design philosophies for HVAC: low-volume, high-velocity; and high-volume, low-velocity. Put simply, you can heat or cool a room by blowing a small column of air very fast or by blowing a large column of air slowly. The high-volume, low-velocity approach creates much less noise. However, changing HVAC system design is usually practical only if you are building or renovating a room. But be aware that excessive HVAC noise will interfere with listeners' ability to hear. This problem is multiplied for hearing-aid wearers.

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