Acoustically dead rooms actually include spaces such as radio, TC, recording and movie studios, where distinct sounds must be capture for reproduction purposes. These are qualitative terms applied to certain aspects of the acoustical properties of a room. The requirements of most rooms will fall somewhere between these two extremes. If the time interval between the original sound and its reflections is short, instead of creating echoes the reflections reinforce the loudness and clarity of the original sound, or , the reflections may persist audibly for a considerable time after teh source sound has stopped. For example, a lecturer may find that some of his words can't be understood because the reflections of one syllable overlap the next syllable.
Reverberation is a basic acoustic property of a room.
In rooms used for speech or music, reverberation time should be long enought to enhance the blending of sounds, but short enough so there is no excessive overlapping and confusion. In general, the larger the room, the longer the reverberation time because sound waves must travel farther between absorptive surfaces. Figure 13 shows the ranges of acceptable reverberation times at 500 Hz. The lower part of the band is best for rooms intended primarily for speech, the upper part is better for music rooms, and the middle portion is recommended for general purpose rooms. Reverberation time can be approximated accurately enough for preliminary study in advance of construction, and acceptable times for rooms of given size and function have been determined with a high degree of certainty.