The Acoustic Ambiance of a room is mainly characterized by its reverberation time or, the time within which the intensity (energy density) of a steady sound field at a particular place has declined by 60 dB after the sound source has been stopped. This time is closely connected to the size of the room and the absorption of sound waves by the walls and furniture in the room. Strong absorption shortens the reverberation time, weak absorption prolongs it.

Long Reverberation Times: Very Echoic Room

Too long reverberation times are found in large, open rooms such as church sanctuaries and also in small, highly reflective rooms such as tiled bathrooms. Most people describe the ambiance of these rooms as being 'live,' or 'echoic.' Speech intelligibility is lower in these rooms and loud, sharp sounds such as clapping or a clicker will cause flickering echoes. These echoes may be more pronounced in various positions in the room.

Short Reverberation Times: Crowded or Stuffy Room

Too short reverberation times are found in rooms that have a lot of sound dampening. This tends to make the room feel smaller than it really is. This causes most people to feel enclosed and describe the ambiance of the room as 'crowded' or 'stuffy.' Libraries are examples of places where this effect is desired.
Human beings can intuitively estimate the size of a room based on its reverberation time. Rooms whose acoustics 'fit' this estimation are considered natural.

Reverberation Times May Lead to Colored Sound

Typical rooms absorb high frequencies much more than low ones. This causes the reverberation times in these rooms to be longer in the low frequency range than in the midrange or high frequency ranges. The thick red line in the diagram above shows an example of a room which would be perceived as ideal. The green bars 'Ideal' indicate the upper and lower limits of the ideal reverberation time across the audible spectrum. Because people tend to expect this behavior in the sounds they are hearing, rooms which deviate from this pattern of frequency dependent reverberation times are felt as being unnatural or strongly sound colored.

CARA Helps to Improve Your 'Acoustic Ambiance'

CARA judges the 'Acoustic Ambiance' of a room by noticing the deviations of the frequency dependent reverberation times from the ideal range. These calculations consider all details of the room construction as well as furniture and materials used. The loudspeaker system used in the room is not relevant to these calculations (except perhaps as pieces of furniture) and is therefore not included.
After the calculations, CARA describes the Acoustic Ambiance of the room and suggests improvements. These changes are often things such as adding or removing furniture or changing the textiles of the room surfaces.