Common requirements for good acoustics include adequate loudness, uniformity, clarity, reverberance, or liveness (the listener should be bathed in sound from all sounds, though still able to localize the sound source), freedom from echoes, and minimal background noise.
The optimum reverberation time of an auditorium is dependent on the use for which it is designed (speaking, chamber music, opera, orchestral music, etc.).
Important subjective attributes of concert hall acoustics include intimacy or presence, reverberance or liveness, spaciousness: auditory source width, spaciousness: listener envelopment, clarity, warmth, loudness, balance, blend, and ensemble.
Intimacy is the subjective impression of the size of a hall. Smaller, more intimate halls are generally preferred over wider, more remote halls.
The acoustical measure of intimacy is called the initial time delay gap (ITDG), which is defined as the interval between the arrival of the direct sound and the first reflection at the listener. From a study by Leo Beranek (1962), a concert hall is considered “intimate” if the ITDG is less than 20 milliseconds.
The early decay time (EDT), or initial rate of sound decay in a room, is perceptually most important to our impression of reverberance. The EDT consists of relatively few isolated early reflections.
Clarity () is defined as the difference (in dB) of the sound energy received at a listener in the first 80 milliseconds minus the (late) reverberant energy (all remaining sound energy). A greater value of gives music a sensation of definition, while decreased definition adds “fullness of tone” (or “muddiness” when excessive). In a study of 22 European concert halls, less definition was preferred.
Optimum reverberation time is a compromise between clarity (requiring short reverberation time), sound intensity (requiring a high reverberant level), and liveness (requiring a long reverberation time).
A two stage decay can satisfy two conflicting, but desirable, attributes of music. A short EDT provides “clarity” and a long RT provides liveness to music.
Spaciousness has two main components: auditory source width (ASW) and listener envelopment (LEV). ASW results from the presence of lateral early reflections, while LEV is primarily influenced by the late reverberation of the room impulse response. Textured room surfaces help sound diffusion. Both ASW and LEV are associated with the degree of interaural cross-correlation at the listener's ears and the degree of lateral energy in the room response.
Reflections from the ceiling or overhead reflectors are not as perceptually desireable as those from side walls. Thus, reverse fan-shaped halls provide better laterality of sound.
Warmth is provided to a music hall via a slight increase in low frequency reverberation.
Excessive high frequency damping can lead to a lack of brilliance. Brilliance is measured by EDT / EDT and EDT / EDT, where EDT is the average of EDT values at 500 and 1000 Hz.
Echoes, flutter echoes, sound focusing, sound shadows, and background noise should be avoided in an auditorium design.
The greater the early decay time (up to two seconds), the greater the preference for the concert hall. Above two seconds, the trend it reversed.
Narrow halls are generally preferred to wide ones.
Preference is shown for halls having a high “binaural dissimilarity”.