Assessing Room Characteristics

• Common requirements for good acoustics in halls include:
  • adequate loudness: the sound source(s) can be heard throughout the room;
  • uniformity: the sound level is similar thoughout the space;
  • clarity: the sound is easily perceived or understood;
  • reverberance or liveness: the listener should be bathed in sound from all directions, though still able to localize the sound source;
  • freedom from echoes: no strong reflections or resonances;
  • minimal background noise: the room should be free from undesirable sounds, such as from fans, electronics, mechanical systems and sounds coming from outside the room.

• Some quantifiable measures of room characteristics include:
  • RT60: quantifies aspects of reverberance or liveness;
  • initial time delay gap (ITDG): the time interval between the arrival of the direct sound and the first reflection at the listener;
  • early decay time (EDT): initial rate of sound decay in a room, based on the early reflections;
  • clarity ($C_{80}$): 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);
  • brilliance: related to high-frequency damping, measured by EDT$_{2000}$ / EDT$_{mid}$ and EDT$_{4000}$ / EDT$_{mid}$, where EDT$_{mid}$ is the average of EDT values at 500 and 1000 Hz;

• Subjective aspects used when assessing rooms include:
  • intimacy or presence: the impression of the size of a space, evaluted using ITDG, more intimate halls (ITDG < 20 ms) are generally preferred over wider, more remote halls;
  • spaciousness: auditory source width (ASW): results from the presence of lateral early reflections;
  • spaciousness: listener envelopment (LEV): primarily influenced by the late reverberant field of the room impulse response;
  • warmth: provided to a music hall via a slight increase in low frequency reverberation.

• A greater value of $C_{80}$ 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.