Early First-Reflected Sound

A large difference between the time of arrival of the direct sound and the first reflected sound is perceived by the listener as a sense of isolation from the performance. Terms used are that the performance lacks "presence" and "intimacy".

Comparative studies of auditoriums indicate a preference for auditoriums in which the first reflected sound arrives within about 30 msec of the direct sound and that a delay of over 50 msec draws negative reactions. For a sound speed of 345 m/s, these delays correspond to pathlength differences of about 10m and 17m respectively. The delay times are included with the measurements for some auditorium examples.

Early First-Reflected Sound

A sometimes popular practice in large auditoriums has been the hanging of reflective "clouds" to provide an early first reflected sound.

While it is not clear that the hanging clouds greatly improve the sense of "presence" of the performance, this practice is certainly preferable to lowering the ceiling, which would lessen the overall reverberation time.

Echoes in Auditoriums

The term echo in an auditorium refers to a reflected sound which stands out significantly over the normal reverberant sound of the auditorium. It is not sufficient in this context to just define an echo as a reflected sound, since the entire reverberant sound field is a collection of many reflected sounds. The term echo in auditorium acoustics may be the prominant reflection from a refective flat back wall, often called a 'slapback' echo.

Distracting echoes arise from focusing surfaces, which should therefore be avoided in auditoriums. In fact, it is good practice to include surfaces or structures which break up any large reflective surface. The columns, decorative woodwork, and general construction of the fine European music halls gives a good mix of reflected sound for more even dispersion of sound. Modern structures tend toward large smooth plaster or concrete surfaces which give problems with echoes.

An Acoustical Nightmare

"According to one parishioner, the echoes were so bad under the egg-shaped dome of Oklahoma City's First Christian Church of Tomorrow that when the minister spoke it sounded 'as though God were repeating every word he said, only much louder.' In the hope of deflecting the echoes, a 20-ft saucer was hung from the apex, but it had no effect. An acoustician finally solved the ploblem by overpowering the echoes with an amplifying system. Carefully filtered sound now come from the round speakers on the walls and spreads evenly-and without echoes-over the congregation." Time-Life, Sound and Hearing, p189.

Google image of dome-shaped building.

Measuring an Auditorium

Though the pleasure of a fine concert in a good auditorium can be considered to be a subjective thing, there are objective measurements which correlate strongly which what persons perceive to be good acoustics in an auditorium. Some of those measurements are: