Acoustics - Extended environments: absorption, reflection and undesired effects

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15.18.1. Absorption

Both the hall's structure and its covering have an essential role in defining the environment's acoustics. The principles we mentioned when we looked at small environments using absorbing and reflecting materials to diffuse sound (the regal draperies hanging in 18th century halls are absolutely essential for acoustics), are also applicable in this case too. The spectators' absorption is a separate matter. If, for example, the stage's height corresponds to that of a horizontal front of house, each single spectator absorbs part of the acoustic energy issuing from the stage. Consequently the spectators in the last rows receive a considerably weaker sound signal. One solution would be to lift the stage in relation to the spectators, so that part of the direct sound can travel without any obstructions above their heads. However, a more effective solution would be to tilt the basement underneath the seats so that each spectator is exposed to the direct sound:

Environmental acoustics - Hall with basement tilted towards the stage

Hall with front of house tilted towards the stage

15.18.2. Absorption by the air

In large halls this is a factor which shouldn't be ignored, due to the large amounts of air enclosed in these environments. Here absorption varies depending on frequency, air humidity and temperature.

15.18.3. Undesired effects

  • Focusing: this takes place when the sound waves are reflected and focused by a concave surface into a specific area.

  • Single echoes: these may occur when two reflecting surfaces positioned at a reasonable distance from one other, repeatedly send each other the same sound.

  • Shaded areas: areas that are inaccessable to the sound due to large obstacles.


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