Chapter-8Room and Auditorium

Acoustics1. Criteria in Acoustical Design

The acoustical quality of a room is determined largely by its Reverberation time.

2. Acoustical Characteristics:LivenessIntimacyFullnessClarityWarmthBrillianceTextureBlendEnsemble

3. Problems in Acoustical Design

4. Calculation of Reverberation Time

Reverberation Time

Reverberation Time

The reverberant sound in an auditorium dies away with time as the sound energy is absorbed by multiple interactions with the surfaces of the room.

In a more reflective room, it will take longer for the sound to die away and the room is said to be 'live'. In a very absorbent room, the sound will die away quickly and the room will be described as acoustically 'dead'.

But the time for reverberation to completely die away will depend upon how loud the sound was to begin with, and will also depend upon the acuity of the hearing of the observer.

Reverberation time is defined as the time for the sound to die away to a level 60 decibels or (1/1,000,000) below its original level.

Ideal Reverberation Times

Liveness and Intimacy

Liveness: A room is said to be “live” when the reverberation time is longer than the average for similar rooms.

Intimacy: Refers to how close the performing group sounds to the listener. Intimacy is achieved whenever the first reflected sound reaches the listener less than about 20 ms after the direct sound.

Fullness and Clarity

Warmth

Brilliance

Texture

Blend and EnsembleBlend:

Blend refers to the mixing of the sound from all the instruments of the orchestra or ensemble throughout the audience.

Proper blend is achieved by mixing the sound from various instruments and voices on the stage before distributing to the audience.

Ensemble:

Ensemble refers to the ability of the members of the performing group to hear each other during performance, enhancing the ability of the players to play together effectively.

Problems in Acoustical Design

The following acoustical problems must be avoided:

Focusing of sound

Echoes

Shadows

Resonances

External noise

Calculation of Reverberation Time

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An approximate formula for the reverberation time, TR is given below:

V = Volume of the room in cubic feet, S = speed of sound in ft/s, A = absorption in sabin.

A1, A2,…are the areas of the various types of absorbing surfaces, and a1, a2, … are the absorption coefficients of the respective surfaces.

Absorption Coefficients.

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Examples of Reverberation Times

One way to respond to the question "What is a good range of reverberation times for concert halls?" is to give examples of some of the most famous halls in the world.

For the overall average reverberation times:

Vienna, Musikvereinsaal : 2.05 seconds Boston, Symphony Hall: 1.8 seconds New York, Carnegie Hall: 1.7 seconds

But the overall average reverberation time does not tell the whole story. The variation of reverberation time with frequency is also important.

Acoustical Ceiling Panels

Over the audience and over the stage, ceiling clouds and panels produce balanced and blended sounds for a variety of venues, resulting in a rich, directed sound that the discriminating audiences have come to expect.

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