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| Reverberation |
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| The persistence of audible sound even after the source has ceased emitting sound is called reverberation. |
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| If the reverberation of a sound pulse persists so long as to prolong the sound while several successive pulses are produced, the intelligibility will suffer, leading to poor acoustic conditions. |
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| The time required for the sound intensity (watts per square
meter) to decrease by a factor of one million so that it reaches the threshold of audibility is called the 'reverberation time'. |
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| If the reverberation time is too short, musical notes are isolated from one another and the music is 'thin'. If, on the other hand, the reverberation time is too long, the sounds from earlier notes clash with the notes being played. For a medium-sized auditorium, the
reverberation time should be of the order of 1 to 2s. Symphony hall in Boston, one of the finest concert halls in the world, has a reverberation time of 1.8s when it is fully occupied. |
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| The reverberation time depends on the volume of the concert hall and the nature of the reflecting surfaces. The larger the volume, the longer it takes for sound
traveling at approximately 345 ms-1, to traverse the distances between the reflecting walls. When surfaces exposed to sound waves are highly absorbent, the rate of energy absorption by all surfaces quickly becomes equal to the rate of energy production by all sources, thus the reverberation time becomes smaller. |
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| The absorption coefficient 'a' of a material is defined as the ratio of the absorbed acoustic intensity to that incident on it. |
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| The absorption coefficient may vary from 0 (no absorption) to 1(complete absorption). The following table gives some typical absorption coefficients of different materials. These are average values for the audible sound range at normal incidence. In general, the absorption coefficient varies with frequency, usually being greater for higher frequencies. |
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