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Although in relation to buildings and suspended ceilings we tend to talk generally about "acoustic performance", we need to
define in more detail what we mean and specifically if we are referring to "sound absorption", "sound reduction" or "sound
attenuation".
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 Sound Absorption |
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Sound Absorption (aw or Alphaw) is the process whereby the floor, wall and ceiling linings of an enclosed space contribute to the absorption of sound within
the room by reducing the amount of sound that is reflected from them. If a room is small and a lot of sound is absorbed at
each reflection then the resulting environment will tend to be quieter and have a shorter reverberation time. Conversely if
the room has a larger volume and little sound is absorbed at each reflection then the room will sound ‘noisier’ and with a
longer reverberation time. Sound absorptive materials therefore affect the aural environment within a room or enclosed space.
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| Sound Reduction |
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Sound Reduction on the other hand, is concerned within the reduction of sound energy as it transmits between adjacent spaces through common
building elements. Sound reduction is influenced by the mass and thickness of the element which is transmitting sound as well
as whether or not there are any gaps or cracks within it or around its perimeter joints. The heavier (thicker) the element
is, the lower will be the sound that is transmitted through it and hence the better will be its sound reduction ability. However
if there are significant gaps or cracks present then the sound transmission will be enhanced and the potential sound reduction
severely reduced.Suspended ceilings are unusual in that the sound reduction through them can be measured in two entirely different
ways depending upon the location of the sound source. Sound Reduction Index (R or Rw) is the measure of when sound passes vertically through the ceiling once, for example when the source of sound is in the
void above the ceiling.
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| Sound Attenuation |
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Sound Attenuation (Dnc or Dncw) however, is the measure when the source of sound is in an adjacent area and the sound transmits horizontally through the
ceiling twice via the common ceiling void.
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| So it is very important to understand the differences between these two measures because each will result in a different tested
value for the same ceiling type.
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| One of the main measures for a successful aural environment is the achievement of an optimum Reverberation Time. This is a measure of how decaying sound persists in a room and it tells us something of how ‘lively’ or ‘dead’ a room’s
acoustics will be and how loud or quiet noise levels will sound. It is related to how much sound absorption is present. For
any room, depending upon its size and whether it is primarily for speech or music, there will be an optimum reverberation
time range.
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For example, the reverberation time for speech must not be too long (0.8 seconds is a good upper limit) otherwise successive
speech sounds will overlap with a consequential loss of intelligibility. However if it is too short (< 0.4 secs) then the
space could seem ‘lifeless’, with no obvious reinforcement from the room, and this will make for extreme difficulty in conversation
particularly when addressing a group of people over distance such as occurs in teaching spaces or in meeting rooms. For teachers
who take classes for perhaps 5 or 6 hours per day in ‘lifeless’ environments, the consequence can be fatigue, sore throats
and loss of inspiration.
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| Music activities however benefit from longer reverberation times with a consequential blending of successive notes and the
resulting fullness of tone. However if the reverberation time is much too long then the received sound will loose clarity
and appear ‘muddy’, and if it is too short, then the sound will be ‘dry’, the performers will seem distant and the sound will
lack ‘warmth’ and ‘envelopment’.
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| So, whatever the application, for any space the reverberation time should be optimised and be neither too long nor too short. Too much sound absorption, when it is not needed, is just as unacceptable as too little when it is!
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| Click here to see the recommended acoustical Armstrong solutions |
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