Question

3. A 16m x 10m x 5m class room has walls, floors and ceilings of reflective material with an average absorption coefficient of 0,05 . This classroom with a capacity of 200 people with absorption per person sitting in a chair is 0,4 m2-sabin (a) Determine the reverberation time in this condition; and
(b) if it is desirable to redesign the classroom to meet the requirements of a 0.6 sec reverberation time, determine the area of ​​the acoustic material that must be added to the classroom. When only acoustical tiles are available with an absorption coefficient of 0,7 (0,8*) that can be applied on wall or ceiling

Answer 1

a) The Sabine formula for reverberation is    

where V is the volume of the room

is the effective cross-section= .

So   and .

So .

b) if cf and cc represents covered floor and covered ceiling (covered by acoustic material) then changed cross-section can be written as

or

if we only cover the ceiling which is more convenient.

So  

. Putting these in Sabin's formula and re-arranging terms we get .