$(1)$ An office is $9$ meters in width, $20$ meters in length, and $2\mathrm{.}8$ meters in height. Because of the reverberant effect, the office will have a suspended acoustical absorbing ceiling and a wood panel covering the entire floor. At a particular sound frequency, the absorption coefficients of the materials in the room are listed as follows. What is the sound level reduced after renovation at that frequency? Suppose that the impact of direct fields can be disregarded.

$\backslash$table$[[$Surface$,$Material,Absorption coefficient$],[$Wall$,$Painted Block,$0\mathrm{.}08],[$Floor$,$Concrete,$0\mathrm{.}06],[$Ceiling$,$Concrete,$0\mathrm{.}06],[$Suspended ceiling,Acoustical panel,$0\mathrm{.}5],[$Floor covering,Wood panel,$0\mathrm{.}3]]$

$(5$ marks$)$