log~~(dB)分貝.....

公式: L=10 log I/I底數0
3&4題,取i(右下方small0) =10^-12 W/m^2

3. 已知在的士高所量度的聲音響度為120dB,求聲音的強度.,,,,,答案:1W/m^2
120 = 10 log ( I /10^-12 W/m^2)
12 = log ( I /10^-12 W/m^2)
( I /10^-12 W/m^2) = 10^12
I =10^-12 W/m^2 x 10^12 (not 10^-12 x 10^12 = 10^0 = 1)
I = 1 W/m^2

4.在午膳時,中四甲班的聲音響度為65dB.求該班的聲音強度(答
案準確至2位有效數字)...............答案:3.2x10^-6 W/m^2
65 = 10 log ( I /10^-12 W/m^2)
6.5 = log ( I /10^-12 W/m^2)
( I /10^-12 W/m^2) = 10^6.5
I =10^-12 W/m^2 x 10^6.5
I =10^-5.5 W/m^2
I = 3.16 x10^-6 W/m^2
I = 3.2 x10^-6 W/m^2 (案準確至2位有效數字)

10.在音樂會&足球場所錄得的聲音響度分別為80dB&85dB.求足球場的聲音強度與音樂會的聲音強度之比為多少?...............答案:3.2:1
85 = 10 log ( I(85) /10^-12 W/m^2)
8.5 = log ( I(85) /10^-12 W/m^2)
I(85) =10^-12 W/m^2 x 10^8.5

80 = 10 log ( I(85) /10^-12 W/m^2)
8.0 = log ( I(85) /10^-12 W/m^2)
I(80) =10^-12 W/m^2 x 10^8.0

The ratio I(85)/I (80)
[10^-12 W/m^2 x 10^8.5]/[10^-12 W/m^2 x 10^8.0]
[10^8.5]/[ 10^8.0]
10^(8.5 – 8.0)
10^0.5
3.16

比為3.2 / 1