已知与一点声源距离为十米声强级是20分贝 1.求距离声源5米处的声强级 2.多远距离就听不到声音.

1.I1=P0/4πr1^2,I2=P0/4πr2^2---->I2/I1=r1^2/r2^2=100/25=4
10Lg(I1/I0)=20dB,距离声源5米处的声强级x=10Lg(I2/I0)-->
x-20=10Lg(I2/I0)-10Lg(I1/I0)=20Lg2---->x=26.02dB
2.距离为十米声强级是20分贝
10Lg(I1/I0)=20dB--->I1=100*I0=P0/4πr1^2
距离x就听不到声音-->I0=P0/4πx^2
--->100*P0/4πx^2=P0/4πr1^2-->x=10r1=100m