A person speaking normally produces a sound intensity of 40 dB at a distance of 1 m. If the threshold intensity for reasonable audibility is 20 dB, the maximum distance at which he can be heard clearly is

dB=10 log10II0; where I0=10−12 Wm−2Since 40=10log10I1I0⇒I1I0=104       ....(i)Also  20=10log10I2I0⇒I2I0=102        ....(ii)⇒I2I1=10−2=r12r22⇒r22=100r12⇒r2=10 m                                                                  {∵r1=1 m}