The intensity I of sound is related to the sound level b through the relationship:
where I0 is the reference intensity = 10-12 W/m2. We can calculate the sound level b at the location r1 using the above relation:
The sound level at the location r2 = r1 + 0.5 m is b2 and is given by
For a source emitting sound isotropically
But r2 = r1 + 0.5 m.