A firework is detonated many meters above the ground. At a distance of 400m from the explosion, the acoustic pressure reaches a maximum of 10N/m2. Assume the speed of sound is constant at 343m/s, the ground absorbs all sound falling on it, and the air absorbs sound energy by the rate of 7dB/km. What is the sound level at 4km from the explosion? I have calculated the sound level at a distance 4km away from the explosion without the absorption. However why do we have to subtract (7*3.6)dB from that answer instead of (7*4)dB ?