Since that is not where your ears are when you fire, the sound is far less. I first fired an M-14 many decades ago and my 17yr old ears only had plugs. All I heard was a loud thump. You should be fine.
If someone else is shooting, stay well behind them for max sound reduction.
Question: How does the sound power decrease with distance"? Answer: "April fool - The sound power does not decrease (drop) with distance from the sound source."
Levels of sound pressure and levels of sound intensity decrease equally with the distance from the sound source. Sound power or sound power level has nothing (!) to do with the distance from the sound source.
Thinking helps: A 100 watt light bulb has in 1 m and in 10 m distance really always the same 100 watts, which is emitted from the lamp all the time. Watts don't change with distance.
A frequent question: "Does the sound power depend on distance?" The clear answer is: "No, not really."
Based upon the above, I'm firing a 308 with a 24 inch barrel, the 24 inch distance doesn't really matter.