The physics
Intensity is the sound power passing through each square metre. Its unit is W/m².
A small speaker spreads its power over a sphere. So the intensity falls as 1/r².
I = P / 4πr² I₁ r₁² = I₂ r₂²
A wave's energy grows as the square of its amplitude. That holds for the pressure swing p₀ and for the displacement swing s₀.
I = p₀² / 2ρv = ½ ρ v ω² s₀²
Our ears hear a huge range of intensities. So we measure loudness in decibels, on a log scale. I₀ is the quietest sound we can hear.
β = 10 log(I / I₀) I₀ = 10⁻¹² W/m²
Each ×10 in intensity adds 10 dB. Twice the amplitude adds about 6 dB. Two equal sounds together add about 3 dB. Add intensities, never decibels.