Answer:
β₂ = 88.8 dβ
Explanation:
The intensity of the sound is
β = 10 log (I / I₀)
The sensitivity threshold (I₀) for humans is 1 10⁻¹² W / m²
I = I₀
I = 10⁻¹² 10^{70/10}
I = 1 10⁻⁵ W / m²
The total intensity for the 76 trombones is
I_total = n I
I_total = 76 1 10⁻⁵
I_total = 76 10⁻⁵ W / m²
Let's find the sound level
β₂ = 10 log (76 10⁻⁵ / 10⁻¹²)
β₂ = 88.8 dβ