• Logarithmic Decibel Addition Formula: Sound pressure levels do not add linearly. Total sound level is given by:

$$L_p = 10 \log_{10} \left( 10^{L_1/10} + 10^{L_2/10} \right)$$

• Calculation: For $L_1 = 70\text{ dB}$ and $L_2 = 80\text{ dB}$:

$$L_p = 10 \log_{10} \left( 10^7 + 10^8 \right) = 10 \log_{10} \left( 1.1 \times 10^8 \right) = 80 + 10 \log_{10}(1.1) \approx 80 + 0.41 = 80.41\text{ dB}$$

• Acoustic Rule of Thumb: When two sounds differ by $10\text{ dB}$ or more, the quieter source contributes less than $0.5\text{ dB}$, yielding a rounded level of 80 dB.

Correct option: (2) 80 dB