The ratio of intensities between two coherent soud sources is $4 : 1$. The differenmce of loudness in $dB$ between maximum and minimum intensities when they interfere in space is:
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$\frac{I_{1}}{I_{2}}=\frac{4}{1}$ or $\sqrt{\frac{I_{2}}{I_{2}}}=\frac{2}{1}$
$\therefore \frac{I_{\max }}{I_{\min }}=\left[\frac{\sqrt{I_{1} / I_{2}}+1}{\sqrt{I_{1} / I_{2}}-1}\right]^{2}=\left[\frac{2+1}{2-1}\right]^{2}=9$
$\therefore L_{1}-L_{2}=10 \log \left(\frac{I_{\max }}{I_{\min }}\right)=10 \log 9=20 \log 3$
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