Discussion
Expressed in decibels, a power ratio of 100 is
Answer: A
For power ratios the decibel is ten times the logarithm:
\[ 10\log_{10}(100) = 20 \ \mathrm{dB} \]
so 20 dB. 10 dB would be a ratio of ten. 40 dB uses twenty times the logarithm, which is the form for voltage or current ratios and would be right only if 100 were a voltage ratio. 100 dB is the ratio quoted as though it were already logarithmic. Keeping the factor of ten for power and twenty for amplitude straight is the single most useful habit in this subject.
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