A decibel value is obtained by comparing the actual level to the starting level.

• A. True
• B. False
• C. Only if the levels are equal
• D. Only if the signal is periodic

Answer: (A)

Decibels express a ratio to a reference level, so you obtain a dB value by comparing the actual level to a defined starting (reference) level. It’s a logarithmic measure of how much larger or smaller the measured quantity is relative to that reference. For power quantities you use 10 log10(P/Pref), and for amplitude-related quantities you use 20 log10(X/Xref). This means the comparison always applies, not only when the two levels happen to be equal. If the actual level equals the reference, the ratio is 1 and the result is 0 dB; if the actual is higher, you get a positive dB value; if lower, a negative one. The concept doesn’t require the levels to be equal, nor does it depend on the signal being periodic.