A -3 dB change indicates the power has changed by what factor?

A -3 dB change indicates the power has changed by what factor?

• A. One-half
• B. Two
• C. One-quarter
• D. Three-quarters

Answer: (A)

The key idea is how decibels relate to power. A change in power in decibels is computed as 10 log10(P2/P1). If the change is -3 dB, then P2/P1 = 10^(−3/10) ≈ 0.501, which is about one-half. So the power has dropped to roughly 50% of its original value. Note that this is about power; if you were talking about voltage amplitude, the relation would be different (about 0.707 of the original in amplitude), but for power the factor is ~0.5. That’s why the correct choice is one-half.

The key idea is how decibels relate to power. A change in power in decibels is computed as 10 log10(P2/P1). If the change is -3 dB, then P2/P1 = 10^(−3/10) ≈ 0.501, which is about one-half. So the power has dropped to roughly 50% of its original value. Note that this is about power; if you were talking about voltage amplitude, the relation would be different (about 0.707 of the original in amplitude), but for power the factor is ~0.5. That’s why the correct choice is one-half.