dB or not dB?

True or false: 30 dBm + 30 dBm = 60 dBm? Why does 1% work out to be -40 dB one time but then 0.1 dB or 0.05 dB the next time? These questions sometimes leave even experienced engineers scratching their heads. What is a decibel and how should we use it in our calculations? This Application Note is intended as a refresher on the subject of decibels.