What are Decibels and why are 3dB and dBm important?

Explains why Decibels are used, and also why the 3dB value is so important. It also explains what dBm is.
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... and sorry everyone. There's a "typo" in this video. I should have used "power" everywhere, not "voltage". Decibels are defined to be 10log_10 of a ratio of powers (not voltages). If you replace all the V's with P's in the video then it's all totally fine. That's annoying. Sorry.
I've fixed it on the Summary Sheet here: drive.google.c...
Note that sometimes people do consider ratios of voltages (as I did here), but then they need to multiply by 20, instead of 10 (which I forgot to do). Why? Because for electrical signals, P=V^2/R. So if, instead of powers, you have a ratio of two voltages (across the same resistor), then the power ratio = (V_2/V_1)^2, and the square comes out the front in the log function, as a multiple - hence the 20, instead of the 10. In other words, 10log_10(P_2/P_1)=20log_10(V_2/V_1).
It's just a factor or 2, which is not important to the general understanding of the log function, but it's annoying I made the mistake in the video. Sorry about that.
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