09-14-11: Almarg
The number "10" in all of your equations that I've quoted above should be "20," since you are computing the number of db based on a voltage ratio. I've explained it, Kijanki provided a proof of it, and Ott provided a proof of it.
kijanki, did you say what almarg says you said? if power is reduced by half is that a 3dB reduction or a 6dB reduction.
the problem is that you're not clear on what ort is talking about. as to ort, when ort talks about dB he is talking about *power*. in the webite that you cited ort says "The dB is a logarithmic unit expressing the RATIO of two powers". what ort also says as an aside "Although the dB is defined with respect to power, it has become common practice to also use it to express voltage or current ratios". so while he is saying that some people compute dB for voltage, or current, ort feels that, properly stated, dB is a ratio of power levels. so when ort talks about dB, he is talking about a ratio of power levels. so when he refers to the 20*log(v2/v1) equation, the dB is a dB in *power*; that's why he calls it a "Derivation of dB as a Voltage Ratio". ort isn't saying that 20*log(v2/v1) is a voltage dB, what he is saying is that it is a power dB expressed as a ratio of voltages.
in ort's view, it is not proper to talk of dB as a ratio of voltages, but he recognizes that many people do so. so what i would expect ort would say about my postings is that it is not strictly proper to talk of a dB in voltage but that it is commonly done even though the only proper dB computation is for power. thus, when you refer to the 20*log(v2/v1) equation, you have to be clear that, in ort's view, "This is only correct, however, when V1 & V2 (I1 & I2) are measured across the same value of impedance".