darrin_h2000 said:
... Root Means Square only refers to measurements of power and not actual sound. So you will never see dB with an RMS rating next to it. Unless its a cheap POS japaneze gear that is only for impressing car stereo kids that dont know any better.
Well, you're sort of headed in the right direction, but that's not right. In fact, it's pretty normal to take a RMS measurement, then describe it in decibels.
RMS does not refer to measurements of power, but to measurements of something like voltage or sound pressure level. The whole reason you need to do a root-mean-square calculation (instead of just a mean, or "average") is because you want to express a varying measurement of something
other than power in terms of the constant measurement that would produce the same power -- or, to put it another way, that will do the same amount of work over a set period of time.
Say you've got a series of voltages: 1, 1, 2, 8, 2, 1. The mean is 2.5. But voltage is related to power by a square function. The mean
power is something like 1 + 1 + 4 + 64 + 4 + 1 / 6 = 12.5 (if 1 volt produces 1 watt in the particular circuit). The square of 2.5 is only 6.25; 2.5 constant volts produces only
half the power (and, over the same amount of time, does only half the work) of the sequence of voltages listed above.
A more meaningful way of describing the effect of a sequence of voltages that changes over time is RMS: you square each value, figure the mean of the squares, then take the square root of that. The RMS of the series of voltages above is the square root of 12.5 = 3.54 (approximately). This series of voltages will produce the same amount of power (and do the same amount of work per unit of time) as a constant 3.54 volts. This is a much more useful thing to know than that the mean voltage is 2.5 volts.
The assertion that "you will never see dB with an RMS rating next to it" kind of makes sense, though it's not right, and you've got it backwards. It isn't because "RMS only refers to power" ... indeed, dB (strictly speaking) is a power measurement, and RMS doesn't refer to power, but a changing value of pressure or voltage, expressed in terms of the constant value of pressure or voltage that would produce the same power (or do the same work per unit time).
Electrical power varies with the square of voltage.
Similarly, the power of sound varies with the square of sound pressure.
This makes sense if you think of voltage as akin to pressure: the "push" that makes current flow.
That's why, when you use a dB scale to describe relative voltage or relative SPL levels, the formula is
20 * log10(measurement/reference).
Bottom line: you can certainly take a varying voltage (or SPL), describe the RMS of that varying voltage, and then relate that value to a reference value with a logarithmic decibel scale. In fact, it's the natural thing to do.
