I know what true-rms is, but I wonder if it is required to measure a real audio signal.
With "real" I mean a mix of many frequencies, not just a test pure sine wave.
I suppose an average meter should work fine, but I've got no true-rms to compare.
Would an average and true-rms meters differ in readings?
Thanks!
Hi,
Not sure if anyone mentioned this yet, but the true RMS value and the average value are in general not the same. To state this in a more definitive way, they are almost never the same. They can get pretty close and maybe we can find a wave that does show the same values, but it would take a little work to find this. This means that a random signal should be considered to have different values for RMS and AVG.
To show how simple this is to prove, all we have to do is find the average and true RMS values for a sine wave. If it doesnt work for a sine wave, then we have to go to great lengths to find a combination that might actually work, and this is not what we normally will see.
The average value for a sine wave mathematically is zero, but for power line and other measurements we usually find the average value of the absolute value of the sine wave, and for a sine wave of amplitude equal to 1 unit this comes out to simply 0.636620 with six digits of accuracy.
The RMS value of that sine wave on the other hand is 0.707107 to six digits. The exact values are 2/pi and 1/sqrt(2).
The ratio of average to RMS is 0.900316 to six digits, so right away we see that they are not the same already.
Since this holds true EVEN for a single sine wave, we will find in general the two will not be equal.
If we look a little we might find some signals that have the same RMS and average values, well, close anyway. I think the signal:
sin(w*t)+sin(3*w*t)/3+sin(5*w*t)/5
comes close but still not exactly the same. This happens to be the first three harmonics of a square wave.
If we do the first 119 harmonics of a square wave we get a ratio of 0.998310 which suggests that a perfect square wave has equal RMS and AVG values. This is not a usual audio signal though, but if you could pass a perfect square wave through an audio amp then you'd see equal RMS and AVG values. Again, this is far from typical.
So the assumption should be that they are never the same, even though on rare occasion we'll see them the same.
Of all the meters i own now i only have one that goes up far above 1kHz. It was not cheap though. The usual meters are used for power line frequencies so they dont have to go up too high.
You can also build your own peak detecting meter. You can calibrate it with a scope and then use it to measure audio and other stuff. The main parts are a very fast diode and resistor and capacitor. If you dont have a scope you can get someone else to calibrate it for you.