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Is true-RMS meter required for audio signals?

Discussion in 'General Electronics Chat' started by Elerion, Aug 28, 2017.

  1. Elerion

    Elerion Member

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    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!
     
  2. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    They would differ in readings, but most meters wouldn't work accurately on audio anyway (they mostly work only with low frequencies) - if you want to measure audio, either use an oscilloscope or an audio millivoltmeter.

    Personally I use a scope (as I have scopes), this makes it easy to use p-p measurements, which is mostly what you want for audio.
     
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  3. Reloadron

    Reloadron Well-Known Member Most Helpful Member

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    Yes, they would differ considerably. Let's use an example. Since I don't have a good audio source we can look at the output of a UPS which when on battery outputs a MSW (Modified Sine Wave) output.

    This first image is looking at mains voltage. A clean sine wave.
    RMS Sine.png

    Now both meters measuring the MSW output of the UPS inverter:
    RMS MSW.png

    Both meters are looking at the same signal. The meter on the left is an Average Responding RMS Indicating meter and while it did well with a nice sine wave it does not fare well at all with the modified sine wave. The meter on the right is a RMS responding RMS indicating meter costing considerably more the the meter on the left.

    Additionally any meter used to try and measure a varying amplitude and varying frequency audio will need a fast sample rate to capture the signal. I believe a good digital scope with a record function would be a better solution. That or a good A/D converter with a storage function.

    <EDIT> I see Nigel addressed this well as I was typing. :) </EDIT>

    Ron
     
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  4. dave

    Dave New Member

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  5. JimB

    JimB Super Moderator Most Helpful Member

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    I guess it all depends on what it is you are measuring and why.

    My understanding is that audio people often use meters which measure the peak voltage of the audio signal.
    This would make sense if you do not want to overload the input of some other device such as an amplifier or radio transmitter.

    Measuring the RMS voltage of the audio signal would give a good indication of the average power of the signal, and hence how "loud" it sounded overall, not just the peaks.

    JimB
     
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  6. crutschow

    crutschow Well-Known Member Most Helpful Member

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    I suspect that an average responding meter would read reasonable close to the RMS value for typical music signals (since it's calibrated to read the RMS of a sinewave), at least close enough for most audio measurements.
    The average responding meter would generally read somewhat lower the a true RMS responding.
    Music with large peaks such as drums or loud bass would be more in error.
     
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  7. Elerion

    Elerion Member

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    Thank you everyone. Very useful.
    An oscilloscope is the obvious piece of equipment, but I was curious if I would benefit form a true-rms meter, as I need to buy a new one soon, and it only is around 25€/$ more.
     
  8. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    It's worth buying a true RMS meter regardless, but neither type is particularly useful for audio signals.

    I mentioned AC millivoltmeters for audio earlier, it might be worth you knowing that those are NOT true RMS. It doesn't really matter for audio, as anything you're measuring usually uses sine waves.
     
  9. alec_t

    alec_t Well-Known Member Most Helpful Member

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    RMS as a measure of audio signals is pretty meaningless unless the interval over which the 'mean' is calculated is known. One cycle? Ten minutes? ...?
     
  10. Elerion

    Elerion Member

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    I never really missed a true-rms. Appart from diode rectification, most situations I faced where AC or just any other kind of sinusoidal wave. Square waves (digital clock signals, etc.) are only 10% off, and most of the time this is fine.
    Are there any common situations I could have missed? Opinions, please :)
     
  11. crutschow

    crutschow Well-Known Member Most Helpful Member

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    As shown in post #3, the average responding meter is nearly 40% low as compared to the true RMS meter when measuring the modified sinewave from a UPS converter.
     
  12. schmitt trigger

    schmitt trigger Active Member

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    If you plan to measure input current on most low power supplies, the input current will not be PF-corrected and there will be significant distortion. A non-true RMS meter will display very significant errors.

    Likewise with SCR or Triac phase-control circuits. The output voltage is distorted, and the readings without a true RMS meter will be meaningless.
     
  13. dknguyen

    dknguyen Well-Known Member

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    Not necessarily. Know that "true-RMS" doesn't always mean true-RMS. At work, we have a fluke meter labelled a "true-RMS", but it's not. It was giving us weird readings with some special switched LCR waveforms so we compared it to a bench meter power meter and an oscilloscope's calculated RMS values (both devices confirmed to numerically calculate RMS from samples). The bench meter and oscilloscope agreed and the Fluke meter was off by a factor of 0.7.

    As asked the guy that owns it and he told us that it "true-RMS" only under certain conditions so it seems that if the waveform strays too far from sinusoid then it's no longer valid since it's working under some set of assumptions rather than numerically calculating the RMS.
     
  14. crutschow

    crutschow Well-Known Member Most Helpful Member

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    The Fluke meter has an AC frequency measurement limit, which may have been the source of your observed large measurement error.
    A Fluke 114 true RMS meter, for example, has an upper frequency limit of 1kHz, which would seriously limit its accuracy when trying to measure high frequency signals, such as from a switching power supply.

    A fundamental rule is, you have to know the limits of the instruments you are using if you want to make accurate measurements.
    The Fluke is indeed a true RMS meter when used within it's specification limits, which you apparently didn't do. :eek:
     
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  15. ronsimpson

    ronsimpson Well-Known Member Most Helpful Member

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    For measuring true power in a resistor .... RMS meter would be nice. Because most meters do not work well above 1khz I can not use the meter.

    For audio, I do not a RMS meter.
    A scope will tell me what level is distorting. (clipping)
    Any meter will measure relative voltage.
    >Example: I am injecting a 100mV 1khz sign wave and getting out a 10V 1khz sign wave. The amp should have a gain of 100. Even if the meter was really strange it would measure both input and output the same way, and give a good gain reading.
    >Example: For testing stereo, you often are just comparing a working right amp against a not working left amp. Again just comparing.
    >Example: I have a 20 input mixer. For a test I can inject the same signal into all inputs. With any meter I can look to see if all channels are working, the same. I really don't care if the voltage reads 1.00V or 0.707V or 1.414V. I just want to know that channel 7 is very weak.

    Many low cost meters don't work well with signals that are not sign wave. In some manuals it may say that signals with 10% duty cycle will not read right.

    Some digital scopes have math functions. (will measure the signal at 10,000 points and do the math) This math is good and works to 100mhz. (top end of your scope) A meter might measure 60hz at 100 points or 600hz at 10 points and 6khz at 1 point.
     
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  16. KeepItSimpleStupid

    KeepItSimpleStupid Well-Known Member Most Helpful Member

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    There is usually something called "Crest factor" and frequency response to consider. The thermal sensors are now obsolete.
     
  17. Elerion

    Elerion Member

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    I finally bought the true-rms version for a little more (by the way Amprobe AM-550, which also has switchable input impedance for VDC/VAC).
    I think the best way to learn is from practice, and it sometimes (always?) has a price.

    The meter seems well built, although it doesn't have this bulky back covers like Fluke's and many other cheap ones, which confer the meter a very sturdy looking. This one is one piece, but very light (<400g).

    Its AC bandwidth is just 1 kHz, but I haven't seen meters which go beyond that, which fit my budget.
    Hopefully, I won't do any switching power supply work.

    If someone considers getting a meter in the series AM-520 up to AM-570, feel free to ask for any technical question.
     
  18. MrAl

    MrAl Well-Known Member Most Helpful Member

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    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.
     
    Last edited: Aug 30, 2017
  19. Elerion

    Elerion Member

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    This seems strange, if you look at this:

    [​IMG]

    Although this, at the same times, is somewhat contradictory:

    [​IMG]
     
  20. Nigel Goodwin

    Nigel Goodwin Super Moderator Most Helpful Member

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    Sorry MrAl, but you need to stop smoking whatever illegal substance you appear to be using :D

    Normal multimeters read the 'average' calibrated solely for a sinewave - on the assumption that it's going to be used for mains measurements - as it's designed to be.

    Obviously measuring a squarewave will be massively incorrect, as it's not calibrated for that.
     
  21. Elerion

    Elerion Member

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    any idea why the two images above disagree?

    I suppose that average multimeters are just calibrated for sine wave, and not for a real average. Maybe that explains what MrAl explained.
     

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