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I can check level audio on a VU Meter or dBFS meter. What's the simplest way to check if an input audio level is line or micro? Can I use a multimeter?

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  • \$\begingroup\$ For sine signals you may have luck with a multimeter, but I dont trust them for anything other than 50 Hz sines. you could connect some speaker/headphone and judge by the loudness or you could judge using an oscilloscope. But besides some difference in amplitude, the main difference between mic and line is the source impedance. you can only judge that well when you add a load e.g. headphones \$\endgroup\$
    – tobalt
    Nov 7, 2021 at 6:34
  • \$\begingroup\$ Add as answer. How to measure using oscilloscope? Do I have to terminate by putting a load or a speaker before taking measurement with oscilloscope? \$\endgroup\$
    – ark1974
    Nov 7, 2021 at 9:54
  • \$\begingroup\$ By "micro" do you mean "mic" which would be shorthand for "microphone?" \$\endgroup\$
    – JYelton
    Nov 8, 2021 at 16:44

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There are four typical audio levels (all values RMS):

  • Mic (microphone) level: 1 to 10 mV 1
  • Instrument level: 100 to 300 mV 2
  • Line level: 300 mV (consumer), 1200 mV (professional)3
  • Speaker level: Tens of volts. Varies by loudspeaker impedance, amplifier type, and volume level. (For example, a 400W (continuous) 8Ω loudspeaker might be driven with ~56 volts. (\$E=\sqrt{PR}\$)).

Anything below 100mV would be considered mic level and from 300 to 1200 mV would be considered line level. If you don't have a test signal, measure with an oscilloscope because your multimeter may only be accurate for 50 or 60 Hz AC measurement.

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  • \$\begingroup\$ Wow! This is exactly what I was looking for.+1 \$\endgroup\$
    – ark1974
    Nov 9, 2021 at 4:11
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Multimeter + cosine test tone will work. Lots of cheap DMMs have ~200-1000 Hz bandwidth on the AC input, but double check yours with a known line level output and verify that you get the expected RMS amplitude.

Otherwise, an oscilloscope would be my first choice if you had one.

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  • \$\begingroup\$ For using oscilloscope, do I have to terminate the audio port with standard load first? What are the expected rms values for line and micro levels? +1 \$\endgroup\$
    – ark1974
    Nov 9, 2021 at 4:11
  • \$\begingroup\$ @ak1974 Use a high impedance (1Meg) probe and do not terminate with any additional resistance. \$\endgroup\$ Nov 9, 2021 at 4:24
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Use cell phone with signal generator app. Measure AC sine wave 50 to 100 hz signal with high impedance instrument, e.g. oscilloscope. Connect the signal to the amp's input, e.g. line, aux, similar. Notice voltage drop. Use Ohm's law to calculate related signals. _ To find out cell phone output resistance (impedance) connect it to the measuring device, read signal value. Then connect a resistor to the output, e.g. a potentiometer of 1000 or 100 ohm. Reduce the resistance until the indicated voltage is half. Disconnect and measure the adjusted resistance of the potentiometer: It equals the output impedance of the cell phone audio connection. _ In a variation of both you can connect line input to cell phone output and in parallel a potmeter with higher resistance as described. Then measure the potmeter's value and calculate what value must be parallel to achieve the combined impedance. This is the value of the line input impedance. Be careful to keep output voltage of generator stable, else you end up with error, may be small, though. For relative / comparative measurements the absolute value is not highly important. An accuracy of 20% might be good enough.

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