Case of the Erroneous Frequency Counter on Audio
It would have been a Fluke if we assumed all DMM's are standard. They're not exactly the same, but pretty standard. One big difference these days is the use of digitizing the inputs and computing true RMS results from instantaneous readings from an ADC. Fluke RMS meters also can measure frequency and the 100 Series Specs have a bandwidth of 50 Hz ~ 50 KHz for voltage and the lowest voltage Full scale is 300mV.
Some even have DSO scope results with limited settings. But not the 100 Series.
But 300mV voltage scale does not mean you can accurately measure frequency at low voltages. Most often counters with analog inputs have hysteresis to avoid displaying random readings from noise for an unconnected input.
As far as I can tell from the Fluke 120 Calibration Manual, it is only calibrated with a standard 6V signal, so the hysteresis may in fact be much larger than expected. But at least I verified it will measure frequency from 50 to 50KHz.
So if the amplitude of the audio changes the frequency of your measurement, chances are good that you have a noisy signal and it is not measuring all the "zero crossings" We normally assuming it is an AC coupled signal or at least no DC offset as this will affect which zero crossings are counted. Now when one looks at music on a Spectrum Analyzer like AC/DC's Hell's Bells, the spectrum may be fairly broad and flat but I saw the peak well below 100 Hz during the chorus.
THis means the average frequency of all the voltages would only be below 100Hz as the higher frequencies do not reach the zero crossing threshold to be counted. They are riding on the crests of each beat and wave. This means EQ and Bass boost will affect the readings as well as amplitude for hysteresis.
Rule of Thumbs
- If changing the threshold or doubling the amplitude has no effect on the frequency count, the result should be triggered properly.
- Do not expect accurate readings from low signals with lots of noise.
- Otherwise, make sure the signal is as strong as the manual's calibration signal.
- remove DC if any present on signal unless input is known to be AC coupled.
To get the OP's 0 to 15 Hz result from a Samsung Galaxy audio jack, I would hypothesize it was counting a percentage of the number of amplitude peaks above and below both +/- hysteresis thresholds that result in a zero crossing. So if the dominant signal was in 100Hz range and broadband noise just below that ( excuse me .. music from 20 ~20KHz) and the signal was a few volts, it might have only counted 0~15% of the zero crossings that were loud enough.
( apologies for goofing assumptions on previous answer. Not all true RMS DMM's are the same and I saw graph and mis-read)
So if you want to count the root of your squared sums, make sure your signals are big enough to overcome the hysteresis for frequency measurements. ( the Voltage and current readings are instantaneous without hystereis ).. I think that covers it.