I tried to connect the analog output of a Sound Pressure Level Meter to a digital oscilloscope to determine the RT60 of a room.

I thought I will somehow have an analog reading proportional to the SPL. But I tried with two different SPL meters, and I must admit I cannot understand the meaning of the signal they send through their analog output.

Here is what I recorded. The room floor noise is at about 50dB SPL, and I send a 3s pink noise signal giving a reading of 100 dB SPL on the SPL meter display. The red bars show the start and end of the generated tone.

enter image description here

  • The first SPL Meter I had was a low-cost "MX Measure MM-SM01" w/USB. It provides both an "AC" and "DC" digital output. On the graph, "AC" signal amplitude is in V. "DC" signal amplitude is in mV.
  • The second SPL Meter is a more expensive EXTECH 407730 providing only an "AC" output (according to the manufacturer). "AC" signal amplitude is in mV.

Notice how the three signals are completely different. How can I interpret those signals? Is this the kind of signal I should expect from the analog output of an SPL meter, or does that indicate the two I tried are just pieces of crap?

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    I've gone through the manuals for a few dB meters, but they don't specify what those AC and DC signals represent... – Hobbes Feb 29 at 14:05
  • Thanks, @Hobbes! I did the same on my side without much more success. At this point, I think the EXTECH output is useless because of the autoscaling. If you look carefully, you see the signal clamps somehow at the start of the test signal, then the dbmeter change scale. And you can see it scaling down after the end of the test signal [...] – Sylvain Leroux Mar 1 at 13:25
  • [...] For both splmeters, I think the AC output is picked at an early processing stage, but I'm not quite sure where exactly it comes from. [...] – Sylvain Leroux Mar 1 at 13:27
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    the only one I can think of is Electronics.SE, but the question is pretty specific to audio... – Hobbes Mar 1 at 19:20
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    @SylvainLeroux: First of all, could it be that you swapped the polarity of DC output? Usually, it's proportional to the SPL reading after all filtering is applied. You should see it changing in similar way to the decibel values displayed on the SLM, i.e. 80mV = 80 dB SPL. However, according to MX Measure, it's 33 mv/dB. Secondly, could you repeat your experiment with switching autoranging off and using sinusoidal signal with stepped amplitude? For example 70, 80, 90 dB? The AC output should have an RMS value proportional to pressure for current range. – jojek Mar 2 at 17:35

Based on a suggestion by Jojek in a comment, I tested the DC output using a sinusoidal signal of stepped amplitude:

enter image description here

The results are given for 90dB SPL, 80dB SPL and 70dB SPL as read on the meter display.

The plot also display the RMS value of the DC signal provided by the SLM. Barring measure error, the result matches the specifications of 33 mV/dB provided by MX Measure.

To answer my question, to externally read measure from that SLM, you must use the RMS value of the DC signal and apply the function:

dBSPL = 18 + 33 × RMS(VDC)

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