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.
- 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?