Quote: Thanks for the useful comments and reference. I'll check it out. dBL as in: "Unweighted sound pressure level is called "linear sound pressure level" and is often written as dBL or just L" (wiki info). I didn't measure the rms voltage with a voltmeter. I wrote an application which uses a high-end sound card and samples at 24bit/96kHz and computes the real rms voltage and calibrated with a precision voltage source. It seems to be accurate (~ 10%) up to 20kHz.