Finally, that one did the job:
http://www.ebay.com/itm/Digital-Noise-Pressure-tester-Level-Meter-30-130dB-Decibel-Sound-Measurement-XP-/112336473823
Be aware that a similar looking model exists with USB connection, which has internally a completely different design, and is unusable for our purpose.
So pay attention on the exact keys arrangement.
That sound pressure level meter delivers an analog output 0-1,1V proportional to dB (10mV/db) , that is bounded to ground, which make it easy to share the power supply with an Arduino.
It requires theoretically a 6-9V power supply and must be started by a pushbutton.
With the following hack, you may however use a plain 5 V supply and the meter will start automatically.
The green wire is soldered to the right inside contact of the DC jack.
The blue wire goes to ground, the red one goes to 5V, the green one to A0 of the Arduino.
Used with analogReferenc(INTERNAL) you get directly a 0,1 dB resolution for your sketch.
You now have got for less than 20 dollar the only perfect Arduino sound level pressure meter available on the market.
Enjoy!