This sound meter or sonometer was found in a Popular Electronics from the 80's. The circuit can be easily implemented today, and the microphone is made of electret or ceramic.
This circuit was found in General Electric's 1965 documentation. It appears elsewhere on the site....
We found this circuit in a 1962 publication by General Electric. The circuit applies to common...
This multi-output oscillator uses CMOS integrated circuits that are uncommon today, but they can...