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.
In the figure we have the typical configuration of a Hartley oscillator. This circuit generates...
Found in an Electronics Now from the 90s this mixer has several inputs. The integrated circuit can...
This useful power supply was found in an 1985 Italian publication, but can still be assembled...