We found this circuit in an English publication from the 60's. The transistors are of germanium of the time and the headphones must be high impedance magnetic.
Found in an American publication from the 1980s, this circuit can be found in commercial sound...
With the use of an additional diode and a resistor, the configuration of the figure can be...
This sound meter or sonometer was found in a Popular Electronics from the 80's. The circuit can be...