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.
Avago Technologies (www.avago.com) suggests the circuit in Figure 9 for amplifying signals of...
We found this circuit in a 1986 publication. However, as it is based entirely on 555 integrated...
This circuit can be used to listen for SSB signals on receivers that do not have the built-in...