This is a nice simple sound meter indicator. Small enough to build directly onto a perf board and all of the components are quite common which will keep the build cost down.

“This project uses an LM3915 bar-graph IC driving two sets of ten LEDs for a 30dB range. The circuit is unique because it has an additional range of 20dB provided by an automatic gain control to allow it to be very sensitive to low sound levels but it increases its range 20dB for loud sounds.”