The circuit was designed to work with an audio power amplifier which operated off +18v-0v-18v power rails. The actual voltage used is not too critical except that the feedback is referred to the LED chain which itself is anchored to a +12v rail, hence the separate stabilizer for this.
http://www.4qdtec.com/avu.html
This unit is designed for monitoring the audio output level across a loudspeaker when carrying out alignment of radios. As no great precision is required, a simple passive circuit arrangement has been used.
http://www.vintage-radio.com/projects/output-indicator.html
Digitally recorded music on CDs offers superior quality to that recorded on vinyl records or tape, but most prerecorded CDs have an annoying characteristic: The average volume levels of the recorded signal can vary by as much as 14 dB from disk to disk.
http://www.edn.com/article/CA90363.html
This version is based on a National Semiconductor IC, and uses the logarithmic version. Each LED operates with a 3dB difference from the previous one, and a jumper is provided to allow dot or bar mode.
http://sound.westhost.com/project60.htm
The LM3916 is a monolithic integrated circuit that senses analog voltage levels and drives ten LEDs, LCDs or vacuum fluorescent displays, providing an electronic version of the popular VU meter.
http://www.national.com/pf/LM/LM3916.html
40 LEDs each accompanied by a comparator, reference voltages generated by a chain of resistors. Linear meter would have been easy to make, but a real VU meter has to be logarithmic. Maybe the meter could be used as a power meter in a power amplifier. VU meter uses formula 20log(V/V0) and power meter formula 10log(V/V0).
http://www.tekniikka.oamk.fi/~archy/vu-meter/index.html
The circuit can be operated from a pair of 9 Volt batteries, or a regulated supply of up to +/-15V. There is no need to use premium opamps unless extremely low noise levels are to be measured, and even then are not needed if there is a gain stage at the front end.
http://sound.westhost.com/project17.htm
This circuit is intended to signal through a flashing LED, the exceeding of a fixed threshold in room noise, chosen from three fixed levels, namely 50, 70 and 85 dB. Two Op-amps provide the necessary circuit gain for sounds picked-up by a miniature electret microphone to drive a LED.
http://www.electronics-lab.com/projects/sensors/012/
The simple peak detector in Figure 1 is the result of a need for a single-5V-supply, level/clipping indicator for a multimedia-PC sound system. The design is unique in that it detects both stereo channels on a single peak-hold capacitor. All the adjustments in the circuit simultaneously apply to both left and right stereo inputs.
http://www.edn.com/article/CA181912.html
This nifty sound level meter is a perfect one chip replacement for the standard analog meters. It is completely solid state and will never wear out. The whole circuit is based on the LM3915 audio level IC and uses only a few external components. This circuit can also be integrated into audio amp projects.
http://www.aaroncake.net/circuits/vumeter.htm