Smaller than the 9-volt battery that powers it, the Musicator Jr. displays the sound it 'hears' (through the Electret Microphone) as fluctuating light bars.
Small enough to fit in your shirt pocket, it can also be placed on a flat surface to monitor the sound levels around it.
An alkaline battery will easily power it 20 or more hours.
Step 1: The parts needed
The 'brains' of this project is a LM358 general-purpose op-amp which costs under 30-cents. The first half of the circuit is an amplifier which boosts the 500-micro-volts from an electret mic to about 1-volt. This level is generally called 'Line-level' and can be used to drive our LEDs, an audio amp, or even the input pins of an Arduino processor.
The second half of the op-amp is used as a voltage-to-current converter, which limits the brightness of the LEDs to 10mA or less.
The complete list of parts is below:
LEDs. Any combination can be used, as long as their total forward voltages is less then 8. For example, you can have up to 4 amber LEDs with 1.8v Vf.
Electret microphone - I got mine on eBay for under 25-cent
LM358 - Op-amp (8-pin DIP). Also available on eBay.
2N4401 - NPN general transistor (other audio NPN-types will probably work as well)
10k resistor x 5
2.2k resistor x 1
470k resistor x 1 (Can also be 330k as labeled in the circuit)
100-ohm resistor x 1
9-volt battery and connector
Perf-board and mounting parts.
Total cost: $3 or less.