Any sound can have more than one frequency components that mixed together. Spectrum analysis is a technique to decompose such complex signals to several individual frequency components which mixed together to give that signal. The decomposing process is done using filters of different center frequencies. In this tutorial, three baseband filters are used to extract three frequency components and display their amplitude on LED bars.
The circuit shown in Fig.(1) performs the spectrum analysis task. The first stage, low noise amplification stage, amplifies the input voice which picked up by Mic, while the second stage is made up of three baseband filter with center frequencies 3KHZ, 5KHZ, 8KHZ respectively. The output of the second stage is delivered to the a LED bar driver that senses the analog input signal level and displaying it on the corresponding connected LEDs.