A spectrum analyzer is basically a device which is technically used for assessing a frequency source with respect to its strength.
Usually this type of circuit will be quite complicated, however here we are interested in getting a visual display for fun and pleasure therefore accuracy may not be so important.
Here we'll discuss only one channel of the spectrum analyzer circuit, any number of such channels can be built and put together for getting the required results.
As can be seen in the figure, the circuit of the proposed audio spectrum analyzer consists of two main stages.
The left stage can be witnessed to be an active tone control stage while the right side IC LM3915 stage is a 10 stage dot/bar LED display stage.
The tone control stage is a simple bass/treble boost circuit which can be set for acquiring the intended magnitude of signal for a particular fed frequency. This can be done with the help of the two pots.
P1 may be set for controlling the bass or the low frequency band, while P2 can be adjusted for achieving the high frequency content from the input.
The led driver stage basically responds to a DC level applied to its pin#5.
This response is converted into a sequencing to and fro movement of the LED connected at its outputs.
For example, at voltage levels around 0 and 2, the first three or four LEDs would respond creating a up/down dancing movement, the subsequent LEDs would respond in similar fashion as the input voltage rises at pin#5 of the IC.
The active tone settings decide which frequency level is allowed to get past to the output or amplified to the output of C3.
Suppose if you adjust P1 such that only frequencies within 200 Hz are allowed to pass, the LEDs will produce maximum rise and fall only for these frequencies, and if the music content lacks these frequencies will result in a lower rise or fall in the sequencing.
Similarly you can adjust different frequency ranges for the additional channels in order to achieve the intended fluctuations over the connected LED driver output.
You can make 3 of these or may be 30 of these, just arrange them serially, adjust the pots as per the required specs and see the LED bars dazzle in a up/down motion producing a stunning audio spectrum graphic analysis.