What I am doing
I built a circuit using a 555 timing chip in astable mode which produces a square wave. There is a potentiometer which allows me to adjust the frequency produced.
One aspect of this circuit is that the square wave is always uneven, i.e., the high and low times are different:
time_high = 0.693 * (r1 + r2) * c1 time_low = 0.693 * r2 * c1 fequency = 1 second / (time_high + time_low)
This difference is smallest when R2 is significantly higher than R1.
What I am observing
When I used an audio microphone to record the output of the circuit. I generated frequency graphs for different sections of the recording. I noticed that at a given R2 position there are two spikes and at other R2 positions there sometimes many more (spikes meaning the circuit is producing a wave at that frequency).
Initially, I was not able to explain this, however, now I am suspecting that these overtones are produced from the relationship of the speaker load and the square wave. (This is why the overtones do not appear in a circuit simulator.)
In other words, when the voltage moves from high to low, the speaker cone is not able to change with the same speed because of its air resistance and mass. If the square wave had a perfect 50% duty cycle this "rounding" effect would be balanced between each push and pull. Since my circuit is asymmetric, perhaps the speaker cone is never pushed fully out when the voltage is high before the voltage goes low and starts to pull the speaker cone back. This, in my head, seems like it would create little humps on the waves.
I kind of just want some thoughts on this. Does what I am saying make sense? If so, is there a name for this effect? If I am correct, could someone point me to an article about this?
Here are the spectrum graphs side by side at two different knob positions. You can see that on the left (238 Hz) there are some overtones but they are modest in relation to the spikes when the circuit is generating a higher frequency wave (771 Hz).
Additionally, considering the formulas, the lower frequency wave is more symmetric than the higher the frequency wave. This is because R2 is > 100x the difference of R1.
I understand that jitters in the square wave will produce "overtones", however, does the speaker load affect it? Can someone speak to that?
This wave form looks nothing like a square wave. It looks like a bunch of sign waves. Which makes sense because a speaker cannot make a "square wave," right?
Also, why are there such strong differences between the graph of 238 Hz and 771 Hz?