Sound hum
Otokun
When you tune a guitar, you adjust the tuning peg on the string you're tuning until the beat between the two notes disappears, right? Why does that beat occur?
father
Ah, guitar tuning. Nowadays, we have tuners, so we don't tune our guitars that way as much anymore. A beat is the interference of sounds with slightly different frequencies, and the difference in those frequencies is what creates the fluctuating frequency.
Otokun
What does it mean that a difference in frequency becomes a fluctuating frequency?
father
Take a look at this diagram (below). Waveform A represents a 200 Hz sound, and waveform B represents a 205 Hz sound. Let's listen to these two sounds first.
Otokun
If you listen carefully, you can hear the difference in pitch.
father
That's right. So let's try combining these two sounds. When you combine the waveforms of A and B, you get the waveform of C.
Otokun
When you superimpose two waveforms, it takes this shape. Does the difference represent the fluctuating frequency? ...
father
First, listen to the sound.
Otokun
Wow. I can clearly hear the growl.
father
That's right. The sound you're hearing now is fluctuating 5 times per second. In other words, the fluctuation frequency is 5 Hz, which is the difference between the frequencies of the two sounds.
Otokun
But why?
father
Figure 2 is a magnified view of this section. Look at waveforms A and B at the point where the amplitude of the superimposed waveforms becomes zero.
Otokun
Waveform A has its maximum value on the positive side, and waveform B has its minimum value on the negative side. Since they are the same value but one is positive and the other negative, do they cancel each other out and become 0?
father
That's right. But when that happens periodically, it creates a beat.
Otokun
Why does this happen periodically...?
father
The ratio of the difference between 200 Hz and 205 Hz, which is 5 Hz, to 200 Hz is 40. Therefore, every 40 waves, it looks like this diagram, and the sum of the amplitudes of the waves becomes ±0.
Otokun
I see. That reminds me of the fluctuating sounds you told me about when we were talking about the "crying dragon."
father
That's right. It's related. Next time, I'll explain the characteristic phenomena that occur under certain conditions when the frequencies of two sounds are far apart.