音のうなり
Oto-kun
When tuning a guitar, you adjust the peg of the string you are tuning until the beats of the two sounds disappear, right? Why do those beats occur?
father
Ah, guitar tuning, right? Nowadays, we have tuners, so it's less common to tune by ear like that. Beats occur from the interference of sounds with slightly different frequencies, and the difference in those frequencies has the characteristic of becoming a fluctuating frequency.
Oto-kun
What does it mean that the difference in frequency becomes the fluctuating frequency?
father
Look at this diagram (below). Waveform A represents a sound of 200 Hz, and waveform B represents a sound of 205 Hz. First, let's listen to these two sounds.
Oto-kun
When you listen attentively, you can tell there are differences in pitch.
father
That's right. Now, let's try overlapping these two sounds. When we overlay the waveforms of A and B, we get the waveform of C.
Oto-kun
When you overlay two waveforms, it takes this shape, right? Does the difference become a variable frequency? ...
father
First, listen to the sound.
Oto-kun
Wow. I can clearly hear the growling.
father
That's right. The current sound fluctuates 5 times per second. In other words, the fluctuation frequency is 5 Hz, which is the difference between the frequencies of two sounds.
Oto-kun
But, why?
father
Figure 2 is an enlarged view of this part, but take a look at waveforms A and B at the position where their combined amplitude is zero.
Oto-kun
Waveform A has a maximum value on the positive side, while waveform B has a minimum value on the negative side. Since they are equal in magnitude but opposite in sign, does that mean they cancel each other out to become zero?
father
That's right. However, it happens periodically, which causes a growl.
Oto-kun
Why does it happen periodically...?
father
The difference of 5 Hz between 200 Hz and 205 Hz has a ratio of 40 to 200 Hz. Therefore, for every 40 waves, as shown in this diagram, the amplitude of the combined waves becomes ±0.
Oto-kun
I see. I remembered the fluctuating sound that you told me about before, during the story of 'The Crying Dragon'.
father
That's right. It's a related topic. Next time, I'll explain the characteristic phenomenon that occurs when two sound frequencies are separated under certain conditions.