the volume of sound
Oto-kun
I noticed something a little interesting.
father
You've been turning the TV volume up and down for a while now, right? Is that what you mean?
Oto-kun
Yeah. When I wear headphones and gradually increase the volume, the high sounds don't change much, but the low sounds become easier to hear. Conversely, when I lower the volume, the bass becomes harder to hear. I wondered if it was a problem with the headphones, so I tried it on the TV as well, but it was the same.
father
You noticed something interesting. The way the loudness of the sound felt is the correct perception.
Oto-kun
I see, that's how it is. It means that louder sounds make lower sounds seem louder.
father
Do you remember when we talked about the noise level *?
Oto-kun
Somehow. The perception of the loudness of sounds varies depending on frequency, and the noise level is an adjustment based on that.
father
That's right. I think I only explained it to that extent the other day, so today, I'll provide a more detailed explanation. As Oto said, human hearing is designed to perceive sounds at the same sound pressure level as varying in loudness depending on the frequency.
Oto-kun
Indeed, people are less sensitive to lower sounds. In other words, unless a loud sound is produced, it won't be perceived as being the same volume as a higher sound.
father
Yes. Look at this graph. This is called the equal-loudness contour for pure tones, and it represents the frequency characteristics created by connecting sound pressure levels at which sounds of various frequencies are perceived to be of the same loudness.
Oto-kun
There are red and blue lines, but I should just look at the red one, right?
father
Yeah. A few years ago, the standards were changed to reflect the results of extensive experiments. The reference is a sound at 1000 Hz, and for example, if you look at the line for 40 phon *1, you can see that the sound pressure level at 1000 Hz is 40 dB, while at 125 Hz it is 60 dB, and at 63 Hz it is around 73 dB.
Oto-kun
This means that in order to hear a sound at 125 Hz as loud as a sound at 1000 Hz at 40 dB, it needs to be at a sound pressure level that is 20 dB higher, and for 63 Hz, it needs to be at a sound pressure level that is about 33 dB higher.
father
That's right. However, what I noticed is that when the volume changed, it felt like the balance between the loudness of high and low sounds changed.
Oto-kun
Yes. So, in this graph, we should compare the 40 phon line and, for example, the 80 phon line.
father
At 80 phons, the sound pressure level at 1000 Hz is 80 dB, while at 125 Hz it is 90 dB, making them the same loudness. Since the difference at 40 phons was 20 dB, there is a 10 dB difference here.
Oto-kun
Conversely, for sounds with frequencies higher than 1000 Hz, they are not significantly affected by the loudness. The feeling that the bass frequencies became more pronounced when the sound was made louder was due to this.
father
That's right. The A-weighted characteristic of noise level, which is a frequency-weighted characteristic, considers 40dB as a representative sound level of the most likely everyday noise, and has been used for many years since the 1930s as a corrected evaluation value of noise using the nearly inverse characteristic of the equal loudness contour of a 40 phon pure tone. *2
Oto-kun
Has it been used for such a long time?
father
That's right. It can be said that it is an excellent evaluation value. It's wonderful that such a complex sense of sound intensity can be expressed with a single number.
Oto-kun
However, since the reference level is set at 40 dB, it means that for sounds quieter or louder than this, the perception of noise levels can be misaligned.
father
That's right. Not only that, but the volume of sound is also related to the effect of masking that we talked about a long time ago.
Oto-kun
That's right. I'd like to hear more about that next time.