Frequency masking

Dad, you told me about sound masking before, right? Today, I was on the train, and while we were above ground, I could hear the announcements clearly, but as soon as we went underground, I couldn't hear them well anymore. I was wondering if that was what you meant by sound masking...

Okay, so today I'll explain masking in a little more detail. It might be a bit difficult, though...
The reason why the train announcements that Oto mentioned became harder to hear when the train went underground is because the noise level inside the train increased, which can also be called a masking phenomenon. This phenomenon is deeply related to the structure of the human ear, and it has two characteristics.
There are sounds of all pitches around us, from high-pitched sirens to deep thuds, but the first characteristic is the masking phenomenon, which is related to the frequency of the sound, or in other words, the pitch of the sound.
If you have a sound at a certain pitch and another sound that's slightly lower, that lower sound makes it harder to hear the higher-pitched sound. When the pitches of the two sounds are further apart, you can hear both sounds again. It also depends on the volume of the two sounds, of course.

What does that have to do with the structure of the human ear?

You probably learned the basics of the ear's structure in science class, but when sound enters the ear canal, it vibrates the eardrum, and the vibration of the eardrum is transmitted to three small bones in the middle ear, and then to a bone called the cochlea, which is shaped like a snail. The cochlea is where the inner ear begins, and its interior is filled with a fluid called lymph, and it is connected to the exit of the cochlea by a membrane called the basilar membrane.

Where does the exit of the snail's shell lead?

I called it an exit, but it's not really an exit; it's blocked there. So, vibrations that enter the entrance of the cochlea have no exit, and are absorbed by traveling through the lymphatic fluid and vibrating the basilar membrane. The part of the basilar membrane closer to the entrance of the cochlea vibrates when high-pitched sounds enter, and the part further away vibrates when low-pitched sounds enter.

Is there a connection between how the membrane vibrates and how the higher-pitched sound of the two sounds Dad said gets drowned out by the lower-pitched sound?

You've noticed something important. You're right, the way the membrane vibrates affects sound masking. When a sound enters, the way the membrane vibrates gradually increases in amplitude from the side closer to the entrance, and then the vibration decreases sharply from the point where it's at its maximum. As I said earlier, higher-pitched sounds are closer to the entrance. So, when there are two sounds that are not far apart in pitch, the higher-pitched sound retains its vibration and gets masked.
The basement membrane contains hair cells, and when these cells are stimulated by vibrations in the basement membrane, the information is transmitted to the brain via nerve fibers, and ultimately, auditory information is recognized.

Wow, so after sound enters the ear, all sorts of processing happens inside. It was a little difficult to understand, but I got the idea that masking is related to the structure of the ear.
So, what's the other feature?

What we just talked about is a phenomenon caused by the pitch of sound, so it's called frequency masking. But there's another type of masking phenomenon called time masking, which occurs when two sounds exist at different times. We'll talk about that another time.