frequency masking
Oto-kun
Dad, you talked about something called sound masking before, right? Today, while I was on the train, I could hear the announcements clearly when we were above ground, but as soon as we went underground, I couldn't hear them well at all. I was wondering if this is related to that sound masking thing.
father
Yes, that's right. So today, let me explain a bit more about masking. It might be a little difficult though...
The announcement inside the train that Oto mentioned became hard to hear when it went underground because the noise level inside the train increased, and this can also be called a masking phenomenon. This phenomenon is deeply related to the structure of the human ear, and it has two characteristics.
In our surroundings, there are various sounds ranging from high-pitched sounds to low rumbling sounds, but the first characteristic is the phenomenon of masking related to the frequency of those sounds, that is, the pitch of the sound.
If there is a sound at a certain pitch and there is a slightly lower sound, the slightly lower sound makes it harder to hear the higher sound. When the pitches of the two sounds are further apart, both sounds can be heard again. It also depends on the volume of the two sounds.
The announcement inside the train that Oto mentioned became hard to hear when it went underground because the noise level inside the train increased, and this can also be called a masking phenomenon. This phenomenon is deeply related to the structure of the human ear, and it has two characteristics.
In our surroundings, there are various sounds ranging from high-pitched sounds to low rumbling sounds, but the first characteristic is the phenomenon of masking related to the frequency of those sounds, that is, the pitch of the sound.
If there is a sound at a certain pitch and there is a slightly lower sound, the slightly lower sound makes it harder to hear the higher sound. When the pitches of the two sounds are further apart, both sounds can be heard again. It also depends on the volume of the two sounds.
Oto-kun
What is the relationship between that matter and the structure of the human ear?
father
I think you have learned about the structure of the ear broadly in science class, but the sound that enters the ear canal (external auditory canal) shakes the eardrum, and the vibrations of the eardrum are transmitted to the three small bones in the middle ear, and then to the cochlea, which is a bone shaped like a snail. From the cochlea, it goes to the inner ear, but the inside is filled with a liquid called lymph, and it is connected to the exit of the cochlea by a membrane called the basilar membrane.
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(Figure 1)
Oto-kun
Where does the exit of the snail lead to?
father
Although it is called an exit, it is actually not an exit, but rather it is blocked there. Therefore, the vibrations that enter the cochlear entrance are absorbed through the lymph fluid because there is no exit, causing the basilar membrane to vibrate. When high sounds come in, the part of the basilar membrane near the cochlear entrance vibrates, while when low sounds come in, the part further away vibrates.
Oto-kun
Is there a relationship between the way the membrane vibrates and the fact that the higher sound among the two sounds mentioned by father is canceled out by the lower sound?
father
You noticed a good point. As you said, 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 sharply decreases from the point where it is largest. As I mentioned earlier, the sounds closer to the entrance are the higher ones. Therefore, when there are two sounds that are not too far apart in pitch, the higher sound remains vibrating and gets masked.
The basilar membrane has hair cells, and the cells stimulated by the vibration of the basilar membrane transmit information to the brain via nerve fibers, ultimately being recognized as sound information.
The basilar membrane has hair cells, and the cells stimulated by the vibration of the basilar membrane transmit information to the brain via nerve fibers, ultimately being recognized as sound information.
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Figure 2
Oto-kun
Wow, after the sound enters the ear, various processes are happening inside the ear. It was a bit difficult, but I understood that masking is related to the structure of the ear.
So, what is the other feature?
So, what is the other feature?
father
What I talked about earlier is a phenomenon caused by the pitch of sounds, which is called frequency masking. There is another phenomenon called time masking, which occurs when two sounds exist at different times. I'll talk about this next time.