time masking
Oto-kun
Dad, when you talked about frequency masking the other day, you mentioned that there is also time masking. What does time masking mean?
father
Frequency masking occurs when sounds of slightly different pitches are present simultaneously, causing the lower sound to make the higher sound less audible. Masking also happens when sounds are misaligned. Sounds that follow a loud noise experience masking for about 0.1 seconds even after the loud sound has ceased, making them harder to hear during that time. This phenomenon is similar to the afterimage effect in vision. This is called temporal masking. Such human masking characteristics are utilized in audio media compression technologies, like those known in MD (MiniDisc). Formats like MD and the recently popular MP3 use these masking effects to avoid recording sounds that are masked, even if they are present. Unlike CDs, which encode all sounds, MP3 can compress data to less than 1/10 of the original size.
Oto-kun
Wow, just by utilizing the properties of masking, the amount of data can be reduced that much, huh? Are there any other familiar applications of this?
father
Hmm, that's right... Background music can be thought of as a way to mask unwanted sounds, right? Actually, I have a place in mind where this could be utilized. Have you ever ridden a bus that doesn't idle while stopped?
Oto-kun
I have it, but how is that related to the use of masking?
father
When the engine is turned off, the sound inside the bus suddenly becomes quiet, right? People who were having normal conversations can be heard whispering, and it becomes so quiet that it feels uncomfortable, doesn't it? I think that when the engine stops, there's no need to produce sound at the same level, but if there is a certain level of masking noise from the engine, that discomfort might disappear.
Oto-kun
That's right, being quiet doesn't necessarily mean it's comfortable.
father
Adding sound can be perceived as noise by those who do not want to hear it, so it should be designed carefully. One successful example is the use of air conditioning noise as masking sound in offices. Offices have various sounds, such as phone conversations and, more recently, noises from printers and copiers. In a quiet environment without other sounds, these noises can become bothersome. Therefore, by skillfully utilizing sounds that are not bothersome, even if they are at a level that is not loud like air conditioning noise (which depends on frequency characteristics and timbre), efforts are made to make the auditory environment in the office more comfortable.
Oto-kun
Creating and designing sound is not just about music, is it?
father
That's right, recently, in a broad sense of industrial products, everything from the sound of closing a car door to the sound of folding a mobile phone is designed as 'sound design.' Computer simulation technology is essential for that. I'll tell you more about it next time.