Time masking

Dad, when you were talking about frequency masking the other day, you also mentioned time masking, right? What does time masking mean?

Frequency masking occurs when two sounds of slightly different pitches are present simultaneously, with the lower sound making the higher-pitched sound harder to hear. Masking also occurs when sounds are out of sync. A sound immediately following a loud sound will remain masked for about 0.1 seconds after the loud sound fades away, making the sound in between difficult to hear. It's similar to the afterimage phenomenon of light. This is called temporal masking. This human masking characteristic is utilized in the information compression technology of audio media such as MDs, which you may be familiar with. MDs and the recently popular MP3s use this masking effect to avoid recording sounds that are actually present but are difficult to hear due to masking. Because MP3s don't encode all the sound like CDs, they can compress the data size to less than 1/10th.

Wow, so you can reduce the amount of data that much just by using the properties of masking. Are there any other things that are commonly used in everyday life?

Hmm, well, you could say that background music is used to mask unwanted sounds, to prevent them from being heard. Actually, there's a place where I think it could be used. Have you ever been on a bus that doesn't idle while stopped?

Yes, but how is that related to the use of masking?

When you turn off the engine, the inside of the bus suddenly becomes quiet, right? It's so quiet that you can hear people who were having a normal conversation whispering to each other. Doesn't that feel uncomfortable? You don't need to produce the exact same level of noise the moment you turn off the engine, but if there was a certain level of engine noise masking, that uncomfortable feeling would disappear, wouldn't it?

That's right, just because it's quiet doesn't necessarily mean it's comfortable.

Adding sound requires careful design, because for those who don't want to hear it, it's nothing but noise. Another successful example is using air conditioning noise as masking sound in offices. Offices have various sounds, such as phone conversations and, more recently, the sounds of office equipment like printers and copiers. In a quiet environment without other sounds, these sounds become more noticeable. Therefore, by cleverly utilizing sounds that aren't loud enough to be bothersome (depending on the frequency characteristics and timbre), like air conditioning noise, efforts are being made to make the office sound environment a little more comfortable.

Creating and designing sounds isn't limited to music, is it?

That's right. These days, "sound design" is being applied to industrial products in a broad sense, from the sound of closing a car door to the sound of folding a cell phone. Computer simulation technology is essential for that. I'll tell you more about it next time.