Ken: Dad, when you told me about the frequency masking, you also mentioned temporal masking, didn’t you? What does the temporal masking mean?

Dad: The frequency masking is caused when a low-tone sound makes the high-tone be heard difficult if there is a certain difference in tones between the two occurred at the same time. The masking occurs also when there is a certain time-delay between the two sounds. The sound occurred right after a loud sound is difficult to hear due to masking for 0.1 second even after the louder has vanished already. This is similar to the light afterimage phenomenon. This is called temporal masking. This kind of masking characteristic of human being is utilized for such information compression technique of audio media as Mini Disc (MD). MD and MP3 which are popular these days utilize the effect of masking, not to record the sound which is hard to hear by masking. MP3 does not encode all the sound as CD does, it can compress data volume to less than one tenth.

Ken: Huh, by utilizing the characteristic of masking, you can reduce the volume of data to that extent. Is there another thing which utilizes its characteristic?

Dad: Well, BGM may be one thing which decreases unwanted sounds utilizing masking characteristic. I have an idea to use its characteristic for a certain place. Have you ever got on a bus without idling while stopping?

Ken: Yes, I have, but how does it relate to the masking utilization?

Dad: When it stops engine, it gets quiet suddenly inside. It is so quiet that you can still hear passengers’ talking even if they start whispering instead. Don’t you think it is uncomfortable? If there is a masking sound for engine sound, not necessary to be the same level, such a discomfort will disappear, won’t it?

Ken: I agree. It is not necessarily true that being quiet around is comfortable in any case.

Dad: Since adding a certain sound is uncomfortable for those who don’t want, it should be designed with care. One case where masking works well is that noise of air conditioning in offices is used for masking. There are varieties of sounds such as talking over phone and OA devices sound such as printer and copier. If there is no noise except these sounds, people get aware of them. So, comfortable sound environment utilizing air conditioning sound makes offices more desirable as far as the utilized sound is too low to be annoyed (though it depends on the frequency characteristic and tone).

Ken: Creating or designing sound is not only for music, is it?

Dad: You are right, Ken. Sound is designed for industrial goods in a broad sense, including sound in shutting automobile doors and folding cell phone. Computer simulation technique is inevitable for those designs. Let’s talk about it next time.