Ken: It was on the news that mosquito sound producing device were placed in a park, but does it have any effects?

Dad: Since vandalism by youth of park facility has continued at night, local government of Tokyo has implemented it in reference to the success in England.

Ken: The news said it's a special noise that only young people can here. But what kind of noise is it?

Dad: The device that is being broadcasted on the news is said to produce a noise higher than 17 kHz. Also, unpleasant tone may be used.

Ken: You taught me before that the frequency range that a person can hear is 20 Hz to 20 kHz. Are frequencies higher than 17 kHz only heard by young people?

Dad: With age, human's hearing ability decreases. That's obviously noticeable in high frequency range. It has been confirmed that there is an apparent difference in hearing ability of high frequency between high-school teens in their late 10's and college students in their early 20's.

Ken: Decreasing of hearing ability starts from that early of an age?

Dad: Yes. With the increase of age, hearing ability of higher frequency gradually drops and with an experiment comparing elderly and somebody in their 20's, there isn't much difference with age for lower frequency but a big difference for the higher frequency.

Ken: How much difference in hearing ability is there?

Dad: When you compare a youngster in their 20's and a senior in their 60's, there's only about 10 dB difference up until 1000 Hz, but at 2000 Hz about 15 dB, 25 dB at 4000 Hz, and 35 dB at 5000 Hz, making it clear the decreasing in hearing power in the higher frequency range.

Ken: Can you hear even less when you are over the 70's?

Dad: Yes. There's not much difference in the lower frequencies, but for example with the 8000 Hz, in comparison to the 20's, 70's shows significant difference in hearing power of 55 dB, and 80's shown greater than 65 dB.

Ken: Elders having difficultly hearing is limited to the high frequency ranges?

Dad: Yes. It's called presbyacusis, but the progress has differences between individuals.

Ken: Do we know the cause?

Dad: I spoke about the structure of the inner ear before. There is an organ called cochlea that analyzes the pitch of a sound. When the sensory cell or blood vessel in it deteriorates with age, it hinders the sound transfer within the inner ear.

Ken: When you can't hear high sound range, I wonder what life obstacle would emerge?

Dad:First, consonants that have large high-frequency component would be difficult to hear. Japanese is vowel based and frequency is low, but if your native language is like English, where its consonant based, influences of presbyacusis can be greater.

Ken: Would it be difficult to hear voices on the cell phone and electronic alarm sounds?

Dad: Warning and notification sounds of many home electrical appliances use the sound of 1000 Hz to 4000 Hz. So it would be difficult to hear them if Presbyacusis progresses. Designers should consider that.

Ken: But the sound for elderly may be too loud for youngsters.

Dad: Yes. For notification sound design, the characteristics of the sound is very important. The warning sound should use higher frequency than the confirmation sound, and it should also make the people feel the sense of urgency. Notification sounds should deliver information to people of all ages, and at the same time they should never be noises to any individual. To cope with such antinomies, further studies will be needed.