mosquito sound
Oto-kun
I saw on TV that a device that emits a mosquito sound was installed in a park, but I wonder if it really works?
father
Due to the continued acts of vandalism by young people in park facilities at night, it was said that measures were introduced in municipalities within Tokyo, referencing a successful case from the UK.
Oto-kun
You said there is a special sound that only young people can hear, but what kind of sound is it?
father
The device reported in the news was said to emit sounds above 17 kHz. I think there might also be some design to make the tone unpleasant...
Oto-kun
Earlier, you told me that the frequency range of sounds audible to humans is from 20 Hz to 20 kHz. Is it true that sounds above 17 kHz can only be heard by young people?
father
Human hearing declines with age, and this is particularly evident at higher frequencies. It has been confirmed that there is a clear difference in the ability to hear very high frequencies between high school students in their late teens and college students in their early twenties.
Oto-kun
Hearing loss begins at such a young age, huh?
father
That's right. As we age, our hearing gradually declines starting from the higher frequencies. Research comparing older adults to young people in their 20s has shown that there isn't much difference in low frequencies due to age, but there is a significant difference in high frequencies.
Oto-kun
How much difference is there in hearing ability?
father
When comparing young people in their 20s to older individuals in their 60s, there is only about a 10 dB difference up to around 1000 Hz, but at 2000 Hz, the difference is about 15 dB, at 4000 Hz it is about 25 dB, and at 8000 Hz it is about 35 dB, showing a significant decline in hearing ability in the high frequency range.
Oto-kun
Does it mean that people in their 70s and older have even more difficulty hearing?
father
That's right. The differences in low frequencies are not that significant, but for example, at 8000 Hz, there is a considerable difference in hearing ability: 55 dB for those in their 70s and over 65 dB for those in their 80s compared to those in their 20s.
Oto-kun
Elderly people experience hearing loss that is limited to high frequencies.
father
That's right. It's called age-related hearing loss, but it seems that the progression varies greatly from person to person.
Oto-kun
Do you know the reason?
father
We talked before about the structure of the inner ear, right? The cochlea is the organ that analyzes the pitch of sound, but it seems that the sensory cells there can develop impairments with age, and vascular issues can occur, leading to poor sound transmission within the inner ear.
Oto-kun
What kind of difficulties will arise in life when high-frequency sounds can no longer be heard?
father
First, it becomes difficult to hear consonants that are prominent in the high frequency range. Fortunately, Japanese is a vowel-based language with lower frequencies, but if one's native language is a consonant-based language like English, which has higher frequencies, age-related hearing loss may have a greater impact. I've heard that the usage rate of hearing aids in the U.S. is about twice that of Japan, which might be one reason for this.
Oto-kun
Do sounds from mobile phones and electronic sounds generated by machines become difficult to hear?
father
It is said that alert sounds such as warning tones, confirmation tones, and end tones in home appliances often use frequencies ranging from 1000 Hz to 4000 Hz, but as age-related hearing loss progresses, it becomes harder to hear them. Manufacturers need to take this into account.
Oto-kun
However, if we adjust for the elderly, it can become too loud for young people.
father
That's right. The design of alert sounds primarily emphasizes 'characteristic' features. Sounds that attract attention use higher frequencies than confirmation sounds, and the temporal aspect is also important. Consideration must be given to ensure that people of all ages can adequately recognize the information without it becoming noise, which means we must reconcile a kind of contradiction. Research in this field will likely continue to be important.