Mosquito tone
Otokun
I saw a news report on TV about a device that emits mosquito sounds being installed in a park, but I wonder if it's actually effective?
father
I heard that because of the continued vandalism of park facilities by young people late at night, local governments in Tokyo have adopted a similar system, referencing a successful example in the UK.
Otokun
They said it's a special sound that only young people can hear, but what kind of sound is it?
father
The device reported on the news was said to emit sounds above 17 kHz. I imagine they've probably taken some measures to make the sound unpleasant...
Otokun
You previously told me that the frequency range of sounds humans can hear is 20 Hz to 20 kHz, right? Does that mean that sounds above 17 kHz can only be heard by young people?
father
Human hearing deteriorates with age, and this is particularly noticeable at higher frequencies. Studies have shown a clear difference in hearing at very high frequencies between high school students in their late teens and college students in their early twenties.
Otokun
So hearing loss can start at such a young age.
father
That's right. As we age, our hearing gradually deteriorates, starting with the higher frequencies. A study comparing older adults with young people in their 20s found that while there isn't much difference in hearing at lower frequencies with age, there is a considerable difference in hearing at higher frequencies.
Otokun
How much difference is there in hearing?
father
When comparing young people in their 20s with older people in their 60s, there's only about a 10 dB difference up to around 1000 Hz, but the hearing loss is significant in the higher frequency range: about 15 dB at 2000 Hz, about 25 dB at 4000 Hz, and about 35 dB at 8000 Hz.
Otokun
Does that mean that hearing deteriorates even more for people over 70?
father
That's right. The difference isn't that big at lower frequencies, but for example, at 8000 Hz, the difference in hearing between someone in their 20s and someone in their 70s is quite significant: 55 dB, and over 65 dB for someone in their 80s.
Otokun
It seems that hearing loss in older people is limited to the high-frequency range.
father
That's right. It's called age-related hearing loss, but the progression seems to vary greatly from person to person.
Otokun
Do you know the cause?
father
We talked about the structure of the inner ear before, didn't we? The cochlea is the organ that analyzes pitch, but apparently, as we age, the sensory cells there can become damaged, or blood vessels can be damaged, which can impair sound transmission within the inner ear.
Otokun
I wonder what kind of problems would arise in daily life if I lost the ability to hear high-pitched sounds...
father
First, it becomes difficult to hear consonants, which are predominantly in the high-frequency range. Fortunately, Japanese is a vowel-based language with low frequencies, but if your native language is a consonant-based language with high frequencies, like English, age-related hearing loss may have a greater impact. I've heard that the hearing aid usage rate in the US is about twice that of Japan, and that might be one reason.
Otokun
Will sounds like cell phone noises or electronic sounds from machines become harder to hear?
father
Many home appliances use sounds in the 1000 Hz to 4000 Hz range for notification sounds such as warning sounds, confirmation sounds, and completion sounds, but these will likely become difficult to hear as age-related hearing loss progresses. Manufacturers should take this into consideration.
Otokun
But if you adjust it to suit older people, it might end up being too loud for younger people.
father
That's right. When designing notification sounds, the primary focus is on making them sound "authentic." Attention-grabbing sounds use higher frequencies than confirmation sounds, and the temporal element is also important. The sound must be easily understood by people of all ages, while also being designed not to be noise—it's a kind of trade-off that must be reconciled. Research in this field will continue to be important.