1. What is the Mosquito Tone, a 17,000-hertz sound that only young people can hear?
The device emits a sound known as the "Mosquito Tone," with a frequency of 17,000 hertz. Hertz measures the number of vibrations per second; a higher number means a higher pitch. 17,000 hertz sits at the very top edge of the range humans can hear [1]. Reports state that most people lose the ability to hear this pitch by age 25. However, hearing varies by individual. This is not a rule that everyone loses hearing on their 25th birthday, but a guideline from 2006 news reports [1]. The sound is unpleasant for teens and almost inaudible for adults. Where does this difference begin deep inside the ear?
2. How the snail-shaped cochlea in the ear handles high sounds
Deep inside the ear is the cochlea, shaped like a snail. Inside it lies the basilar membrane, lined with hair cells. Hair cells at the wide side of the snail, which is the entrance, detect high sounds, while cells in other locations detect different pitches. For example, high-pitched sounds like a baby’s cry are caught at the entrance [2]. This is similar to how a music box works. Just as the length of each tooth determines its pitch, specific spots in the cochlea are assigned to specific pitches. But unlike a music box where the tooth itself vibrates to make sound, hair cells in the cochlea create signals in response to sound. Thanks to this division of labor, the brain knows the pitch of a sound by identifying which location sent the signal. So, how do hair cells turn sound into nerve signals?
3. How hair cells in the cochlea turn sound into electrical signals
When sound reaches the cochlea, the hairs on the hair cells bend. At the tips of these hairs are channel-like holes. When they bend, these channels open. Chemicals then flow into the cell, creating an electrical signal [2]. This signal travels through nerves to the brain, where we perceive it as sound. Bending the hairs is the first step in converting sound to a signal. If cells decrease, sounds at the pitch that location handled become harder to signal. In age-related hearing loss, the main cause is thought to be the gradual loss of sensory cells in the cochlea [3]. This loss does not happen everywhere at once, but starts in specific places.
4. Why the hair cells for high-pitched sounds are more easily damaged
Age-related hearing loss is generally said to start with high sounds. A public relations magazine from Tochigi Prefecture explains that high sounds become gradually harder to hear [4]. Research abstracts suggest that hair cells tuned to high frequencies are also more vulnerable to damage from noise or drugs, and are the first to be lost with aging [5]. However, this information comes from a search result excerpt. Why are high-frequency cells weaker? One hypothesis involves differences in calcium regulation. But this is not a confirmed cause. It is safer to conclude that the reason is not fully understood. What is clear is that we face the problem of how to replace cells that have been damaged and lost.
5. How losing high-pitched hearing makes speech harder to understand
When high sounds become hard to hear, sounds containing many high-frequency components, such as the "s" and "k" sounds in Japanese, often become harder to distinguish than others [3]. It is not that the sound disappears completely, but that parts of words get buried. Japanese medical information states that symptoms often appear around the late 60s. By age 80, over 80% of men and over 70% of women show signs of age-related hearing loss [3]. However, these numbers depend on diagnostic criteria and do not apply to "everyone." A US national agency explains that about 1 in 3 people aged 65–74 and nearly half of those 75 and older have hearing difficulties [6]. These are US figures, and because the methods differ from Japan, they cannot be simply compared. One question remains: Can damaged hair cells return?
6. Why birds can recover hair cells but humans cannot
Research news from Kyoto University explains that auditory hair cells in mammals, including humans, do not regenerate once damaged. Therefore, hearing loss caused by hair cell damage is considered difficult to restore naturally [7]. On the other hand, in birds, when hair cells are damaged, new hair cells regenerate from surrounding support cells [7]. Even though they are both "ear hair cells," the fate after injury differs between birds and humans. Why can humans not regenerate? Exploring this difference is a major question in hearing research. If an answer is found, it could open new paths for ears damaged by aging or noise.
7. How to ask people close to you about their hearing
First, check if it is okay to ask people close to you about their hearing. If they are willing, ask, "What helps when you’re listening to someone?" Answers will vary. Some people have trouble with high sounds, while others do not. If you want to read the original text, Kyoto University’s research news is published in Japanese [7]. Compare how experts describe the difference between bird regeneration and mammals. Materials also include the Tochigi Prefecture magazine and medical information [4]. Comparing explanations from public institutions with medical information reveals that even the same phenomenon can be described in different ways.