How is hearing loss caused?
Hearing loss can have many causes, the most common of which is aging and noise damage.
Genetic factors (both congenital and acquired), ototoxic medications, and many ear diseases can cause hearing loss to occur.
Of these, 90% of hearing loss is sensorineural. In this hearing loss condition, otherwise, sound hair cells in the cochlea are lost. In these damaged areas, the nerve cells no longer produce electrical impulses. As a result, information is missing or erroneous.
Typically, the higher the frequency, the more likely the area is to be damaged. Therefore, high-frequency hearing loss is one of the most common types of hearing loss.
High-frequency hearing loss is a loss of hearing in the 2000-8000 Hz range, making it more challenging to hear already soft high-frequency sounds.
These high-frequency sounds include t, sh, f, p, and s. It can manifest as insensitivity to female and children's voices, inability to hear birdsong or even doorbells, etc.
Why are the higher frequency areas more susceptible to damage?
The higher frequency areas are more susceptible to damage due to several objective factors.
The main elements are the natural resonance effect of the external auditory canal, the cochlea's anatomy, and the auditory nerve's distribution.
The natural resonance effect of the external auditory canal
In the frequency range of 2 kHz to 4 kHz, the external auditory canal's gain to sound is the greatest. It can be amplified by 10-15 dB at the tympanic membrane.
Studies have shown that noise-induced hearing loss is often located at an octave above the noise frequency.
The external auditory canal's natural resonance peak is 2kHz-4kHz, so it is most likely to cause hearing loss in the 4kHz-6kHz frequency range.
Anatomy of the cochlea
The outer shape of the cochlea resembles that of a snail, with a spiral shape. Different cochlear basilar membrane areas sense different frequencies of sound, with the bottom of the cochlea sensing high frequencies and the top of the cochlea sensing low frequencies.
However, both high-frequency and low-frequency sounds need to be transmitted from the bottom up through the basilar membrane's vibrations around the cochlea base, so the basilar membrane, which senses high frequencies, is more likely to be tired and damaged.
The base of the cochlea is also more susceptible to ototoxic drugs.
Distribution of the auditory nerve
The auditory nerve's outer layer is composed of nerves from the cochlea base, transmitting high-frequency sounds. The central part is composed of nerves from the apex of the cochlea, which share low-frequency sounds.
Changes in the physical and chemical environment, such as trauma, ischemia, and hypoxia, are the first to affect the high frequencies' outer nerves.
In summary, it is easy to understand why many hearing losses start in the high frequencies first.
Why is it difficult to detect hearing loss in time?
Mild to moderate hearing loss is less noticeable in a quiet environment because most sounds can still be heard.
However, when the environment is filled with noise, it becomes difficult and strenuous for them to understand conversations.
And interestingly, the perception of loud and complex sounds is the same for people with normal hearing and hearing loss. For the hearing impaired, if the sound is loud, they are also afraid of noise. This is an important reason why hearing loss is not usually detected immediately.