The ear is a wonderful organ, and as we have seen very complex and capable of hearing a wide range of sounds. Despite our hearing threshold is pretty low (which means we can hear very soft sounds), we cannot hear anything that provokes less pressure than 20µPa on our eardrum (approximately the sound of a mosquito flying at a distance of three meters). Of course there is also a limit to hearing loud sounds: this limit depends on the physical resistance of the eardrum. High pressure, as you can imagine, can break the thin membrane.
When the pressure is too high, the first thing that happens is that we feel pain: this is a natural mechanism that should make us aware that we could incur in damages if the pressure persists. The threshold of pain is subjective and also depends on one's age (the eardrum is more elastic in younger subjects, and tends to harden with age), but it is usually around 120-140 dB(SPL). 120 dB(SPL) is approximately the noise of the first row of a very loud rock concert. A prolonged exposure to such pressure without protection can lead to permanent hearing impairment which may be of two types:
1. Eardrum-related damage: high pressure can break or damage the eardrum membrane, which (once healed) would be thicker and more rigid, which would prevent us from hearing particularly soft sounds.
2. Cilia-related damage: high pressure provokes larger waves of fluid inside the cochlea which expose the cilia to a much larger mechanical damage. They also are damaged by the neurotransmitter substance which is toxic in large quantities.
Initially, if the pressure exposure was not limited, the cilia are able to heal themselves over time: this is why after, say, a loud concert we experience a 'temporary threshold shift'. In other words, our hearing threshold raises temporarily until the inner hair cells are reconnected. In other cases we may also experience a temporary 'tinnitus': a tinnitus is the phenomenon that makes us hear a tone, a ring or a whistle inside our ears. Basically, the dislocation of the cilia makes them send signals to our brain even in total absence of sound. This may last from a few hours to a few days, then it disappears.
However, higher level of pressure and/or a longer exposure make those symptoms permanent, so it is imperative that we take care of our hearing with prevention!
Ageing also plays a role in our hearing: even without the above external circumstances, we progressively lose our sensitivity. A normal healthy 10 years old kid can listen to the full frequency range from 20Hz to 20KHZ, but adolescents start losing sensitivity to high frequencies and a middle age person usually hear up to 15KHz. Elderly people may have their hearing reduced to 10KHz. Low frequencies are not that subject to loss of sensitivity since we are able to feel them through resonances of our cranial and other bones. Our eardrum also goes through a process of sclerosis, which means that over the years it naturally becomes thicker and more rigid.
Please note that the contents of these pages is only an explanation of few of the factor that can provoke hearing loss. These cannot under any circumstance substitute medical advice, always consult a specialist for ear-protection or hearing loss related pathologies.