The human ears are two, paired vibration sensors, particularly sensitive to the vibration of air molecules between the frequencies of 20-20,000 Hertz: what we interpret as sound.
The ear is made up of three composite parts:
The outer ear is made up of the external ear or pinna (the serpentine shape on the side of your head) and the ear canal. The outer ear essentially acts as a funnel to focus sound down into the eardrum.
The middle ear is made up of the eardrum and three little bones – malleus, incus and stapes – in an air-filled space that connects to the back of the nose via the Eustachian tube. It is this tube that keeps the pressure in the middle ear the same as atmospheric pressure and allows your ears to ‘pop’. The middle ear transforms the low pressure, high amplitude vibrations of the air molecules into lower amplitude but higher pressure vibrations which are then transferred to the fluids of the inner ear.
The inner ear is made up of two parts: the vestibule (which includes the semi-circular canals) for balance and the cochlea for hearing. The cochlea is a transducer and converts the mechanical movements of sound into a neuro-electrical signal that passes through the cochlea nerve to the brain. This transduction is performed by vibrations of hair cells, both inner and outer, in a part of the cochlea called the Organ of Corti.
Unfortunately the ear can be damaged by high intensities of the very energy source it was designed to detect. This high intensity is measured in the form of volume: if it is too loud, your are at risk of hearing damage. The outer hair cells are more metabolically active than the inner hair cells and so more susceptible to damage. As a result individuals may suffer problems such as tinnitus or difficulties with conversation in the presence of background noise long before they reach the level of noticeable hearing loss.
How Loud is Too Loud?
Sound is measured in decibels and the safe noise level is around 85 decibels. Constant exposure to loud noise (in excess of 85 decibels) can cause hearing loss over time. The human ear can tolerate loud noise for a period of 2-3 hours provided it doesn’t have to do this often, but many people don’t realize that they can permanently change their hearing after just 15 minutes exposure to loud noise or music that exceeds this level.
- Raindrops = 40 decibels
- Normal Conversation = 60 decibels
- Busy City traffic = 85 decibels
- Hair Dryers = 90 decibels
- Rock Concerts = 105 decibels
- Chainsaws = 110 decibels
- An iPod at full volumes = 115 decibels
- Jackhammers = 120 decibels
- Gunshot, Fireworks = 140 decibels
How are the ears damaged?
After relatively short bursts of loud noise exposure, the ear suffers from something called “Temporary Threshold Shift” (TTS). Most people have experienced this short-lasting upset to hearing and ringing in their ears (tinnitus) after say an evening in a loud club or bar. Usually by the next day everything is back to normal. It is believed that TTS is due to metabolic exhaustion of the hair cells of the cochlea. This can recover with a period of ‘rest’ as the background noise levels fall.
However if this type of noise exposure continues over a prolonged period of time, permanent changes may occur. This is called ‘Permanent Threshold Shift (PTS) and typically affects the higher speech frequencies around 4-6000 Hertz. Initially the sufferer may not notice a hearing loss as such but rather a difficulty in understanding what people are saying, particularly in the presence of background sound. As the high-tone hearing loss worsens, eventually an obvious hearing loss appears. PTS is due to actual damage or even death of the delicate hair cells of the cochlea.
One of the additional and often more troublesome problems that follow permanent noise damage to the inner ear is tinnitus. This tinnitus is due to damage to the hair cells which the leads to an upset or alteration in the tiny electrical signals that pass up the auditory nerve to the brain. If the affected individual is unlucky, this new pattern of electrical signals may be recognised buy the brain as sound. The sound itself is as individual as the person who is suffering from it, but common descriptions include a whistle, a whine, a high-pitched ringing or even buzzing.
Information Supplied By: Brittish Tnnitus Association