People living in industrialized countries lose their hearing earlier than their non-industrialized counterparts, and men lose their hearing earlier than women.
Men might begin to notice a loss in hearing sensitivity around age 32. For women, noticeable hearing loss doesn't occur until age 37. This gender difference in hearing loss is partially due to the fact that men are generally exposed to louder noise for longer periods than women.
The progressive loss of high frequencies has been found in non-industrialized countries as well as in noisy, industrialized ones, so presbycusis is age-related and is not only due to long term exposure to loud noise. The gender difference disappears in non-industrialized societies.
Since those working in high-noise environments experience the onset of presbycusis sooner, using hearing protection may preserve your hearing a little while longer.
AGING EAR: Presbycusis
is the progressive, age-related loss of hearing and is caused by the
atrophy of the basal hair cells.
This process starts far earlier than most people suspect. A young child can hear tones up to 20,000 Hz. However, a twenty-something can hear only tones up to 18,000 Hz.
Most people in their 60's cannot hear above 8,000 Hz.
In human speech, there is a difference in the frequency of vowels and consonants. Most of the energy for vowels is between 100-4,000 Hz.
But consonants have higher frequencies, as high at 8,000 Hz. Furthermore consonants such as c, ch, f, s, sh, and z are typically spoken with less power than vowels, making them harder to detect.
Because consonants have higher frequencies than vowels, it is possible for high frequency hearing loss to make it hard to understand some parts of speech.
AGING EAR: Older people can
the speech sounds but take longer to process the message. This reduced
comprehension is due to the slowing of nerves as we age.
The 8th cranial nerve, the vestibulocochlear nerve, shows age-related changes: the diameter shrinks and the conduction speed decreases.
Slower nerve conduction can affect speech intelligibility and comprehension.
Older people depend more heavily on context to comprehend speech. Context helps them to overcome the handicap of high frequency hearing loss.
Older people also watch the
lips of their conversation partners. This helps them to fill in the
parts of speech that they might have missed hearing. So visual
factors such as good lighting and an unobstructed line-of-sight can
help older people to keep up with the conversation.
There are two different modes by which sound is conducted: air and bone.
Air conductive hearing refers to the sound waves that travel through the air. The sound waves pass through the outer ear to the middle ear which transmits to the inner ear structures. The outer and middle ears are filled with air; the inner ear is fluid-filled. Air conduction is best for high frequencies.
Bone conductive hearing relies on transmission of sound waves directly to the temporal bone which contains the cochlea. Bone conduction works better for low frequencies; it does not transmit frequencies above 125 Hz.
THE AGING EAR:
Bones often thin and become more fragile as we age. This is caused by
Osteoporosis can affect the density of the temporal bone, which is normally the strongest bone in the body.
Low density temporal bone can reduce the transmission of sound through bone. This reduces bone conductive hearing. This is how osteoporosis can be a factor in hearing loss.
Certain types of drugs have ototoxic properties, that is, they destroy hair cells. These include antibiotics in the myocin family (streptomyocin), some types of diuretics and chemotherapy drugs. The effect is cumulative -- and irreversible. The greater the exposure to the ototoxic drug, the greater hearing loss. Furthermore, ototoxicity can be cumulative between medication or greatly increase the amount of damage from loud noises. There are several hundred drugs, aspirin is one of them, that can cause tinnitus in high doses.
Otosclerosis is a hereditary problem that cause deafness. The tiny bones in the inner ear stiffen which makes it harder to hear. Otosclerosis affects ~10% of the population, but causes a serious hearing loss for only 1% of the population. Some people think that famous composer Ludwig van Beethoven's deafness was due to otosclerosis.
Hearing loss can be treated with hearing aids. Hearing aids contain a microphone to collect sound waves, an amplifier to boost the strength of the electrical signals, and a receiver that converts the electrical signal to vibratory energy.
These devices can be customized to meet the specific needs of the wearer. For example, the hearing aid can be tuned to the specific frequencies that need to be boosted.
If the person has suffered loss of air-conductive hearing, an air-conductive hearing aid can be prescribted. This type is placed inside the ear canal so it can deliver sound vibrations to the eardrum. If the hearing impairment is due to loss in bone conductive hearing, bone-conduction hearing aids (also called postauricular receivers) are the appropriate type of hearing aid. This type is worn behind the ear and delivers sound vibrations to the temporal bone.
Even if hearing loss is in only one ear, it can be beneficial to use hearing aids in both ears. Binaural hearing helps with sound location and also improves speech comprehension in noisy environments.
Sound localization -- the ability to tell where a sound is coming from -- depends on accurate hearing from both ears. The brain compares the input from each ear and based on the difference from the time delay, determines the direction of sound. The ability to locate sound helps us maintain our spatial orientation.
AGING EAR: Older people may have
more difficulty with locating sound if there is damage in either ear,
or a change in the nervous system processing.
This may lead to confusion and disorientation, especially in noisy environments with multiple sources of sounds, such as a crowded shopping mall or airport terminal.