Age Related Hearing Loss
When people age, their hearing degrades. Considering the complexity of the hearing system, this should not come as a surprise. The main symptom is difficulty in hearing high‑frequency sounds. This problem originates from the destruction of the more fragile hair cells associated with high frequencies. Why are those hair cells more fragile than the others? Because they have to be much more sensitive: in a normal environment, high frequencies are much quieter than low frequencies. Figure below, shows the spectral density of a male human voice. Frequencies above 8kHz are 45dB below frequencies around 200Hz, and don't forget that ‑45dB means 32,000 times quieter. When listening to someone talking, hair cells associated with frequencies above 8kHz have to be 32,000 times as sensitive as the ones associated with frequencies around 200Hz: no wonder they're more fragile.Figure below: Spectral density of a male human voice.
Indeed, those particular hair cells are subject to much stress. Listen to loud music and they get destroyed. Eat too much fat or smoke too much, your arteries become clogged, the hair cells don't get enough oxygen, and they die. This concerns the inner hair cells as well, and since those behave like a dynamic compressor, when they die, your tectorial membrane becomes less able to handle high signal levels — so even more outer hair cells die as a result! It's no wonder so many people become deaf after 60.
Trouble doesn't stop at the inner ear. Muscles and body tissues in general get less elastic with age, and this naturally applies to both tensor tympani and stapedian muscles. Since they don't work as well, they're less able to perform dynamic compression. This results in louder signals getting to the inner ear, and even more hair cells being destroyed. Incidentally, this also explains why some older people sometimes hear sounds as overly loud: it's when dynamic compression of the middle ear doesn't work as it should.
On a brighter note, for some time this loss of high‑frequency hearing can be compensated for, thanks to a very interesting phenomenon. Suppose that, at some point, a group of high‑frequency‑specific hair cells are destroyed. The brain will reconfigure all the remaining hair cells, so that hair cells that were previously dedicated to lower frequencies will now handle the missing higher frequencies. Naturally, this has drawbacks. To begin with, it reduces the number of cells dedicated to each frequency band. This loss of definition reduces our ability to discriminate between different frequencies that are close together.
For people with perfect pitch, this reconfiguration of the inner hair cells brings annoying consequences. Think about it: once reconfigured, the hair cells that were previously associated with a given pitch are now associated with a different one, so the brain is going to confuse one pitch with the other. To give a practical example, for the last five years, I have consistently heard everything approximately half a tone lower than standard 440Hz tuning. If you ask me, an A is really a B-flat. As I said before, it's really annoying.
A final consequence of hearing impairment is an increase in 'otoacoustic emissions' — in other words, the ear's background noise. Older people sometimes claim they hear 'ringing'. Such ringing originates from the hair cells dedicated to the highest frequencies these people can still hear. Those cells being in bad shape, their signal‑to‑noise ratio diminishes considerably, to the point that it's practically 0dB: background noise gets confused with actual audio signal.
Temporary Threshold Shift
A temporary threshold shift is a temporary shift in the auditory threshold. It may occur suddenly after exposure to a high level of noise, a situation in which most people experience reduced hearing. A temporary threshold shift results in temporary hearing loss. People who experience a temporary threshold shift may often also experience temporary tinnitus.
Causes of temporary threshold shift
A temporary threshold shift which results in a temporary hearing loss is normally caused by exposure to intense and/or loud sounds or noise for a shorter or longer time. This could be e.g. an explosion or a concert.
Treatment of temporary threshold shift
If you experience a temporary threshold shift, it is recommended that you spend some time in a quiet place and not expose yourself to loud sounds. The recovery time for a temporary threshold shift varies. It may take only a few hours for the symptoms to subside but it can also last for days. If your hearing does not recover within a few days, you should seek medical advice.
Permanent threshold shift
The opposite of a temporary threshold shift is a permanent threshold shift. A permanent threshold shift is when the ability to hear is reduced permanently, which causes a permanent hearing loss.
Auditory fatigue is defined as a temporary loss of hearing after exposure to sound. This results in a temporary shift of the auditory threshold known as a temporary threshold shift (TTS). The damage can become permanent (permanent threshold shift, PTS) if sufficient recovery time is not allowed for before continued sound exposure. When the hearing loss is rooted from a traumatic occurrence, it may be classified as noise-induced hearing loss, or NIHL.
There are two main types of auditory fatigue, short-term and long-term. These are distinguished from each other by several characteristics listed individually below.
full recovery from TTS can be achieved in approximately two minutes
TTS is maximal at the exposure frequency of the sound
recovery requires a minimum of several minutes but can take up to several
Wind Related Hearing Loss
Wind-Related Noise Experienced by Cyclists Can Contribute to Noise-Induced Hearing Loss (September, 2016)
DETROIT – A new study led by otolaryngologists Michael Seidman, M.D., director of Otologic / Neurotologic / Skull Base Surgery at Florida Hospital Celebration Health and Anna Wertz, M.D. from the Henry Ford Hospital Department of Otolaryngology – Head and Neck Surgery found that the wind-related noise experienced by cyclists can reach levels loud enough to contribute to noise-induced hearing loss.
Using the Ford Motor Company aero-acoustic wind tunnel, researchers generated wind speeds ranging from 15 - 60 mph and used microphones attached to a cyclist’s ears to measure the noise level at various speeds. Sound measurements were taken with the head positioned at 15 degree increments relative to the wind from 0 degrees to 180 degrees.
Wind noise ranged from 85 decibels at 15 mph and increased proportionally with speed to a maximum of 120 decibels at 60 mph. “These findings are important because noise induced hearing loss can begin with sounds at or above 85 decibels,” said Anna Wertz, M.D. Henry Ford otolaryngologist and coauthor of the study. “Short term exposure to loud sounds isn’t likely to have a lasting effect on hearing, but prolonged or repeated exposure can lead to permanent damage."
The study also found that the noise level was greater in the downwind ear, which was facing away from the wind. The researchers say this finding may be due to air turbulence caused from eddy currents observed on the downwind side. For more information, contact Cat-Ears or J. Adkins at Henry Ford Health Systems.
For more information on noise induced hearing loss, see the US Department of Health and Human Services article from the NICDC.
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