TTS imperceptibly gives way to PTS.

In addition to hearing loss, other external symptoms of an acoustic trauma can be:

- Tinnitus

- Some pain in the ear

- Hyperacusis

- Dizziness or vertigo; in the case of vestibular damages, in the inner-ear

Occupational hearing loss (OHL) is hearing loss that occurs as a result of occupational hazards. OHL, damage to one or both ears from exposures related to one's occupation, is a large but preventable problem. Organizations such as the Occupational Safety and Health Administration (OSHA) and the National Institute for Occupational Safety and Health (NIOSH) work with employers and workers to reduce or eliminate completely hazards to hearing. Occupational hearing loss is one of the most common work-related illness in the United States. Hazards to the hearing of workers include industrial noise, and exposure to various ototoxic chemicals. These exposures may also interact to cause more damage than either one would by itself. Many chemicals have not been tested for ototoxicity, so unknown threats may exist.

A 2016 study by NIOSH found that the mining sector had the highest prevalence of hearing impairment at 17%, followed by the construction sector (16%) and the manufacturing sector (14%). The public safety sector had the lowest rate of hearing impairment, at 7%.

Personal protective equipment, administrative controls, and engineering controls can all work to reduce exposure to noise and chemicals, either by providing the worker with protection such as earplugs, or by reducing the noise at the source or limiting the time or level of exposure.

The first symptom of noise-induced hearing loss is usually difficulty hearing a conversation against a noisy background. The effect of hearing loss on speech perception has two components. The first component is the loss of audibility, which is something like a decrease in overall volume. Modern hearing aids compensate this loss with amplification. However, difficulty in understanding speech represents selective frequency loss for which hearing aids and amplification do not help. This is known by different names such as “distortion,” “clarity loss,” and “Signal-to-Noise-Ratio (SNR)-loss.” Consonants, due to their higher frequency, seem to be lost first. For example, the letters “s” and “t” are the common letters that are difficult to hear for those with hearing loss due to them being our highest frequency sound in our language. Hearing loss can affect either one or both ears. When one ear is affected it causes problems with directional hearing. Directional hearing provides the ability to determine from which direction a sound came. Lacking this ability can cause confusion within individuals who have hearing loss in one ear.

Primary symptoms:

- sounds or speech becoming dull, muffled or attenuated

- need for increased volume on television, radio, music and other audio sources

- difficulty using the telephone

- loss of directionality of sound

- difficulty understanding speech, especially women and children

- difficulty in speech discrimination against background noise (cocktail party effect)

Secondary symptoms:

- hyperacusis, heightened sensitivity to certain volumes and frequencies of sound, resulting from "recruitment"

- tinnitus, ringing, buzzing, hissing or other sounds in the ear when no external sound is present

- vertigo and disequilibrium

Usually occurs after age 50, but deterioration in hearing has been found to start very early, from about the age of 18 years. The ISO standard 7029 shows expected threshold changes due purely to age for carefully screened populations (i.e. excluding those with ear disease, noise exposure etc.), based on a meta-analysis of published data. Age affects high frequencies more than low, and men more than women. One early consequence is that even young adults may lose the ability to hear very high frequency tones above 15 or 16 kHz. Despite this, age-related hearing loss may only become noticeable later in life. The effects of age can be exacerbated by exposure to environmental noise, whether at work or in leisure time (shooting, music, etc.). This is noise-induced hearing loss (NIHL) and is distinct from presbycusis. A second exacerbating factor is exposure to ototoxic drugs and chemicals.

Over time, the detection of high-pitched sounds becomes more difficult, and speech perception is affected, particularly of sibilants and fricatives. Patients typically express a decreased ability to understand speech. Once the loss has progressed to the 2-4kHz range, there is increased difficulty understanding consonants. Both ears tend to be affected. The impact of presbycusis on communication depends on both the severity of the condition and the communication partner.

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.

Short-term fatigue

- full recovery from TTS can be achieved in approximately two minutes

- the TTS is relatively independent of exposure duration

- TTS is maximal at the exposure frequency of the sound

Long-term fatigue

- recovery requires a minimum of several minutes but can take up to several days

- dependent on exposure duration and noise level

Symptoms of ototoxicity include partial or profound hearing loss, vertigo, and tinnitus.

The cochlea is primarily a hearing structure situated in the inner ear. It is the snail-shaped shell containing several nerve endings that makes hearing possible.

Ototoxicity typically results when the inner ear is poisoned by medication that damages the cochlea, vestibule, semi-circular canals, or the auditory/ vestibulocochlear nerve. The damaged structure then produces the symptoms the patient presents with. Ototoxicity in the cochlea may cause hearing loss of the high-frequency pitch ranges or complete deafness, or losses at points between. It may present with bilaterally symmetrical symptoms, or asymmetrically, with one ear developing the condition after the other or not at all. The time frames for progress of the disease vary greatly and symptoms of hearing loss may be temporary or permanent.

The vestibule and semi-circular canal are inner-ear components that comprise the vestibular system. Together they detect all directions of head movement. Two types of otolith organs are housed in the vestibule: the saccule, which points vertically and detects vertical acceleration, and the utricle, which points horizontally and detects horizontal acceleration. The otolith organs together sense the head’s position with respect to gravity when the body is static; then the head’s movement when it tilts; and pitch changes during any linear motion of the head. The saccule and utricle detect different motions, which information the brain receives and integrates to determine where the head is and how and where it is moving.

The semi-circular canals are three bony structures filled with fluid. As with the vestibule, the primary purpose of the canals is to detect movement. Each canal is oriented at right angles to the others, enabling detection of movement in any plane. The posterior canal detects rolling motion, or motion about the X axis; the anterior canal detects pitch, or motion about the Y axis; the horizontal canal detects yaw motion, or motion about the Z axis.

When a medication is toxic in the vestibule or the semi-circular canals, the patient senses loss of balance or orientation rather than losses in hearing. Symptoms in these organs present as vertigo, difficulties walking in low light and darkness, disequilibrium, oscillopsia among others. Each of these problems is related to balance and the mind is confused with the direction of motion or lack of motion. Both the vestibule and semi-circular canals transmit information to the brain about movement; when these are poisoned, they are unable to function properly which results in miscommunication with the brain.

When the vestibule and/or semi-circular canals are affected by ototoxicity, the eye can also be affected. Nystagmus and oscillopsia are two conditions that overlap the vestibular and ocular systems. These symptoms cause the patient to have difficulties with seeing and processing images. The body subconsciously tries to compensate for the imbalance signals being sent to the brain by trying to obtain visual cues to support the information it is receiving. This results in that dizziness and "woozy" feeling patients use to describe conditions such as oscillopsia and vertigo.

Cranial nerve VIII, is the least affected component of the ear when ototoxicity arises, but if the nerve is affected, the damage is most often permanent. Symptoms present similar to those resulting from vestibular and cochlear damage, including tinnitus, ringing of the ears, difficulty walking, deafness, and balance and orientation issues.

Hearing loss is classified as mild, moderate, severe or profound. Pure-tone audiometry for air conduction thresholds at 500, 1000 and 2000 Hz is traditionally used to classify the degree of hearing loss in each ear. Normal hearing thresholds are considered to be 25 dB sensitivity, though it has been proposed that this threshold is too high, and that 15 dB (about half as loud) is more typical. Mild hearing loss is thresholds of 25–45 dB; moderate hearing loss is thresholds of 45–65 dB; severe hearing loss is thresholds of 65–85 dB; and profound hearing loss thresholds are greater than 85 dB.

Tinnitus occurring in only one ear should prompt the clinician to initiate further evaluation for other etiologies. In addition, the presence of a pulse-synchronous rushing sound may require additional imaging to exclude vascular disorders.

SSHL is diagnosed via pure tone audiometry. If the test shows a loss of at least 30db in three adjacent frequencies, the hearing loss is diagnosed as SSHL. For example, a hearing loss of 30db would make conversational speech sound more like a whisper.

Many people notice that they have SSHL when they wake up in the morning. Others first notice it when they try to use the deafened ear, such as when they use a phone. Still others notice a loud, alarming "pop" just before their hearing disappears. People with sudden deafness often become dizzy, have ringing in their ears (tinnitus), or both.

Patients with unilateral hearing loss have difficulty in

- hearing conversation on their impaired side

- localizing sound

- understanding speech in the presence of background noise.

- interpersonal and social relations

- difficulty concentrating in large, open environments

In quiet conditions, speech discrimination is no worse than normal hearing in those with partial deafness; however, in noisy environments speech discrimination is almost always severe.

Conductive hearing loss makes all sounds seem faint or muffled. The hearing loss is worse in low frequencies.

Congenital conductive hearing loss is usually identified through newborn hearing screening or may be identified because the baby has microtia or other facial abnormalities. Conductive hearing loss developing during childhood is usually due to otitis media with effusion and may present with speech and language delay or difficulty hearing. Later onset of conductive hearing loss may have an obvious cause such as an ear infection, trauma or upper respiratory tract infection or may have an insidious onset related to chronic middle ear disease, otosclerosis or a tumour of the naso-pharynx. Earwax is a very common cause of a conductive hearing loss which may present suddenly when water gets behind the wax and this blocks the ear canal.

Ototoxicity is the property of being toxic to the ear ("oto-"), specifically the cochlea or auditory nerve and sometimes the vestibular system, for example, as a side effect of a drug. The effects of ototoxicity can be reversible and temporary, or irreversible and permanent.

It has been recognized since the 19th century.

There are many well-known ototoxic drugs used in clinical situations, and they are prescribed, despite the risk of hearing disorders, to very serious health conditions.

Ototoxic drugs include antibiotics such as gentamicin, loop diuretics such as furosemide and platinum-based chemotherapy agents such as cisplatin. A number of nonsteroidal anti-inflammatory drugs (NSAIDS) have also been shown to be ototoxic. This can result in sensorineural hearing loss, dysequilibrium, or both. Some environmental and occupational chemicals have also been shown to affect the auditory system and interact with noise.

Occupational hearing loss is defined as damage to either or both ears, at the inner ear or auditory nerve, that results from an exposure in a person's occupation. Although high levels of noise are the main cause of occupational hearing loss (also called noise-induced hearing loss) there are also other factors in the work environment that can result in it. Chemicals, foreign bodies, vibration, barotrauma, along with other hazards can result in hearing loss. These losses that these workers obtain, affect many aspects of their life, mainly social interactions.

Within the United States of America, approximately 10 million people have noise-related hearing loss. Over twice that number are occupationally exposed to dangerous noise levels. Hearing loss accounted for a sizable percentage of occupational illness in 2007, at 14% of cases. As in many countries, in the US organizations like the Occupational Safety and Health Administration (OSHA) and the National Institute for Occupational Safety and Health (NIOSH) are working to understand the causes of occupational hearing loss and how it can be prevented. They work to produce regulations and guidelines to help protect the hearing of workers in all occupations.

Hearing loss is sensory, but may have accompanying symptoms:

- pain or pressure in the ears

- a blocked feeling

There may also be accompanying secondary symptoms:

- hyperacusis, heightened sensitivity to certain volumes and frequencies of sound, sometimes resulting from "recruitment"

- tinnitus, ringing, buzzing, hissing or other sounds in the ear when no external sound is present

- vertigo and disequilibrium

- tympanophonia, abnormal hearing of one's own voice and respiratory sounds, usually as a result of a patulous eustachian tube or dehiscent superior semicircular canals

- disturbances of facial movement (indicating possible tumour or stroke)

Known causes include physical trauma, acoustic neuroma, measles, labyrinthitis, microtia, meningitis, Ménière's disease, Waardenburg syndrome, mumps (epidemic parotitis), and mastoiditis.

Fluid accumulation is the most common cause of conductive hearing loss in the middle ear, especially in children. Major causes are ear infections or conditions that block the eustachian tube, such as allergies or tumors. Blocking of the eustachian tube leads to decreased pressure in the middle ear relative to the external ear, and this causes decreased motion of both the ossicles and the tympanic membrane.

- acute or serous otitis media

- otitis media with effusion or 'glue ear'

- Perforated eardrum

- Chronic suppurative otitis media (CSOM)

- Cholesteatoma

- Otosclerosis, abnormal growth of bone in or near the middle ear

- middle ear tumour

- ossicular discontinuity as a consequence of infection or temporal bone trauma

- Congenital malformation of the ossicles. This can be an isolated phenomenon or can occur as part of a syndrome where development of the 1st and 2nd branchial arches is seen such as in Goldenhar syndrome, Treacher Collins syndrome, branchio-oto-renal syndrome etc.

- Barotrauma unequal air pressures in the external and middle ear. This can temporarily occur, for example, by the environmental pressure changes as when shifting altitude, or inside a train going into a tunnel. It is managed by any of various methods of ear clearing manoeuvres to equalize the pressures, like swallowing, yawning, or the Valsalva manoeuvre. More severe barotrauma can lead to middle ear fluid or even permanent sensorineural hearing loss.

Human hearing extends in frequency from 20-20,000 Hz, and in amplitude from 0 dB to 130 dB or more. 0 dB does not represent absence of sound, but rather the softest sound an average unimpaired human ear can hear; some people can hear down to -5 or even -10 dB. 130 dB represents the threshold of pain. But the ear doesn't hear all frequencies equally well; hearing sensitivity peaks around 3000 Hz. There are many qualities of human hearing besides frequency range and amplitude that can't easily be measured quantitatively. But for many practical purposes, normative hearing is defined by a frequency versus amplitude graph, or audiogram, charting sensitivity thresholds of hearing at defined frequencies. Because of the cumulative impact of age and exposure to noise and other acoustic insults, 'typical' hearing may not be normative.

Due to variations in study designs, data on the course of tinnitus showed few consistent results. Generally the prevalence increased with age in adults, whereas the ratings of annoyance decreased with duration.

In hyperacusis, the symptoms are ear pain, annoyance, and general intolerance to many sounds that most people are unaffected by. Crying spells or panic attacks may result from the experience of hyperacusis. It may affect either or both ears. Hyperacusis can also be accompanied by tinnitus. Hyperacusis can result in anxiety, stress and phonophobia. Avoidant behaviour is often a response to prevent the effects of hyperacusis and this can include avoiding social situations.

Tinnitus can be perceived in one or both ears or in the head. It is the description of a noise inside a person’s head in the absence of auditory stimulation. The noise can be described in many different ways.

It is usually described as a ringing noise but, in some patients, it takes the form of a high-pitched whining, electric buzzing, hissing, humming, tinging or whistling sound or as ticking, clicking, roaring, "crickets" or "tree frogs" or "locusts (cicadas)", tunes, songs, beeping, sizzling, sounds that slightly resemble human voices or even a pure steady tone like that heard during a hearing test. It has also been described as a "whooshing" sound because of acute muscle spasms, as of wind or waves. Tinnitus can be intermittent or continuous: in the latter case, it can be the cause of great distress. In some individuals, the intensity can be changed by shoulder, head, tongue, jaw or eye movements. Most people with tinnitus have some degree of hearing loss.

The sound perceived may range from a quiet background noise to one that can be heard even over loud external sounds. The specific type of tinnitus called pulsatile tinnitus is characterized by hearing the sounds of one's own pulse or muscle contractions, which is typically a result of sounds that have been created by the movement of muscles near to one's ear, or the sounds are related to blood flow of the neck or face.

Hyperacusis (or hyperacousis) is a debilitating hearing disorder characterized by an increased sensitivity to certain frequencies and volume ranges of sound (a collapsed tolerance to usual environmental sound). A person with severe hyperacusis has difficulty tolerating everyday sounds, some of which may seem unpleasantly or painfully loud to that person but not to others.

"Hyperacusis" is often coincident with tinnitus. Both conditions have a prevalence of about 10–15% and hearing loss as a major risk factor. However, there also appear to be important differences between the mechanisms involved in tinnitus and hyperacusis.

Note: "The complete anatomy of the ear is extensive, and can be divided into the inner ear and outer ear. The remainder of this article mainly references the cochlea, outer hair cells, and organ of Corti."

In general, structural damages to any anatomical part of the ear can cause hearing-related problems. Usually, minor bending of the stereocilia of the inner ear is associated with temporary hearing loss and is involved in auditory fatigue. Complete loss of the stereocilia causes permanent hearing damage and is more associated with noise-induced hearing loss and other auditory diseases.

The outer hair cells, or OHCs, can be thought of as microamplifiers that provide stimulation to the inner hair cells. The OHCs are the most fragile of the hair cells, hence their involvement in auditory fatigue and other hearing impairments.

Similarly to vision loss, hearing loss can vary from full or partial inability to detect some or all frequencies of sound which can typically be heard by members of their species. For humans, this range is approximately 20 Hz to 20 kHz at ~6.5 dB, although a 10 dB correction is often allowed for the elderly. Primary causes of hearing loss due to an impaired sensory system include long-term exposure to environmental noise, which can damage the mechanoreceptors responsible for receiving sound vibrations, as well as multiple diseases, such as HIV or meningitis, which damage the cochlea and auditory nerve, respectively.

Hearing loss may be gradual or sudden. Hearing loss may be very mild, resulting in minor difficulties with conversation, or as severe as complete deafness. The speed with which hearing loss occurs may give clues as to the cause. If hearing loss is sudden, it may be from trauma or a problem with blood circulation. A gradual onset is suggestive of other causes such as aging or a tumor. If you also have other associated neurological problems, such as tinnitus or vertigo, it may indicate a problem with the nerves in the ear or brain. Hearing loss may be unilateral or bilateral. Unilateral hearing loss is most often associated with conductive causes, trauma, and acoustic neuromas. Pain in the ear is associated with ear infections, trauma, and obstruction in the canal.

In some cases, the loss is extremely sudden and can be traced to specific diseases, such as meningitis, or to ototoxic medications, such as Gentamicin. In both cases, the final degree of loss varies. Some experience only partial loss, while others become profoundly deaf. Hearing aids and cochlear implants may be used to regain a sense of hearing, with different people experiencing differing degrees of success. It is possible that the affected person may need to rely on speech-reading and/or sign language for communication.

In most cases the loss is a long term degradation in hearing loss. Discrediting earlier notions of presbycusis, Rosen demonstrated that long term hearing loss is usually the product of chronic exposure to environmental noise in industrialized countries (Rosen, 1965). The U.S. Environmental Protection Agency has asserted the same sentiment and testified before the U.S. Congress that approximately 34 million Americans are exposed to noise pollution levels (mostly from roadway and aircraft noise) that expose humans to noise health effects including the risk of hearing loss (EPA, 1972).

Certain genetic conditions can also lead to post-lingual deafness. In contrast to genetic causes of pre-lingual deafness, which are frequently autosomal recessive, genetic causes of post-lingual deafness tend to be autosomal dominant.

Signs and symptoms of AIED are:

- Progressive hearing loss in both ears

- Typically will begin in one ear and gradually affect the other

- Hearing loss may begin suddenly

- Tinnitus (ringing or buzzing in ears)

- Decrease in word recognition capability

- Loss of balance (vestibular symptoms)

- Degree of balance loss can change throughout the course of the disease