Acoustic trauma is the sustainment of an injury to the inner ear as a result of being exposed to very loud noise[1]. The ear is composed of three sections: outer, middle, and inner. The outer ear includes the pinna, ear canal, and tympanic membrane (otherwise known as the eardrum)[1][2]. The middle ear includes the ossicles: the malleus, incus, and stapes (also known as the hammer, anvil, and stirrup)[1][2]. The inner ear includes the cochlea and semicircular canals. The cochlea contains outer hair cells, or sensory cells, that register sound signals and send them to the auditory nerve[1]. When an excessively loud sound or signal occurs, there can be lasting damage on the outer hair cells, negatively affecting an individual's hearing[1]. This occurrence is known as acoustic trauma.
Cause
Acoustic trauma is caused by exposure to excessive levels of sound. Some examples of excessive sound levels include a gunshot, an explosion, horns, car backfire, etc. These sounds are typically short in duration, but their intensity or loudness is extreme.
Acoustics of Sound
Sound occurs when an object is set into vibration. This vibration causes an acoustic wave to move through or propagate through a medium[1]. Air is a very common medium which sound propagates through; however, sound waves can also propagate through water and solid objects, such as steel[1][3]. A wave propagates through air by the movement of air molecules[4]. These molecules move back and forth when an object begins to vibrate, beginning with the molecules closest to the source of vibration and spreading out from there[4][3]. As the vibration moves outward, the air molecules are pushed closer together; this is known as condensation[1][3]. When the vibration moves in the opposite direction, though, the air molecules are not forced together by sound pressure, which is called rarefaction[1][3]. Some important properties of a sound wave include the amplitude, wavelength, and period. Amplitude refers to the height of the wave, wavelength refers to the distance between the beginning and end of a wave, and period refers to the number of cycles per second and is also known as frequency. Amplitude is perceived as loudness and frequency is perceived as pitch. The units for amplitude are decibels (dB) and for frequency is Hertz (Hz).
Sound and the Auditory System
In order for humans to hear a signal, the sound wave must propagate through air or another medium to the ear[1]. There, the sound is funneled by the pinna into the ear canal and down to the tympanic membrane[2]. The acoustic vibrations of the wave set the tympanic membrane in motion and, consequently, the ossicles immediately behind the ear drum into motion. This process is the main action and purpose of the middle ear: to act as a transducer[1]. The sound is converted from an acoustic vibration at the tympanic membrane into a mechanical vibration at the ossicles. The ossicles move in and out to pass the signal along to the cochlea via the oval window[2]. Specifically, the mechanical vibration begins with the malleus, which is attached to the tympanic membrane, and passes to the incus, and finally to the stapes. The footplate of the stapes is attached to the cochlea via the oval window. As the stapes moves in and out with the mechanical vibration, the fluid inside the cochlea is displaced[1]. In this process, the mechanical vibrations are transduced into hydromechanical vibrations[1].
| External videos | |
|---|---|
 Auditory Transduction (2002), YouTube video |
The cochlea is filled with two fluids: endolymph and perilymph[1]. These fluids are located within different sections of the cochlea. The cochlea is arranged into three sections: the scala vestibuli, scala tympani, and scala media[1]. Perilymph is located in the scala vestibuli and scala tympani, whereas endolymph is located in the scala media. When the stapes pushes into the oval window, perilymph is displaced within the scala vestibuli. This movement causes the membranes separating the three sections and their fluid to move as well[2]. The organ of corti is a structure located inside the scala media, which contains the sensory cells for hearing: inner and outer hair cells[1]. The hydromechanical movement of the fluids stimulates these cells, which now passes the signal to the auditory nerve and up to the brain.[2]
Damage to the Auditory System
Very loud sounds (or sounds with a large amplitude) can cause temporary or permanent damage to the auditory system[5]. Greater amplitude in the sound wave causes a greater impact on the movement of the ear drum, the ossicles, and on the components of the inner ear. Specifically, issues that can occur include a burst ear drum, damage to the ossicular chain, and damage to the inner hair cells[5]. Breakdowns in any of these areas result in hearing loss and is known as acoustic trauma[1]. Hearing loss severity may vary depending on the how long an individual is exposed to the damaging sound and how loud the sound is[1].
Damaging sound may be short in duration, which is known as impulse sound[5]. Some examples of impulse sound include explosions, gunfire, or a burst of loud shouting[5]. This kind of exposure often results in a temporary hearing loss, also known as a temporary threshold shift (TTS). However, longer exposure to loud sounds can result in permanent hearing loss, as well. Individuals experiencing acoustic trauma may experience physical pain as well.
Symptoms
- Tinnitus[5]
- Defined as the presence of ringing in your ears[6]. Individuals with tinnitus report varying descriptions of their tinnitus. Tinnitus can be very loud to very soft, high pitched or low pitched, in one ear or in both ears, be bothersome or not impactful, as well as constant or fluctuating[6]. The Mayo Clinic lists several descriptors for how tinnitus may sound to an individual: ringing, humming, hissing, roaring, clicking, or buzzing[6].
- Trouble hearing in noise[5]
- Trouble understanding speech signals[5]
- Fatigue (see Auditory Fatigue)[5]
- Trouble with school and/or work performance[5]
- Ear pain
Diagnosis
If an individual feels they are experiencing acoustic trauma or any other symptoms of hearing loss, they may pursue a proper hearing evaluation. Full hearing evaluations can be obtained from an audiology or Ear Nose and Throat (ENT) clinic. Hearing screenings may also be completed
Treatment
Insert info
See also
References
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