Hearing loss is commonly categorized by why hearing no longer works, either as sensorineural or conductive.Sensorineural hearing loss means that hearing was lost by damaging the nerves or inner ear structures. Conductive hearing loss means transmission of sound to the inner ear is impaired. Either of these types of hearing loss can come from multiple causes, such as auditory fatigue, trauma, disease, drugs, or genetics. In many cases, the cause is simply unknown. Sound exposure is an important cause of hearing loss and is the focus of the discussion below.

      A typical hair cell. Photo provided by kamares.ucsd.edu/~ffilimon/107B/107b.html.

       Hair cells are sensory receptor cells for hearing and are the structures most easily damaged by sound. They are part of theorgan of Corti, which is housed in the cochlear duct inside thecochlea (see Hearing in Mammals for an introduction to how mammals hear).

       The hair cells rest on top of the basilar membrane. Small hair-like extensions called stereocilia at the top of the hair cells are embedded in the tectorial membrane. Sound waves cause the basilar membrane to vibrate. This creates a differential motion between the basilar and tectorial membranes, causing the hair cell stereocilia to bend. This leads to internal changes within the hair cells that release chemicals that create electrical signals. Auditory nerve fibers synapse (connect) with the hair cells and pass these signals on to the brain.

       The physical properties of the basilar membrane (its thickness and width at each end) make it particularly good at detecting frequencies. Since the width and thickness of the basilar membrane varies from the top to the bottom, it does not vibrate evenly all over from a sound wave. The apical end of the basilar membrane vibrates most at low frequency tones, and the basal end vibrates most at high frequency tones. The basilar membrane is, therefore, tonotopic, or responds in order from high (base) to low (apex) frequencies. Therefore, based on which hair cells are responding along the basilar membrane, a specific group of ganglion cells becomes active, and the brain is able to perceive what tones have been received.

       The minimum level of sound that is detectable is defined as the threshold of hearing and will vary with each frequency. Thresholds are typically measured over many frequencies with repeated trials and may vary by individual over time (Hearing Sensitivity Studies). They are usually displayed as a "hearing curve" which is a graph of thresholds for the frequencies tested. An animal's hearing can be impaired temporarily or permanently by overexposure to certain characteristics of sound.

       Estimates of the hearing thresholds for some groups of marine mammals along with typical ambient noise levels. The y-axis (vertical) for the hearing thresholds is relative intensity in underwater dB. The y-axis for the ambient noise curve is spectral level in 1Hertz frequency bands with units of dB re 1 µPa2/Hz. The x-axis (horizontal) is the frequency of a sound on a logarithmic scale. (Figure is adapted from Office of Naval Research, 2001. See notes regarding this figure.)

       Hearing loss from sound exposure actually depends on the sensitivity of the animal to a sound and the interaction of three characteristics of the sound: the frequency of the sound (for more information on frequency, see How do you characterize sounds? Frequency), the intensity of the sound (for more information on intensity, see How do you characterize sounds? Intensity or Loudness), and the duration or how long the animal is exposed to that sound. Exposure to a short, very loud sound or to some lower level sound over a long period of time can damage the hair cells by over-extending the stereocilia, shrinking the cell, or breaking connections between cells and cilia. If hair cell damage is minor, hair cells can recover and hearing will return to normal. If the hair cells are severely damaged, they will not recover. These effects range from minor auditory fatigue to major cell death.

       Auditory Fatigue

       If the hair cells and stereocilia do not have sufficient time to recover between sounds, the ear experiences auditory fatigue, which is also known as noise-induced hearing loss (NIHL). An animal can accommodate sounds that are above its hearing threshold until a certain combination of intensity and duration is reached. Above this limit, the animal's hearing threshold will be elevated. If the hearing threshold returns to baseline levels then it is known as atemporary threshold shift (TTS). Temporary threshold shift (TTS) studies have been conducted with several species of marine mammals, (See TTS Studies)

       The degree of any threshold shift depends on the three characteristics of the sound that the animal is exposed to along with the sensitivity of each individual to the sound. These factors may vary by individual and by species (see Hearing Loss). Currently there are some generalities about the effects of sound on animal hearing thresholds:

     1. the degree of hearing loss from any sound is likely to differ among species and among individuals

     2. for pure tones, hearing loss centers around the frequency of the tone

       3. even for sounds that cause TTS, there may be a point at which the interaction of frequency, intensity and duration of exposure will cause an irreversible hearing loss.
Cell Death and Permanent Loss

       If the hearing threshold does not return to baseline levels, the effect is called a permanent threshold shift (PTS). PTS can occur as a result of repeated occurrences of TTS, or it can occur catastrophically as a result of a single exposure to a very intense sound.

       Acoustic trauma is severe traumatic injury from sound. Ears are especially subject to these injuries which result from the inability of a tissue to tolerate very high, sudden pressures like impulse noise from gunshots. Damage due to acoustic trauma is distinct, and scientists studying the ears of whales can identify its characteristics. Injury to theauditory system from extreme events like explosions, blows to the head, or other concussive forces are sometimes mistakenly called acoustic trauma. These injuries are not caused by sound.