Hearing LossHearing 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 the organ of Corti, which is housed in the cochlear duct inside thecochlea (see Hearing in Mammals for an introduction to how mhear).The hair cells rest on top of the basilar membrane. Small hair-like extensions called stereociliaat 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 1 Hertz frequency bands with units of dB re 1 Pa2/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