An acoustic signal from a sound source, such as a radio, enters into the external auditory canal (ear canal), and is funneled through to the tympanic membrane (eardrum), causing it to vibrate. The vibration of the tympanic membrane drives the middle ear ossicles, the malleus, incus, and stapes to vibrate in sync with the eardrum. The middle ear ossicles transfer mechanical energy to the cochlea by way of the stapes footplate hammering against the oval window of the cochlea, effectively amplifying the sound signal. This hammering causes the fluid within the cochlea (perilymph and endolymph) to be displaced. Displacement of the fluid causes movement of the hair cells (sensory cells in the cochlea) and an electrochemical signal to be sent from the auditory nerve (CN VIII) to the central auditory system within the brain. This is where sound is perceived. Different groups of hair cells are responsive to different frequencies. Hair cells at or near the base of the cochlea are most sensitive to higher frequency sounds while those at the apex are most sensitive to lower frequency sounds. There are two known biological mechanisms of NIHL from excessive sound intensity: damage to the structures called stereocilia that sit atop hair cells and respond to sound, and damage to the synapses that the auditory nerve makes with hair cells, also termed "hidden hearing loss". Physiological response
The symptoms mentioned above are the external signs of the physiological response to cochlear overstimulation.
The over-exposure to excessive loud noise is partially attributed to recreational exposure, such as the use of personal audio devices with music at high volumes for long durations, or social settings such as bars, entertainment and sporting events.The International Organization for Standardization (ISO) developed the ISO 1999 standards for the estimation of hearing thresholds and noise-induced hearing impairment. They used data from two noise and hearing study databases, one presented by Burns and Robinson (Hearing and Noise in Industry, Her Majestys Stationery Office, London, 1970) and by Passchier-Vermeer (1968). As race and ethnicity are some of the factors that can affect the expected distribution of pure-tone hearing thresholds several other national or regional datasets exist, from Sweden, Norway, South Korea, the United States and Spain. In the United States hearing is one of the health outcomes measure by the National Health and Nutrition Examination Survey (NHANES), a survey research program conducted by the National Center for Health Statistics. It examines health and nutritional status of adults and children in the United States. While there is no perfect way to pinpoint hearing loss from excessive noise, researchers look for audiometric notches in a hearing test—dips in the ability to hear certain frequencies—as signs of possible NIHL. As of 2011 data, approximately 24% adults age 20–69 in the United States has an audiometric notch. This data identified differences in NIHL based on age, gender, race/ethnicity, and whether or not a person is exposed to noise at work.