decrease in the loudness heard over time, even when the actual intensity of the noise has not changed.[1] This is caused by the exhaustion of the active process.Active process[edit]The active process is also known as the cochlear amplifier. This amplification increases vibrations of the basilar membrane through energy obtained from the Organ of Corti.[3] As the stimulation increases, it is assumed that basilar membranedisplacement, caused by the traveling wave, becomes continually more basal in regards to the cochlea.[4] A sustained low-level stimulus can cause an energetic exhaustion of the active system which in turn prevents the passive system from activating.Excessive vibrations[edit]Currently it is believed that auditory fatigue and NIHL are related to excessive vibrations of the inner ear which may cause structural damages.[5][6][7] Metabolic activity is required in order to maintain the electrochemical gradients used in mechano-electrical and electro-mechanical transduction during noise exposure and sound recognition.[5] The metabolic activity is associated with active displacements which are components of the sound-induced vibration involving prestin, a motor protein that causes OHC motility.[5] Excess vibrations require increased metabolic energy.In addition, these extra vibrations can cause the formation of free radicals known as reactive oxygen species or ROS.[8][9]Elevated levels of ROS continue to increase the metabolic demands of the system. These increasing demands fatigue the system and eventually lead to structural damages to the Organ of Corti.[5][10]Recovery[edit]In all cases of auditory fatigue, sufficient recovery time should allow full correction of the hearing impairment and return threshold levels to their baseline values.[1] There is currently no way to estimate the amount of time needed to recover from auditory fatigue because it is not usually detectable until after the injury has already occurred. Studies that measured recovery time have noted that the time required is related to the magnitude of the initial hearing loss.[11] The most significant recovery was found to occur during the first 15 minutes following cessation of the noise exposure.[12][13] When sufficient recovery time is not allotted, the effects become permanent and cause acquired noise-induced hearing loss.[11] Up to 120 minutes of recovery time can be required of noises of only 95 dB.[11] For comparison, common items that can produce noise at this level are motorcycles and subways.[14]Protective measures[edit]Toughening and energy spread[edit]Two protective measures have been investigated related to the amount of noise exposure and the duration of that exposure. Although these would be hard to regulate in spontaneous occurrences, they may could have a positive effect on work conditions if guidelines could be set for machining times or for other systems that produce loud noises over a long period of time. The toughening effect is put in place by increasing the system's resistance to noise over time.[15] Currently, the specific mechanisms that cause the cochlear toughening are not known. However, the OHCs and related processes are known to play a role.[16] The other toughening measure is to spread a given amount of energy to the system over a longer amount of time. This would allow recovery processes to take place during the quiet interludes that are gained by increasing the exposure duration.[15] So far,