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There are many qualities of human hearing besides frequency range and amplitude that can't easily be measured quantitatively.
But for many practical purposes, normative hearing is defined by a frequency versus amplitude graph, or audiogram, charting sensitivity thresholds of hearing at defined frequencies.
For workplace noise regulation, the "safe" daily exposure amount at 85 d B A, known as an exposure action value, is 8 hours, while the "safe" exposure at 91 d B(A) is only 2 hours.
Different standards use exposure action values between 80d BA and 90d BA.
If one is exposed to loud sound (including music) at high levels or for extended durations (85 d B A or greater), then hearing loss will occur.
Sound intensity (sound energy, or propensity to cause damage to the ears) increases dramatically with proximity according to an inverse square law: halving the distance to the sound quadruples the sound intensity.
As noise damage progresses, damage spreads to affect lower and higher frequencies.
On an audiogram, the resulting configuration has a distinctive notch, called a 'noise' notch.
There is a progressive loss of ability to hear high frequencies with aging known as presbycusis.Studies of primitive peoples indicate that much of what has been attributed to age-related hearing loss may be long term cumulative damage from all sources, especially noise.People living in preindustrial societies have considerably less hearing loss than similar populations living in modern society.Note that for some people, sound may be damaging at even lower levels than 85 d B A.Exposures to other ototoxins (such as pesticides, some medications including chemotherapy agents, solvents, etc.) can lead to greater susceptibility to noise damage, as well as causing its own damage. Since noise damage is cumulative over long periods of time, persons who are exposed to non-workplace noise, like recreational activities or environmental noise, may have compounding damage from all sources.