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For example, aging typically leads to hearing thresholds which get poorer as test frequencies get higher. [10] Noise induced hearing loss is typically characterized by a "notch" in the audiogram, with the poorest threshold occurring between 3000 and 6000 Hz (most often 4000 Hz) and better thresholds at lower and higher frequencies. [11]
On an audiogram, the resulting configuration has a distinctive notch, called a 'noise' notch. As ageing and other effects contribute to higher frequency loss (6–8 kHz on an audiogram), this notch may be obscured and entirely disappear. Various governmental, industry, and standards organizations set noise standards. [10]
This "protective effect" had long been thought to involve the active mechanisms of the outer hair cells and the efferent system commanding them. [80] The contractile effect of the outer hair cells, activated by the efferent nervous system has been proven to provide a protective effect against acoustic trauma. [119] Cross-section of the cochlea.
Noise-induced hearing loss is a permanent shift in pure-tone thresholds, resulting in sensorineural hearing loss. The severity of a threshold shift is dependent on duration and severity of noise exposure. Noise-induced threshold shifts are seen as a notch on an audiogram from 3000 to 6000 Hz, but most often at 4000 Hz. [16]
However, the usable dynamic range may be greater, as a properly dithered recording device can record signals well below the noise floor. The 16-bit compact disc has a theoretical undithered dynamic range of about 96 dB; [20] [d] however, the perceived dynamic range of 16-bit audio can be 120 dB or more with noise-shaped dither, taking advantage ...
Of teenagers, 20-50 percent experience exposure to noise levels high enough to cause acute acoustic trauma. [10] Hearing loss due to noise is the second most common sensorineural hearing loss, after age-related hearing loss (presbycusis). Of more than 28 million Americans with some degree of hearing impairment, as many as 10 million have ...
Thus, more noise has to be applied, to produce a masking effect. At the right hand side of the graph, to identify 50% of the speech correctly, the speech needs to much more intense than in the quiet. This is because at this end of the graph, the noise is very loud whether the person has a hearing loss or not.
Psychoacoustic tuning curves can be measured using the notched-noise method. This form of measurement can take a considerable amount of time and can take around 30 minutes to find each masked threshold. [10] In the notched-noise method the subject is presented with a notched noise as the masker and a sinusoid (pure tone) as the signal.