Hearing loss is generally divided into two types: conductive and sensorineural. Conductive hearing loss occurs when conditions in the outer or middle ear prevent sound from being conducted to the cochlea in the inner ear. Sensorineural hearing loss involves problems with sensory sensor cells in the cochlea, or, less commonly, nerve pathways in the brain.
In some cases, both conductive and sensorineural hearing loss occur together, resulting in what is known as mixed hearing loss. Conductive hearing loss can usually be corrected surgically and is relatively common in childhood, while sensorineural hearing loss is usually permanent. Therefore, it is important for audiologists to distinguish between the two conditions.
One way to distinguish conductive hearing loss from sensorineural hearing loss is to compare air and bone conduction hearing threshold levels.
This involves measuring hearing sensitivity by using two different types of headphones. In an air conduction test, pure tones are presented through headphones (or speakers). The signal travels through the air in the outer ear to the middle ear and then to the cochlea in the inner ear. In a bone conduction test, instead of using headphones, electromechanical headphones are placed on the skull. This allows stimulation of the cochlea through mechanical vibrations of the skull with little stimulation of the outer and middle ears.
Normal hearing people typically have hearing threshold levels close to 0 dB for both air and bone conduction. People with hearing impairment in any part of the auditory pathway have poor air conduction thresholds. A poor air conduction threshold is a major indicator of conductive hearing loss because abnormalities in the conduction mechanism have relatively little effect on bone conduction measurements.
In sensorineural hearing loss, both air and bone conduction thresholds are affected, such that the air-bone gap (air minus bone conduction) is close to zero. The presence of an air-bone space implies conductive hearing loss.
In normal hearing listeners, the dynamic range between hearing threshold and loudness discomfort level is approximately 100 dB. Listeners with inductive hearing loss had improved hearing thresholds, but their levels of loudness discomfort were essentially similar to those of normal hearing listeners.
Listeners with sensory hearing impairment have a reduced dynamic range and experience loudness recovery, or an abnormally high rate of loudness growth, characterized by an abnormally disproportionate increase in loudness and a slight increase in sound intensity.
This has implications for the design of hearing aids, as non-linear amplification is required, where soft sounds require greater amplification than loud sounds.
While nonlinear hearing aids can compensate by increasing the amplification of soft sounds, it cannot compensate for the loss of transthreshold capabilities such as impaired frequency resolution. As a result, background noise remains a problem for many listeners.
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