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• Air conduction hearing thresholds:
When a child is tested with sound presented through earphones, air conduction hearing thresholds are measured, representing hearing sensitivity for the entire hearing system. Because sound is presented to each individual ear, information can be gathered about the hearing in each ear, separately. The symbols used to represent air conduction testing are an X for the left ear and an O for the right ear. Sometimes colors are used for all of the different symbols: red for the right and blue for the left. If the child was tested with sounds presented from a loudspeaker, also called sound field, the hearing thresholds will be plotted with an S. Sound field hearing thresholds will reflect the hearing of the better ear, if there is a difference between ears. Ideally, children are tested under earphones to obtain information about each ear individually. However, in infants and toddlers, even small insert earphones are often rejected by the child and testing can only be conducted from the loudspeaker. If the child does not hear the sound at the loudest level of the audiometer (the machine used to test hearing), it may be indicated several different ways, with a NR (no response), or an arrow downward from the X or O.
• Bone conduction hearing thresholds:
Auditory stimuli can also be presented by bone conduction, a small box that sits behind the ear on the mastoid bone and presents sound by vibration to the inner ear. Bone conduction thresholds are represented by the symbols > and < or [ and ] on the audiogram. Bone conduction represent the responses from the inner ear, as the bone conducted signal bypasses the outer and middle ear systems. A comparison of bone and air conduction thresholds allows the audiologist to determine the type of hearing loss (conductive, sensorineural, or mixed).
This audiogram shows air conduction thresholds for a person with a moderate hearing loss:
Types of hearing Loss
Permanent hearing loss
Conductive Hearing Loss
If there is a problem in the external or middle ear, a conductive hearing loss exists. This means sound is not being conducted properly to the inner ear. Common causes of conductive hearing loss are ear wax (cerumen) in the external ear, fluid in the middle ear, or a hole or tear (perforation) in the eardrum. Most types of conductive hearing loss can be treated medically or surgically.
Sensorineural Hearing Loss
If a problem occurs in the inner ear, the hearing loss is sensorineural. Sensorineural hearing loss usually cannot be cured medically or surgically, but the use of hearing aids or other amplifying systems can help children hear and develop speech and language.
Neural Hearing Loss
If the outer, middle, and inner ear are functioning normally, but the auditory nerve and neural pathways to the brain do not function properly, the disorder is called a neural hearing loss, or
Mixed Hearing Loss
Children with sensorineural hearing loss also can develop middle ear problems (such as fluid in the middle ear) This can make the infant's hearing loss worse. When there is a combination of two problems (sensorineural and conductive hearing loss), this is known as a mixed hearing loss.
Degrees of hearing loss
Follow this link for recorded simulations of different degrees of hearing loss:
http://www.babyhearing.org/HearingAmplification/HearingLoss/audiogram.asp.
Mild hearing loss
The softest sounds a child hears are at levels of 25 dB to 40 dB. Without amplification, the child can hear most conversations up close and in quiet environments, but is likely to miss parts of words. A child with mild hearing loss will have trouble hearing faint or distant speech, and may have trouble hearing in a noisy environment. A child with mild hearing loss will benefit from amplification and should be able to hear all sounds with hearing aids.
Moderate hearing loss
The softest sounds a child hears are at levels of 40 dB to 65 dB. Speech can only be understood if it is loud. A child may have limitations in vocabulary, language comprehension and language usage. Without amplification, the child will have difficulty hearing spoken conversation such that 50 to 100% of spoken conversations may be missed. With amplification and intervention, a child with moderate hearing loss should be able to hear and recognize all sounds.