What do hearing tests measure?

WHAT DO HEARING TESTS MEASURE?

Intensity and Frequency

Hearing tests measure loudness in decibels (intensity) and pitch in Hertz (frequency). The lowest number of decibels (dB) you can hear at different frequencies (Hz) is measured and plotted on a graph called an audiogram.

Frequency: Frequency is the number of vibrations per second, expressed as Hertz (Hz). The sounds of speech are in the range of 250 Hz to 4000 Hz. These are commonly tested in a hearing test.

Intensity: Sound waves made by something vibrating gently have little intensity (the sound is soft). If an object vibrates strongly, sound waves have greater intensity (the sound is loud).

When sound waves are made by something vibrating slowly, a foghorn or a drum, a low frequency sound occurs.
When sound waves are made by something vibrating quickly, a whistle or a small bell, a high frequency sound occurs.

TYPES OF STANDARD HEARING TESTS

Pure tone audiometry
You listen to a range of beeps and whistles, called pure tones, and indicate when you can hear them. The loudness of each tone is reduced until you can just hear the tone. The softest sounds you can hear are your hearing thresholds marked on an audiogram.

Air conduction
Hearing is measured with pure tones through headphones. The sounds go via the air, down the ear canal, through the middle ear, and to the cochlea in the inner ear. The air conduction hearing thresholds are indicated on the audiogram by X for left and 0 for right.

Bone conduction
Placing a small vibrator on the mastoid bone behind the ear measuring the softest sounds that can be heard can test the sensitivity of the cochlea. Sounds travel through the bones of the skull to the cochlea and hearing nerves, and sidestep the middle ear. Unmasked bone conducted thresholds are indicated by Δ, while masked bone conducted thresholds are indicated by [ for right and ] for left.

Speech discrimination tests
The range of audible sounds, not just the degree of hearing loss, can vary considerably from person to person. Unfortunately when hearing is damaged it is usually not just the volume or quantity of sound heard that is lost. Often the quality of the sound is also distorted. The amount of distortion can be measured using speech discrimination tests. Poor speech discrimination means that voices are distorted and, not loud enough.

Recording the results of a hearing test

Audiologists use different symbols on a chart called an audiogram to show which ear was being tested and how the sound was presented to the ear.
Down the side of the audiogram, the intensity of the sound is shown from soft sounds at the top to loud sounds at the bottom. Across the bottom of the audiogram, the frequency is shown from low-pitched sounds on the left to high-pitched sounds on the right.

INTERPRETING THE RESULTS OF A HEARING TEST

The results of a hearing test will indicate the ...

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Recording the results of a hearing test

INTERPRETING THE RESULTS OF A HEARING TEST

The results of a hearing test will indicate the degree and type of hearing loss.

Mild: 21-45 dB
You would have some difficulty hearing soft speech and conversations but would manage in quiet with clear voices. Hearing aid will assist most hearing problems in this range.

Moderate: 46-60 dB
You would have difficulty understanding conversational speech and more so in the presence of background noise. TV and radio would be turned up. Hearing aid will assist most hearing difficulties if speech discrimination, is good and background noise low.

Moderately severe: 61-75 dB
The clarity of speech heard may be significantly affected and this can restrict the benefit of a hearing aid. However, with appropriate training a hearing aid should allow conversational speech to be heard in quiet listening.

Severe: 76-90 dB
Normal conversational speech is inaudible. A hearing aid will amplify many speech sounds. Yet visual cues will assist in understanding speech.

Profound: 91 dB +
There is great inconsistency in the benefit derived from a hearing aid. Some can understand clear speech face to face in places with good auditory conditions when wearing a hearing aid. Others find it impossible.

Types of hearing loss

Conductive hearing loss
This type of hearing loss is indicated on the audiogram by normal hearing thresholds obtained via bone conduction and a hearing loss obtained via air conduction (air/bone gap is present). This means that the cochlea is normal and healthy however the hearing loss is caused by blockage or damage in the outer and/or middle ear. Middle-ear infection is a common cause of a conductive hearing loss in young children. It leads to a reduction in the loudness of sounds. It may be possible to treated this type of hearing loss medically or surgically.

Sensorineural hearing loss
This type of hearing loss is indicated on the audiogram by the hearing thresholds obtained via air conduction and bone conduction being the same yet below normal levels (no air/bone gap). Results from damage or malfunction of the cochlea or the auditory nerve. It can be present at or soon after birth (congenital) or acquired later in life. Exposure to excessive noise and aging process can cause a sensorineural hearing loss. It leads to a loss of loudness as well as a lack of clarity. The quantity and the quality of sound are affected and, sometimes may limit the benefit of a hearing aid.

Mixed Hearing Loss
This type of hearing loss is indicated by a gap between the hearing thresholds obtained via air conduction and bone conduction yet both are below normal levels (air/born gap is present). Results when there is a problem in both the conductive pathway (in the outer or middle ear) and in the nerve pathway (the inner ear). An example of a mixed hearing loss is a conductive loss due to a middle-ear infection combined with a sensorineural loss due to damage associated with aging.

Congenital hearing loss
Present at, or soon after birth and caused by damage to the unborn baby due to a virus such as CMV (Cytomegalovirus) or German Measles (Rubella) or by Atresia (partial or complete closure of the ear canal). In Australia, 20 children per 10,000 live births will be born with a congenital sensorineural hearing impairment.

Pre-lingual hearing loss
Affects how well a child learns to speak. The hearing is lost before a child has completely developed speech and language. It may be congenital or conductive in the first few years of life.

Post-lingual hearing loss
The hearing loss is acquired after speech and language has developed. 12 children per 10,000 will acquire at least a moderate hearing impairment by the age of 17.

HEARING TESTS FOR YOUNG CHILDREN

The selected test to assess hearing in infants and young children depends on

The child's age and ability to undertake the test.
The information that is needed about the child's hearing.

Nearly all audiological tests are divided into either Behavioral tests or Physiological tests. Most children will be assessed using a combination of behavioral and physiological tests.

Behavioral tests

Behavioral Observation Audiometry or BOA: Noisemaker testing is carried out with infants less than 7 months of age and with older children who cannot respond when they hear a sound. It is often performed by health visitors in the community.

Visual Reinforcement Audiometry or VRA: This is used to test the hearing of children between 7 months and 3 years of age. By altering the frequency and intensity of the sounds, it is possible to find out about the child's ability to hear sounds across a range of frequencies.

Play Audiometry: Play audiometry is used to test the hearing of children from 3 years of age. This also is used to test the child's hearing when hearing aids are worn. In play audiometry, the child listens to sounds and is taught to make a response every time a sound is heard. It is similar to pure tone audiometry however instead of pressing a button to respond the child performs a playing task.

Physiological tests

Results help determining which part of the auditory system is involved in the hearing loss.
Physiological tests measure a physical response of a specific part of the auditory system. They can be performed on both children and adults. OAE and BSERA are typically tests performed on child/infants because they do not require the individual to respond and so can be performed at very young ages, practically straight after birth. .

Oto-acoustic Emission Testing – OAE: It is possible to get an idea about how hair cells in the cochlea are working. A probe that produces a sound is inserted into the ear and measures the very soft sound (oto-acoustic emission) produced by the hair cells as a result. If the OAE is present it indicates normal cochlea function.

Brainstem Evoked Response Audiometry – BSERA: This technique provides information on electrical activity generated in response to sound along the nerve pathway, also called brainstem, to the brain. It takes approximately one hour and is usually carried out while the baby is in natural sleep. If this is not possible occasionally it is performed while the person is under GA.

Tympanometry and Acoustic Reflex: Tympanometry gives information about the middle ear.

Normal - Compliance: 30 to 150, Middle ear Pressure: -150 to 50
Middle ear dysfunction - no peak compliance recorded (flat trace)
Eustachian tube dysfunction - Compliance: 30 to 150, Middle ear Pressure: < -150 or > 50
Perforation/grommet: either no seal obtained or large cavity volume

When a person has a normal tympanogram it may be possible to test for the presence of a muscle reflex - acoustic reflex - in the middle ear.