Hearing Assessment
Behavioural Testing
"Behavioural" hearing tests require the participant to reliably demonstrate a change in behaviour when a test sound is heard. For adults and older children, the most commonly used forms of behavioural tests are pure tone audiometry and speech discrimination testing. These two measures provide information on the degree and nature of any hearing loss.
Pure Tone Audiometry
Pure tone audiometry involves listening to beeps across a range of pitches, or notes, and responding when the stimulus is heard. The examiner systematically finds the softest sounds the participant can hear across a range of frequencies and determines the hearing thresholds - the softest sounds one can hear across the range of pitches that are important for speech understanding. The hearing thresholds are plotted on a graph called an audiogram (below). Frequency ("pitch") is measured in Hertz and is plotted on the x-axis with low frequencies on the left increasing to high frequencies on the right (like a piano keyboard). Loudness ("volume") is measured in decibels and is plotted on the y-axis, with very soft sounds at the top of the graph increasing to very loud sounds at the bottom.
The Audiogram
The beeps used in pure tone audiometry may be presented via:
- headphones that are placed over the ears,
- insert earphones - small foam tips that sit in the ear canal, or
- bone conductor - a small vibrating cube on a metal headband. The vibrator can be positioned either on the mastoid bone behind the ear or on the forehead.
Headphones and insert earphones test the entire hearing system, from the ear canal, through the middle ear to the brain. Thresholds obtained with these are called air conduction thresholds and indicate one's functional hearing sensitivity. Some hearing aids incorporate a signal generator which makes it possible to test air conduction hearing through the hearing instrument. This is called "in-situ audiometry".
The bone conductor directly stimulates the cochlea (organ of hearing), by-passing the ear canal and middle ear. Thresholds obtained with the bone conductor are called bone conduction thresholds.
Occasionally a wind-like noise called masking is used to occupy one ear while the tester determines in which ear the beeps are being heard.
Together, air and bone conduction thresholds help indicate where a hearing problem may be. If air conduction thresholds indicate a hearing loss, it may be:
- Conductive: air conduction thresholds indicate a hearing loss but bone conduction thresholds are normal. Possible causes include obstruction of the ear canal by wax, accumulation of fluid behind the ear drum, a hole in the ear drum. Conductive hearing loss can often be treated medically.
- Sensorineural: bone conduction thresholds match air conduction thresholds. This shows that the cochlea or hearing nerve is causing the hearing loss. Sensorineural hearing loss is almost always permanent. Possible causes include the ageing process, exposure to loud noise, use of ototoxic medication.
- Mixed: both air and bone conduction thresholds are elevated, but air conduction thresholds are poorer than bone conduction thresholds. As the name implies, this indicates a combination of conductive and sensorineural hearing loss with multiple underlying causes.
Both adults and children can be tested with pure tone audiometry, but use different means of indicating when they hear a tone. Adults usually press a button to indicate they have heard a tone, while children give other behavioural responses described below.
Play Audiometry
Play audiometry is suited to children aged around 3 to 7 years developmentally. The child is taught to respond, using a pre-determined task, whenever they hear tonal stimuli that are introduced through headphones or through a bone conductor placed behind the ear on the mastoid. The task may be anything that will maintain the child's attention. For example, the child may be taught to put a colourful peg into a peg board, or drop a marble into a marble-ladder game. Older children may be interested in putting a stamp or sticker on a piece of paper each time they hear the sound.
Given sufficient cooperation from the child it is usually possible to produce a complete and accurate "audiogram" that illustrates their threshold of hearing for a pre-determined frequency range. By the age of around 7 years many children will be happy to perform standard audiometry, that is, simply press a button when they hear the sound.
Play Audiometry
Visual Reinforcement Orientation Audiometry (VROA)
This test technique is suited to infants aged 7 or 8 months to 3 years developmentally. It is traditionally performed in a sound-field but may also be performed while the child is wearing headphones. The child is taught (i.e., conditioned) to turn their head when a sound is heard. Initial conditioning is achieved by the introduction of stimuli at moderately high levels and the audiologist waits until the child looks for the source of the sound. They are then shown a colourful, moving puppet or toy under illumination as a reward. This puppet or toy "reinforces" the child's turning behaviour or orientation and gives rise to the term Visual Reinforcement Orientation Audiometry. Once this conditioned response is reliably observed, the stimuli can be presented at ever decreasing levels until auditory threshold or minimum audible levels have been reached.
While most infants find the "puppet show" fun and surprising, the success of the technique will depend very much on the child's overall development and state on the day. For example, if the child is tired or unwell, reliable results may not be obtained. VROA may not be suitable for children with some disabilities, particularly those that affect vision or movement. In such situations other types of testing may be indicated.
Visual Reinforcement Orientation Audiometry
Behavioural Observation Audiometry (BOA)
This test technique is suited to infants aged less than 7 months developmentally. It might also be used for older children who cannot be taught to respond for VROA or play audiometry because of disabilities that affect motor skills or cognition (learning). Young infants generally show little interest in tonal stimuli even when they have good hearing. BOA testing therefore is performed using stimuli, or "noisemakers", such as baby rattles, drums, squeaking toys, or cellophane paper cracking.
During the test one audiologist makes a sound, making sure that the child cannot see them, while a second audiologist watches for any change in the child's behaviour (e.g., a "startle" or sudden reflexive movement, eye blinks or changes in sucking patterns). The type of sound is recorded together with its intensity and the nature of the behaviour change.
There are some significant difficulties with BOA testing. First, the infant's response to these sounds is strongly influenced by their age and state on the day of the test. If they are sleepy, hungry, sick or crying, then outcomes will be unreliable. Second, many infants (even those known to have good hearing) may not show behavioural changes to sounds until they are quite loud. Third, some infants may be more interested in some types of sounds than others. These difficulties mean that the results of BOA must be interpreted cautiously, with the knowledge that many factors other than hearing may affect the child's responsiveness during the test. BOA cannot therefore be used to estimate a child's audiometric threshold and is likely to be used in conjunction with other more objective hearing tests (see "electrophysiological tests").
Special note: Sometimes if a parent or child carer is concerned about their infant's hearing they may repeatedly perform their own "noisemaker" tests around the home by banging a lid or slamming the door to see if the child responds. Children quickly grow accustomed to sounds and may stop showing a response if they hear the same sound often enough. For this reason, it is recommended that repeated "testing" at home is avoided prior to formal BOA testing with the audiologist.
Behavioural Observation Audiometry
Further Reading:
1. Diefendorf, AO (2002). Detection and assessment of hearing loss in infants and children. In: Katz J (Ed), Handbook of Clinical Audiology. Lippincott, Williams & Wilkins, Maryland, Philadelphia, United States of America. Chapter 23, pp 469-480.
2. Gans DP (1987). Improving behaviour observation audiometry testing and scoring procedures. Ear & Hearing, 8(2):92-100.
3. Gans D and Flexer C (1982). Observer bias in the hearing testing of profoundly involved multiply handicapped children. Ear & Hearing, 3(6):309-313.
4. Gerber S (1982). The use of noise-making toys as audiometric devices. International Journal of Paediatric Otorhinolaryngology, 4: 309-315.
5. Lovegrove R and Birtles G (1983). Measurements of noisemaking toys and their application in infant hearing assessment. Australian Journal of Audiology, 5(2): 47-54.
6. Moore JM, Wilson WR and Thompson G (1977). Visual reinforcement of head turn responses in infants under 12 months of age. Journal of Speech and Hearing Disorders, 42:328-334.
7. Thompson M and Thompson G (1972). Responses of infants and young children as a function of auditory stimuli and test methods. Journal of Speech & Hearing Research, 15:699-707.
8. Thompson G and Weber B (1971). Responses of infants and young children to behaviour observation audiometry (BOA). Journal of Speech & Hearing Disorders, 39:140-147.
Speech Discrimination Testing
Involves listening to speech and repeating what one hears. The speech material is usually either single words or sentences, and is often presented at more than one volume.
Speech discrimination testing is not suited to younger children and infants and therefore other techniques have been developed.
Further information on behavioural hearing assessment in adults can be found in:
Katz J (2002). Handbook of Clinical Audiology. 5th Ed. Baltimore, Lippincott, Williams and Wilkins, pp 71-88, 124-142.
