Hearing Aid Verification
Prescriptive Fitting Strategies
A primary reason for using a prescriptive hearing instrument fitting approach with infants and young children is that they cannot give the audiologist information about how well the hearing instrument performs or how much of speech is audible through the hearing instrument. By using a prescriptive strategy, the audiologist can use any audiometric threshold information that is available to determine targets for hearing instrument performance. Even ABR threshold information can be used, enabling audiologists to begin the fitting process as soon as hearing loss is confirmed. Prescriptive targets can be compared to measurements of hearing instrument performance in order to choose appropriate amplification, evaluate hearing instrument settings and ultimately ensure that use of the hearing instrument will facilitate auditory skill development.
Historically, amplified sound field and/or functional gain measures were used to verify the adequacy of a hearing instrument fitting. These test procedures have numerous shortcomings including the fact that the measures are not sensitive to many of the signal processing differences across hearing aids. Scollie and Seewald (2002) provides a comprehensive discussion of the efficacy of the two most common techniques used to verify hearing instrument performance (amplified sound field thresholds [ASFT] and probe microphone measures). For a verification technique to be effective for children it must be able to accomplish the following goals:
- Assess how the hearing instrument processes speech
- Determine the levels at which output is limited
- Be efficient, reliable, and valid
- Be capable of being used with infants
- Be meaningful
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Table 1 summarizes a comparison between the different evaluation techniques.
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| Evaluates with input levels comparable to speech |
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| Maximum output in the ear is measured / estimated |
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| Variability of the measure |
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| Frequency Resolution of the measure |
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| Age constraints for test procedure (particularly < 6 months) |
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ASFT
(Amplified Sound
Field Thresholds)
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NO
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NO
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HIGH
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SLOW
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POOR
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YES
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Probe Microphone Measures
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YES
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YES
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LOW
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FAST
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HIGH
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NO
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