Inspire offers a variety of prescriptive approaches for fitting hearing aids. These approaches can be categorized as being either Linear or Non-Linear in nature. Linear fitting formulas were designed to prescribe gain for hearing instruments with fixed gain and frequency responses across input levels. Non-Linear fitting formulas prescribe different amounts of gain depending upon input level, and were developed to fit wide dynamic range compression hearing instruments.
The Non-Linear and Linear fitting formulas available within Inspire are described below. Only the fitting formulas selected via Target Preferences are available in the drop down list from any Fitting screen.
e-STAT (default) - This formula represents an intelligent combination of the best, and most well-validated, published fitting formulas with data-based conclusions from mining our vast database of hearing aid fittings (literally 100s of thousands of fittings). e-STAT is coupled to an active and growing database and will continue to evolve and improve over time. e-STAT is more closely tailored to our compression architecture rather than a generic approach.
DSL v5 (DSL m[i/o]) - Specific objectives of DSL v5 include:
Avoidance of loudness discomfort during hearing aid use
Hearing instrument prescription that ensures audibility of important acoustic cues in conversational speech as much as possible
Prescription of hearing instrument compression that is appropriate for the degree and configuration of the hearing loss, but that attempt to make a wide range of speech inputs available to the listener
Accommodation for the different listening requirements within quiet and noisy listening environments
Widely recognized fitting formula for pediatrics
A detailed description can be found at http://www.dslio.com.
Real Ear Aided Response (REAR) pure-tone DSL v5 targets that are generated in Inspire can be expected to closely match the DSL v5 stand alone version. Slight discrepancies can be accounted for by the fact that Inspire always uses soft speech compression thresholds whereas DSL v5 may prescribe slightly different compression thresholds.
NAL-NL1 - The established NAL-R procedure (Byrne & Dillon, 1986) aims to produce all bands of speech at equal loudness. For people with profound hearing loss, research with linear amplification shows us that a systematic departure from equal loudness is necessary: High frequency bands are made less loud than the others, because of the diminished contribution of the high frequency bands to intelligibility, even when the information within those bands is audible. This departure from equal loudness is the basis of the NAL-RP prescription procedure (Byrne, Parkinson and Newall, 1990). The NAL-NL1 procedure is an extension of the NAL-RP formula, based on the following assumptions:
The relative loudness of different speech bands should be such as to maximize speech intelligibility
The aid wearer should perceive speech to be no louder than that which a normal hearing person would perceive
NAL-NL1* - National Acoustic Laboratories is in the process of deriving a new prescription formula, NAL-NL2, for non-linear hearing aids by improving upon the established NAL-NL1 prescription. In anticipation of the release of NAL-NL2, and evidence from our studies that NAL-NL1 targets often prescribe more gain than desired, Inspire has added NAL-NL1*. This formula is modeled on the established NAL-NL1 with less aggressive gain targets.
NAL-NL2 - This formula maintains the same aim as NAL-NL1, which is to make speech intelligible and overall loudness comfortable. In general, NAL-NL2 provides more gain in low and high frequencies, and less gain through the mid-frequency region. The National Acoustics Laboratory reports that NAL-NL2 will provider better speech intelligibility without increasing loudness.
BERGER - This linear formula overlays on the hearing threshold level a gain-frequency response, which mirrors the speech spectrum. Berger is designed for use with linear and compression limiting hearing aids.
NAL-R - The goal of this linear prescriptive formula is to amplify speech so that all frequency bands contribute equally to its loudness. Modifications for listeners with severe to profound hearing loss is also included.
WFA - This linear formula provides more high frequency gain and is a proprietary fitting formula (William F. Austin) that's only available in Inspire.