History

The earliest versions of the DSL Method used tables of values that specified target sensation levels for amplified speech as a function of frequency and hearing level. These desired sensation levels, or DSLs, were based on data describing the speech sensation levels that were associated with comfortable listening levels across hearing levels (e.g., Kamm, Dirks and Mickey, 1978; Pascoe, 1978) and, more importantly, ceiling speech recognition performance in children with sensory hearing impairment (e.g., Gengel, Pascoe and Shore, 1971; Erber and Witt, 1977; Macrae, 1986; Smith and Boothroyd, 1989). The DSL Method also provided hearing instrument output limiting targets appropriate for use with young children that also varied as a function of frequency and hearing level (Seewald, 1991; Seewald, Ramji, Sinclair, Moodie and Jamieson, 1993). These look-up table of values and accompanying paper/pencil worksheets made clinical implementation of the DSL Method cumbersome. In 1991, the DSL Method (DSL v3.0) was made available as a software program making it the first published computer-assisted implementation for hearing instrument fitting for young children (Seewald, Zelisko, Ramji and Jamieson, 1991).

In 1995, Cornelisse, Seewald and Jamieson described an electroacoustic fitting algorithm called the DSL input/output formula (DSL[i/o] v4.0) (Cornelisse, Seewald and Jamieson, 1995). This device-independent enhancement of the original DSL Method provided prescriptive targets for the fitting of wide-dynamic-range compression hearing aids which had become readily available by this time. The DSL[i/o] algorithm applied loudness data and a curvilinear fit to map a wide range of input levels to target hearing instrument output levels across frequencies. It has been used in DSL software systems for v4.0 and v4.1 and in most hearing instrument and real-ear system manufacturers software implementations.

Recently several factors have made our laboratory consider a number of modifications and elaborations to both the DSL Method and the DSL[i/o] algorithm. First, children with hearing loss are being identified at birth and amplification is being provided to infants by 6 months of age (Joint Committee on Infant Hearing, 2000; American Academy of Audiology, 2003). These infants will wear their hearing instruments at settings determined by clinicians for at least the first few years of life increasing the importance of continued research and development on an objective, evidence-based procedure like the DSL Method for hearing instrument fitting. Secondly, improvements in auditory brainstem response (ABR) testing procedures, and significant advances in hearing instrument technology make continued development both desirable and necessary. In addition, since the release of the computer-assisted implementation of DSL v4.1 in 1997 anecdotal reports from clinicians, research studies in our laboratory and published studies indicated some modifications could be applied to the algorithm for more appropriate adult application (e.g., Moore, Alcantara, and Marriage, 2001). Finally, clinicians still desire access to generic prescriptive algorithms relative to manufacturer-specific proprietary fitting algorithms, especially for their pediatric clients. This is understandable in light of recent published studies which have shown that adult clients with similar hearing losses might be fitted with substantially different amplification characteristics depending on the hearing instrument proprietary fitting method chosen (Smeds and Leijon, 2001; Keidser, Brew and Peck, 2003; Killion, 2004). Given all these considerations, work was initiated in the late 1990s on a new version of the DSL Method (DSL v5.0) which includes the DSL multi-stage input/output algorithm, referred to as DSL m[i/o].

Understanding an SPLogram

One of the primary traits of the DSL Method is the now well-recognized SPLogram display. The goal of packaging amplified speech within the residual auditory area can be best observed by plotting all thresholds using an ear canal dB SPL reference scale. This is a sample SPLogram for a child with a moderate hearing loss from .25 to 6 kHz. Unaided hearing thresholds and predicted thresholds of discomfort define the residual auditory area in a dB SPL reference level. Targets for maximum power output of a hearing aid and for aided conversation-level speech are also plotted. Measured aided responses for soft, average and loud conversational speech inputs are shown. Note that the extent of the child's residual auditory area and resulting location of aided conversational speech and hearing aid maximum output are easily observable and compared with each other.