FONIX FP40 Portable Hearing Aid Analyzer
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Appendix I: DSL Programming Notes
Notes on the DSL
®
Programming
The DSL
®
(Desired Sensation Level) software in the FONIX FP40/FP40-D program is based
on the latest copyrighted I/O computer program. The DSL method was originally described
by Seewald, Ross & Stelmachowicz in 1987. Its purpose was to provide amplified speech that
is consistently audible, comfortable, and undistorted across the broadest relevant frequency
range. Recently this method has been elaborated to attempt to provide audibility for as broad
a range of inputs as possible. The goal is to make soft sounds audible, bring average speech
sounds to the most comfortable level to maximize speech discrimination, and to keep loud
sounds from exceeding the comfort level of the hearing aid wearer. These goals are addressed
by the WDRC formula.
The original purpose of the DSL method was to fit hearing aids on children. Children present
particular obvious problems in testing. Since it is often impossible to get as much reliable data
as needed from the child, methods were developed to predict audiometric and acoustic infor-
mation and to be able to do most of the fitting of the hearing aid in a sound chamber rather
than in the real ear. Nevertheless, the DSL programming in the FP40 is found in the Probe sec-
tion. Verification of the fitting is done whenever possible in the real ear.
The size of the ear canal, a function of the age of the child, affects the frequency response of
the aid. Therefore, corrections are built into the FP40 target curves based on age. These are in
addition to the corrections for microphone placement that are determined by the type of hear-
ing aid. Frye electronics has long had its own corrections for microphone placement. The DSL
corrections are used instead when the DSL formula is chosen so that the results are equivalent
to those found in the DSL I/O computer program (If another formula is used, our original cal-
culations are used. If desired, users may apply the DSL age corrections to the target coupler
conversions that originated from existing insertion gain fitting formulas).
Sensation Level refers to an amount of sound above threshold. Frequency-specific target val-
ues, or Desired Sensation Levels, provide guidance in choosing and setting the hearing aid.
Research findings* show that the required sensation levels for maximum speech discrimination
vary with thresholds. Adjustments are made to the formula to reduce the upward spread of
masking from the lower frequencies. Finally, predictions of uncomfortable levels are made from
threshold data. The high level limits in DSL are not the traditional UCLs, but are one standard
deviation below. They are designed to be at the upper limit of comfort (when formulas other
than DSL are chosen, the SPL display in the FP40’s Probe Option reflects the traditional UCL
numbers).
Two important considerations in prescribing WDRC hearing aids are the compression threshold
and compression ratios. The compression threshold should correspond to the hearing aid being
fitted. The DSL creators recommend setting the compression threshold at the lowest possible
setting. Compression ratio targets are given for each audiometric frequency. This information
can be used in selecting a hearing aid. If all the compression ratios are very similar, a single-
*Erber, NP & Witt, LH (1977) Effect of stimulus intensity on speech perception by deaf children. Journal of Speech
and Hearing Research, 42, 271-278
