In development since the 1980s, the DSL Method is a family of hearing aid prescriptions spanning five versions to date. A hearing aid prescription is a set of computations that recommend the amount of amplification that a hearing should provide. These computations consider the amount of hearing loss that a person has, his or her ear canal acoustics, and many details about the hearing aid and how the person’s hearing was tested. The most recent version of DSL is Version 5.0, which allowed DSL to support accurate hearing aid prescription for infants. We also included prescriptions for adults, allowing DSL targets to address the different listening needs of people across the lifespan. Today, DSL is transferred to the hearing instrument industry as software, allowing it to be widely accessible by clinicians all over the world. Proceeds from transfer of DSL are returned to our university, for continued use in research.
DSL Version 5.0 was the first generic hearing aid prescription that adjusts for the different listening needs of children and adults, and that provides different amounts of hearing aid gain for use in quiet versus noisy places.
Dr. Richard Seewald
Dr. Richard Seewald is a Distinguished University Professor Emeritus at the National Centre for Audiology and in the School of Communication Sciences and Disorders, Faculty of Health Sciences, Western University. For 35 years, Seewald’s work focused on issues pertaining to the selection and fitting of amplification and early habilitation for infants and young children with hearing loss. He is known internationally for his work in developing the Desired Sensation Level (DSL) Method for pediatric hearing instrument fitting.
Rubella Outbreak: An impetus for habilitation
An epidemic of rubella reached its peak in the Atlantic Provinces of Canada in 1974. Parents of infants aged one to three arrived at the Children’s Hospital in Halifax, Nova Soctia, seeking help. As one of only three audiologists in all of the Atlantic Provinces, Dr. Seewald and his peers were under-prepared to deal with the aftermath of the damage caused by the rubella outbreak. Programs and processes were not in place to diagnose, fit hearing aids and habilitate these infants.
Motivated to support these families, Dr. Seewald would commit a lifetime of research to hearing aid prescriptive procedures for infants with the goal to diagnose by three months of age.
Development of DSL
With the arrival of personal computers in 1979, Dr. Seewald set out to develop a computer-assisted prescriptive procedure for the fitting of amplification in pre-verbal children. He developed a conceptual model of the hearing aid fitting process was developed, which began the vision for DSL as a comprehensive fitting method. Although Seewald wished to support early intervention for children who wear hearing aids, the technical advances that led to today’s procedures for infant hearing assessment were not fully available in the early years of DSL. Dr. Seewald looked to the literature for studies of loudness discomfort, speech perception and preferred listening levels in children. These results, together with the advent of the first probe-microphone systems in the 1980s allowed development of the earliest versions of DSL. Clinicians had to look-up the table of values and complete paper and pencil worksheets, which made the DSL method cumbersome.
Desired Sensation Level Software
The Desired Sensation Level (DSL) Method, developed by Seewald and colleagues, became available as the first software program to assist with hearing aid fitting for young children in 1991. DSL formulae version 3.1 worked with linear hearing aids and included a prescription for hearing aid output limiting (Seewald, Ross, & Spiro, 1985; Seewald, 1991). Similar to the NAL-NL formulae, Seewald sought to prescribe a comfortable level of speech that was associated with maximum speech sound recognition performance in children (based on studies by Erber & Witt, 1977; Kamm, Dirks, & Mickey, 1977; Macrae, 1986; Pascoe, 1978; Pascoe, 1988). These early versions of DSL incorporated children’s ear canal acoustics, and child-friendly verification procedures.
Advancing the DSL Software
By 1995, wide dynamic range compression technology came out and offered a range of input signals for soft, average and loud speech. This required a major overhaul of the DSL software. Seewald’s team responded with the DSL[i/o] algorithm formulae. DSL[i/o] prescribed targets for gain and output across frequencies, and could be adjusted for use with linear or amplitude compression hearing aids (Cornelisse et al, 1995).
With support to Seewald from the Canada Research Chair program, development of DSL Version 5 commenced in 2002. This team included Susan Scollie, Sheila Moodie, Marlene Bagatto, Leonard Cornelisse, and John Pumford, as well as software engineer Steve Beaulac. Their revision included the incorporation of multichannel amplitude compression targets, infant-focused features, corrections for conductive and binaural fittings, and targets for adult listeners (Bagatto et al., 2005; Scollie et al., 2005). Upon Dr. Seewald’s retirement, Drs. Scollie, Bagatto, Glista and Moodie have carried on development and implementation of procedures for intervention for pediatric hearing impairment, including ongoing development and support for DSL v5.0 and related procedures. Dr. Scollie leads the development of DSL5, Dr. Bagatto leads the development of audiological monitoring and infant hearing protocols, Dr. Glista leads a program of outcome measurement in frequency lowering, and Dr. Moodie leads initiatives for collaboration with clinicians in development new knowledge in pediatric audiology.
The DSL family of formulae has evolved over decades, responding to advancements in technology and breakthroughs in research. They continue to be the most common prescriptive formulae used in pediatric hearing aid research and in clinical practice (McCreery et al., 2013; Jones & Launer, 2011).