And How the Desired Sensation Level (DSL) Approach Can Help (Part 1)
Whatever happened to the aided audiogram? Teachers and parents, accustomed to using the aided audiogram as an assessment, counseling and programming tool, often wonder why these test results are rarely seen. Most clinical audiologists have replaced aided audiograms, which indicate the child's hearing thresholds after hearing aid amplification, with real ear measurement technology, but teachers and parents/ caregivers may not have learned to interpret and apply these results to everyday life. The first part of this article will describe why aided audiograms are no longer considered the measurement tool of choice by clinical audiologists, and the second part will describe how parents/caregivers and teachers can interpret a different kind of graph, the SPLogram, to obtain the same information.
An aided audiogram is obtained in essentially the same way as a standard audiogram. Sounds are presented in the sound suite or booth, and the child's responses to increasingly softer sounds are recorded; the main difference is that the sounds are presented through speakers because the child is wearing his or her amplification. For infants and young children, these responses might be a head turn towards the source of the sound; for older children, these responses might be putting a block in a bucket or raising a hand when sounds are presented. Obtaining a complete aided audiogram requires a significant amount of time during which the child needs to be paying close attention. Real ear measurement technology, on the other hand, allows the audiologist to measure the characteristics of the hearing aid in the child's ear canal very quickly and accurately. One of the most commonly used software programs for real ear measurement technology, called the Desired Sensation Level (DSL) program, produces a different type of audiogram. This "SPLogram" looks a little different than a traditional audiogram, and with a little practice parents and teachers can use SPLograms to understand what a child can be predicted to hear with his or her hearing aids.
How do clinical audiologists measure how well a hearing aid is working without an aided audiogram or SPLogram? One way is an electroacoustical evaluation where the hearing aid is connected to a metal container (called a 2 cc coupler), placed in a small, sound proofed chamber and measured. This is often referred to as an "ANSI" test, meaning that the procedures and the measurements have been standardized by the American National Standards Institute (ANSI, 1996). These results can be compared to specification sheets from the manufacturers that provide expected values for each hearing aid. The measurement is typically done by clinical audiologists to ensure that the electronic components are working; however, it cannot be used as an indication of how a child will perform with this hearing aid.
Why not? The values obtained when measuring the hearing aid coupled to a small metal chamber and the values obtained when the same hearing aid is placed in a child's ear are quite different. Sound behaves very differently in a metal chamber than in an ear canal made of skin, cartilage and bone. Therefore, the "ANSI" measurement may overestimate or, more typically, underestimate the sound levels the child receives. Aided audiograms, on the other hand, do test hearing aid performance on the child's ears. However, they are time consuming, tiring for children and yield information about only a few frequencies. With the introduction of real ear measurement systems in the late 1980s, a more accurate way to measure hearing aid benefit was possible.
Real ear measurement systems provide a direct measure of sound at an individual's eardrum. Our ear canals actually shape sound as it passes through them to the eardrum. A phenomenon called "resonance" causes certain frequencies in the speech signal to be enhanced or amplified by the ear canal. …