In our clinic, we use prescriptive methods when fitting hearing aids, such as the Desired Sensation Level (DSL) and National Acoustics Laboratories (NAL) formulae. Prescriptive targets provide a consistent and validated approach for determining the amount of amplification needed to make speech audible while minimizing the likelihood of loudness discomfort.
Children with hearing loss encounter a wide range of listening environments every day, from quiet to very loud. While hearing aid signal-processing strategies, such as wide dynamic range compression and digital noise reduction, have been designed to enhance comfort across different listening situations, questions remain as to whether or not a single amplification prescription is optimal.
Children's Speech Recognition and Loudness Perception with the Desired Sensation Level v5 Quiet and Noise Prescriptions
Crukley J, Scollie SD Am J Audiol 2012;21:149
The study by Jeffery Crukley and Susan Scollie sought to explore this issue by determining the optimal hearing aid frequency response in quiet and in noise for school-age children.
Two different gain prescriptions from DSL version 5 were compared for 11 children age 8 to 17 who had stable moderate-to-severe sensorineural hearing loss and wore hearing aids. DSL Quiet is a prescription developed for quiet listening situations, while DSL Noise increases the prescribed compression threshold by 10 dB, resulting in less gain for lower-level sounds in noisy situations, where the input to the hearing aid is presumed to be primarily composed of noise.
Consonant recognition in quiet and sentence recognition in noise were measured for both versions of the prescription. The participants rated loudness on a categorical scale; text descriptions were illustrated by pictures of faces to help the younger children with the task. Sentences in noise were presented at 52 to 80 dB SPL to approximate the speech range.
For consonant recognition in quiet, the DSL Noise prescription resulted in a small but statistically significant decrease at 50 dB SPL compared with DSL Quiet—about four percent—but there were no differences at 70 dB SPL, nor did sentence recognition in noise vary between the two prescriptions.
These findings matched what the authors predicted would happen based on the characteristics of the two approaches. Specifically, since the DSL Noise program decreases gain for soft sounds, the potential exists for reduced speech recognition, particularly for children. However, the reduction in speech understanding was small overall and limited to the softest input level in quiet.
The loudness perception data were also in line with the researchers' predictions. For average levels—52 to 68 dB SPL—there were no differences between prescriptions. For higher input levels—72 to 80 dB SPL—DSL Noise resulted in lower categorical loudness ratings compared with DSL Quiet. The reduction in gain had the anticipated effect of improving listening comfort for loud sounds without affecting loudness ratings for levels approximating soft and average speech.
SECONDARY NOISE SETTING
What does this mean for fitting hearing aids in children? The authors suggest that DSL Noise could be implemented as an alternative setting in a secondary program of the hearing aid, to be used specifically for situations with significant background noise. This approach seems promising.
Drs. Crukley and Scollie also point out that the average reduction in consonant recognition observed at 50 dB SPL may have been driven by the performance of one specific listener in the study. The fact that children did not experience a difference across the two prescriptions for sentence recognition in noise could suggest that any reductions in gain are offset by the availability of linguistic context from sentences. The authors advise that clinicians assess children's aided speech recognition for different prescriptions and, when choosing materials for the assessment, consider the availability of linguistic context.
The authors also raise important points about the potential use of digital noise reduction or other hearing aid signal-processing features to achieve the same goal as the DSL Noise prescription—i.e., reduced gain for listening environments with noise. Not surprisingly, patterns of improved listening comfort and preserved speech recognition in noise have been observed with digital noise reduction in adults, as described by Ruth Bentler and Li-Kuei Chiou ( Trends Amplif 2006;10:67-82).
While Drs. Crukley and Scollie did not compare the DSL Noise program with digital noise reduction, a consideration of the two approaches does raise some important points for the clinic. For example, the DSL Noise prescription was developed to provide a weighted gain reduction so that gain is preserved for the frequency bands most important to speech understanding. Many digital noise reduction systems use similar methods to preserve gain for speech, but the systems' effects on audibility can be difficult to verify and may not consistently occur when speech and noise are both present. While further research in this area is needed, the authors make a strong case for considering and clinically evaluating alternative hearing aid prescriptions for non-quiet situations.