Background: Prescriptive methods have been at the core of modern hearing aid fittings for the
past several decades. Every decade or so, there have been revisions to existing methods
and/or the emergence of new methods that become widely used. In 2001 Byrne et al provided
a comparison of insertion gain for generic prescriptive methods available at that
time.
Purpose: The purpose of this article was to compare National Acoustic Laboratories—Non-linear
1 (NAL-NL1), National Acoustic Laboratories—Non-linear 2 (NAL-NL2), Desired Sensation
Level Multistage Input/Output (DSL m[i/o]), and Cambridge Method for Loudness Equalization
2—High-Frequency (CAMEQ2-HF) prescriptive methods for adults on the amplification
characteristics of prescribed insertion gain and compression ratio. Following the
differences observed in prescribed insertion gain among the four prescriptive methods,
analyses of predicted specific loudness, overall loudness, and bandwidth of cochlear
excitation and effective audibility as well as speech intelligibility of the international
long-term average speech spectrum (ILTASS) at an average conversational input level
were completed. These analyses allow for the discussion of similarities and differences
among the present-day prescriptive methods.
Research Design: The impact of insertion gain differences among the methods is examined for seven
hypothetical hearing loss configurations using models of loudness perception and speech
intelligibility.
Study Sample: Hearing loss configurations for adults of various types and degrees were selected,
five of which represent sensorineural impairment and were used by Byrne et al; the
other two hearing losses provide an example of mixed and conductive impairment.
Data Collection and Analysis: Prescribed insertion gain data were calculated in 1/3-octave frequency bands for
each of the seven hearing losses from the software application of each prescriptive
method over multiple input levels. The insertion gain data along with a diffuse field-to-eardrum
transfer function were used to calculate output levels at the eardrums of the hypothetical
listeners. Levels of hearing loss and output were then used in the Moore and Glasberg
loudness model and the ANSI S3.5-1997 Speech Intelligibility Index model.
Results: NAL-NL2 and DSL m[i/o] provided comparable overall loudness of approximately 8 sones
for the five sensorineural hearing losses for a 65 dB SPL ILTASS input. This loudness
was notably less than that perceived by a normal-hearing person for the same input
signal, 18.6 sones. NAL-NL2 and DSL m[i/o] also provided comparable predicted speech
intelligibility in quiet and noise. CAMEQ2-HF provided a greater average loudness,
similar to NAL-NL1, with more high-frequency bandwidth but no significant improvement
to predicted speech intelligibility.
Conclusions: Definite variation in prescribed insertion gain was present among the prescriptive
methods. These differences when averaged across the hearing losses were, by and large,
negligible with regard to predicted speech intelligibility at normal conversational
speech levels. With regard to loudness, DSL m[i/o] and NAL-NL2 provided the least
overall loudness, followed by CAMEQ2-HF and NAL-NL1 providing the most loudness. CAMEQ2-HF
provided the most audibility at high frequencies; even so, the audibility became less
effective for improving speech intelligibility as hearing loss severity increased.
Key Words bandwidth - CAMEQ2-HF - compression - DSL m[i/o] - hearing aid - loudness - NAL-NL2
- prescription - speech intelligibility
