The NAL- NL 1 procedure has been shown to be a suitable starting point for fitting hearing aids to children and adults who wear cochlear implants in the opposite ear. Individual preferences for hearing aid frequency response and overall gain may differ from average requirements and research has shown that it is beneficial to fine-tune the hearing aid according to individual needs. On average, children who have had their hearing aid fittings optimised with their cochlear implants have better speech perception for sentences, localisation and do better in everyday life than they do with their cochlear implant alone or with an unadjusted hearing aid and implant. (Ching, Psarros, Hill, Dillon and Incerti, Ear Hear 22, 365-380, 2001; Ching, Psarros, Incerti and Hill, The Volta Review 103, 39-57, 2003).

There are two parts to the hearing aid optimisation procedure. For both parts, the child/adult wears a hearing aid that is connected to a programming device. The first part aims to determine the best frequency response for understanding speech. This involves doing a paired-comparisons test with audio-visual presentation of speech (Ching, Hill, Birtles, Beecham, Aus & NZ J Audiol 21, 51-63, 1999). The child/adult wears a hearing aid that is connected to a programming device. The NAL-NL1 frequency response, and alternative frequency responses that either provides a 6dB/octave boost (250 – 2000 Hz) or a 6dB/octave cut relative to the NAL-NL1 frequency response are set up. A story is presented audio-visually at an average conversational level, and the child/adult listens to the story first using one frequency response, then a second frequency response, and then decides which one is better for understanding the story. Each alternative is compared with the NAL response for a number of times until a significant preference is obtained. The cochlear implant is switched off during this test. The second part aims to equate the loudness of speech in the ear with a hearing aid to that in the ear with a cochlear implant. This involves the child/adult listening to the story using the preferred frequency response in the hearing aid only, and then using the cochlear implant only. The child/adult needs to compare the loudness in both ears, and decide whether the hearing aid side is louder, softer, or similar to the cochlear implant side. The hearing aid gain is then adjusted until the child finds both sides to be similar in loudness. For non-linear hearing aids, this test can be carried out at more than one input level, and the compression characteristics of the hearing aid can be adjusted accordingly.

To perform the hearing aid optimisation procedure you will need the equipment shown in the block diagram.

Prior to hearing aid optimisation, the child/adult needs to have used the hearing aid for at least a few hours a day on a regular basis. People who are newly fitted with a hearing aid will need to have used the hearing aid for about four weeks on a regular basis.

The procedures, together with the instructions and score sheets, are provided in the following linked documents (the procedures are applicable to the Bernafon AF120 behind-the-ear hearing aid and the Oasis software. The hearing aid is a single channel hearing aid with wide dynamic range compression, and has a fixed compression ratio of 2:1, and an adjustable compression threshold):

• A paired-comparisons test to determine the best frequency response for understanding speech;
• A loudness balancing task to equate the loudness of speech in the ear with a hearing aid to that with a cochlear implant.

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