Performance of children fitted with multi-channel, non-linear hearing aids

Abstract

The basic goal of amplification is to provide audibility for speech without making sounds uncomfortably loud. These requirements must be followed with special care with pediatric patients to ensure they have the opportunity to develop their speech, language, and auditory skills to their full potential. This is because young children have more difficulty filling in missing information than do older individuals whose hearing loss occurred after they had acquired language. Before discussing the fitting of non-linear instruments on a child, let’s examine how a linear hearing aid meets the basic amplification goals. The hearing aid gain is set so that conversational speech is audible and the maximum output is set below the wearer’s discomfort level. When the listeners are trying to hear soft speech (e.g., a whisper), they are instructed to turn up the volume control (VC). When listening to loud or shouted speech, they are instructed to turn down the VC. Unfortunately, until children are old enough to understand the proper use of the VC, they will be unable to make the appropriate volume adjustments to their hearing aids. This is one practical reason why all young children should be fitted with non-linear hearing aids. A non-linear hearing aid adjusts its gain based on the input level. In single-channel, non-linear instruments, the loudest input signal controls the overall gain of the hearing aid. For example, in a room where an air conditioner generates loud, low-frequency noise, the single-channel processor will turn down the gain across all frequencies. This, of course, reduces the audibility of all inputs, including high-frequency speech sounds. To allow for independent gain control in separate frequency regions, the hearing aid must have multiple channels. In the case of the air conditioner, a three-channel, non-linear hearing aid would reduce gain in the low-frequency channel without changing the gain in the midand high-frequency channels. This processing may allow more gain for soft inputs such as high-frequency speech sounds (e.g., /s/ and /f/). Another consideration with hearing aids is ensuring the audibility of soft sounds. One approach is to lower the compression threshold of the non-linear hearing aid. When this is done, only the gain for the low-input sounds is increased, while gain for conversational and loud speech remains as prescribed for the hearing loss.1 Frequently, soft speech and soft conversational speech sounds are as low as 20 dB HL.2 To make these sounds audible, maximum gain should be provided for soft sounds so as to produce aided thresholds in the vicinity of 20 dB HL. Therefore, a desirable compression threshold would be close to 20 dB HL. Such a threshold allows high gain for soft sounds in order to ensure the audibility of soft inputs but does not compromise performance at higher input levels. While non-linear hearing aids seem like a logical choice for pediatric patients, their use has not been widespread. A survey published in 2001 reported that more than 50% of hearing aids dispensed to children were linear.3 There are at least two possible explanations for these findings. One is the relative novelty of fitting children with non-linear signal processing compared to the long history of giving them linear hearing aids. The other may be that third-party payers frequently require additional documentation before they will reimburse for non-linear hearing aids.