Your child's audiologist will carry out various measures to determine the level of sound the ear is receiving and what your child is hearing with their hearing aid. These tests are carried out to ensure your child's hearing aids are set for optimal speech intelligibility and comfort according to the NAL hearing aid selection procedure.
In addition to these measure, feedback from you, your child and your child's teacher is essential in determining that the hearing aids have been fitted appropriately.
The main measures audiologists can use to assess aided performance are:
For older children and adults, we measure the level of a reference sound in the ear canal without the hearing aid, and then repeat the measurement with the hearing aid in the ear and switched on. The difference between these two measurements is the insertion gain of the hearing aid.
This measurement gives an accurate indication of whether the prescribed insertion gain (targets) is being achieved in the real ear. If not, the audiologist can further adjust the hearing aid.
|Example of insertion gain measurement results
Left is an example of an insertion gain measurement. By comparing the hearing aid response with the NAL-NL1 Target (solid black line), the audiologist can see that additional amplification is required in the high frequencies to optimise intelligibility and comfort.
Accurate measurements can be tricky to achieve with young children and the two measurements required for insertion gain often cannot be obtained. Real-Ear-to-Coupler Difference (RECD) measurements are preferable for younger children and infants because they only require one real ear measurement.
In this procedure, the level of a reference sound is measured in the child's ear with the aid connected to a standard coupler (connector) in a hearing aid test box. The difference between these two measurements (real ear and coupler) is the Real-Ear-to-Coupler Difference.
Using this measure, the audiologist can accurately predict the level of sound in the child's ear when the aid is worn. Therefore, the hearing aids can be adjusted in the test box to achieve the best possible match to targets.
Sound field evaluation involves obtaining the hearing thresholds for your child's aided hearing. The sounds are presented through a speaker instead of headphones and the softest sounds your child can hear with their hearing aid are recorded. Responses are then compared to target aided thresholds using the NAL-NL1 procedure.
With the use of digital hearing, which uses compression circuits (see hearing aid section), this type of evaluation is used less than it used to be to determine the way a child is able to use aided hearing. As the procedure relies on detecting the softest sound or threshold, the hearing aid is most likely to provide more gain at this level than it may for normal speech inputs.
More commonly, a speech-o-gram will be predicted from the NAL-NL1 prescription algorithms. See below for an example of a speech-o-gram.
Speech Perception Testing tells us how clearly a child can hear speech, and can be used to develop a more comprehensive picture of the child's listening ability with the hearing aids. Australian Hearing audiologists have a range of formal speech materials they can use depending on the child's age and linguistic skills.
The Aided Cortical Testing means of evaluation is still very new and highly specialised. It is just being developed as a clinical test and equipment is not available in most hearing centres. It can be used where children are not able to have reliable evaluation through other means. It involves attaching electrodes (small metal disks) to the child's head and presenting some speech sounds through a speaker at known levels. The electrodes are trying to pick up the electrical activity of the brain that is created by these sounds. (This is a type of EEG). This helps us to determine whether the sound is being detected and may tell us how to adjust a hearing aid even for a very young baby.
The speech-o-gram is a way of displaying information about your child's hearing aid fitting. It gives a picture (for each ear individually) of what sounds the child should detect in speech when using their hearing aids, including information about how much of average speech is audible and the effects of changes in the hearing aid fitting.
The part of the spectrum that is above threshold is indicated with darker shading than the part that is inaudible. For people with mild and moderate hearing loss at all frequencies, more audibility generally means more intelligibility, but this is not necessarily the case at any frequency where the loss exceeds about 60dB HL Also more audibility always means more loudness, and this may not always be desirable.
Remember that the speech signal going through a hearing aid is constantly changing as the speaker talks. Also, people may speak with soft or loud voices, and may be up close or far away. So in real life the speech signal going into the hearing aids will often be different from what is shown on the speech-o-gram - it is only a guide.
After adjusting the hearing aids to get the best possible match to the amplification targets, the audiologist continues to monitor and evaluate the fitting to see if any fine-tuning needs to be done to account for individual factors. This can be done in a number of ways:
Asking the child's opinion
This has obvious limitations with babies and children who have limited speech and language abilities. Even if a child is able to tell the audiologist what they think about the sound of their hearing aids, it can still be very difficult to interpret the comments to determine what adjustments should be made. However, a child's feedback about the sound of their hearing aid is always taken seriously.
This procedure is most appropriate for children over six years old with severe and profound losses, and is available at some Australian Hearing centres. The child is required to listen to a simple story that is recorded on video. They are asked to compare speech amplified in different ways, and to tell us which one sounds best. If the child prefers an alternative setting to what is prescribed, the hearing aids can be adjusted accordingly.
Asking the parent's and teacher's opinion
Gathering information from parents and teachers is vital to understanding how the child is hearing and functioning in the real world. This feedback is particularly valuable in evaluating if the hearing aid is set to a comfortable loudness, and how well the child is accepting and using the aids. The information may be gathered using formal questionnaires or by discussion with the parent or teacher. If behavioural test information is limited, observations from parents and teachers help to build up a more complete picture of the child's hearing abilities.
14-Nov-2015 7:51 PM (AEST)