Written by Pam Millett, PhD
It is a stressful journey through unfamiliar territory, from a child’s first hearing screening, through hearing tests, specialist appointments, and talks about amplification, to earmold impressions and anticipating getting those first hearing aids. Hearing aid fitting day is a day everyone has waited for, usually with a mixture of nervousness, relief and sadness. No matter the child’s age or hearing loss, all families have the same questions: “So how well can she hear now?” How does the audiologist know that these hearing aids are the right ones?” “How do we know that we have done the best we can?” This article describes the three steps involved in finding the answers to these questions.
Step 1: Selecting the right hearing aid
A recent Google search for “hearing aid manufacturers” yields over 3 million results, so how do audiologists even start to choose the best hearing aids? Audiologists start by gathering information from three sources:
i) First, audiologists need to gather as much information as possible about the child’s hearing and about the physical characteristics of the child’s ear (size, shape, etc.). This is done by using hearing test results and results from a test known as real ear measurement (sometimes also called probe tube or probe mic measurement), which measures how each individual ear shapes the sound coming into it.
ii) Second, they need to consider the circuits, features and accessories we want (What kind of microphone? Should there be a volume control? What about those noise reduction circuits? Do we need a telephone program?, etc.). Cost, is an important consideration, but more “bells and whistles” do not necessarily mean better hearing aids.
iii) Third, audiologists need to calculate targets for the hearing aids for how much volume needed at each frequency (pitch) and what the maximum allowable sound levels should be so the hearing aid doesn’t make sound too loud and damage the child’s remaining hearing. These targets are generated by a software program known as the Desired Sensation Level Approach (DSL), which is the most extensively researched program for children’s hearing aids available today.
The DSL program needs two pieces of information to generate hearing aid targets. The first piece of information is hearing thresholds from the hearing test. DSL does not require a full audiogram, so audiologists can start with partial information and add more information as they get it (for example, partial information from the first hearing tests of babies). The second piece of information is a real ear (or probe tube/mic) measurement of the child’s ear canal. Our ear canals actually shape sound as it passes through them to the eardrum, giving a little boost in volume at certain frequencies. The amount of this boost changes as children grow to adulthood. Knowing all about how each individual’s ear shapes sound provides a better hearing aid fitting because sound levels from the hearing aid will differ for different sized ears. Think of how loud a balloon sounds when it is popped in a small room versus outside – it sounds much louder in a smaller space. In this same way, a particular hearing aid on an adult can have a maximum sound output of 120 dB. When the same hearing aid at the same settings is placed on a child’s ear, the maximum output can be closer to 130 dB because the child’s ear canal is much smaller than the adult and could possibly damage hearing.
The DSL program then allows the audiologist to evaluate many different hearing aids and hearing aid settings using the measurement of how the child’s ear size and shape boosts sound that is stored in the computer. The audiologist can then choose what he/she thinks will be the best possible hearing aid for that child. There is no one perfect hearing aid for each child; the most important thing is to have carefully fit hearing aids that are worn all of the child’s waking hours, each and every day. Expensive hearing aids, even when well fit to the child’s hearing loss, will provide little benefit to learning if they sit unworn in a box most of the day.
Step 2: Proving that the hearing aids are providing the right amount of amplification
The day you pick up your child’s new hearing aids, your audiologist will need to verify the aids’ performance, making sure that the hearing aids are set properly and working the way they should. Years ago, audiologists did this by doing a hearing test with the child wearing his/her hearing aids in the sound room. While it seems to make sense to do this, and to compare a child’s unaided and aided thresholds on the same audiogram, these “aided audiograms” are not accurate enough for today’s sophisticated hearing aids. Instead, verification using the DSL program provides results in an “SPLogram”.
Let’s look at an example of an SPLogram. All hearing aid data is measured in a different kind of decibel, dB sound pressure level (or SPL) so the hearing test information and hearing aid information can be compared directly. The DSL program first converts the child’s hearing threshold information into dB sound pressure level (SPL). You will see that the SPLogram looks like an upside down audiogram. Frequency (pitch) is still read from left to right along the bottom. Loudness values are now reversed – very soft sounds are at the bottom of the graph and loud sounds are at the top. This picture shows the SPLogram for an audiogram with a 50 dB hearing loss at all frequencies.
The DSL program then calculates targets for different sounds, or how much we want the hearing aid to improve the child’s ability to detect sound at each of the frequencies. Our SPLogram shows a child’s hearing thresholds (as circles), targets for conversational speech (as +’s) and targets for the maximum output, or the ceiling loudness, of the hearing aid (as *’s). The key to understanding the DSL printouts is to realize that the hearing aid targets are meant to ensure that all speech sounds at that frequency can be heard comfortably in quiet. The levels of sound produced by the hearing aid can then be measured and the audiologist can see if the hearing aid meets all of the DSL targets. If so, this implies that the child should be able to hear all speech sounds in quiet. We always have to keep in mind that being able to hear a sound doesn’t necessarily mean being able to understand and make sense of it, but hearing is the first step.
The SPLogram shows what actually happens – that the hearing aid makes speech and other sounds loud enough to be heard. We are not improving the child’s hearing levels. A traditional aided audiogram, which shows aided hearing levels as “A’s” on a standard audiogram format, often gave the impression that a child’s aided hearing was normal, or close to normal. However, we cannot change a child’s hearing ability, nor can we restore normal hearing – we can only change how loud the hearing aid makes sounds. The sounds of speech typically are at a loudness of 50-60 dB SPL. If this is too soft for the child to hear clearly, we must make the speech sounds loud enough to fall within the child’s range of hearing. The SPLogram shows how well the hearing aid is doing this. It also shows the child’s “range of hearing” (the difference between the softest sound one can hear and the loudest sound one can tolerate) can be very small. Some people with sensorineural hearing loss can actually be more sensitive to loud sounds than someone with normal hearing. The SPLogram shows the maximum loudness that a hearing aid should produce, so that the hearing aid is comfortable and does not risk damaging the child’s hearing. When we verify a hearing aid using DSL, the graph shows how the hearing aid performs for soft sounds, sounds at the volume of a conversation, loud sounds and very loud sounds.
Step 3: Confirming that the hearing aids are really helping the child as expected
Validating a hearing aid fitting means making sure the child is hearing and learning language with the hearing aids as expected, based on how well she should be able to hear speech with the hearing aids. Validation relies entirely on the eyes and ears of the people in the child’s environment. Before leaving the clinic, parents and families should ask the audiologist to explain the testing in terms of how well the hearing aids are able to make speech audible. With more severe hearing losses, sometimes it is impossible for any hearing aid to meet DSL targets, and this is important information to know too so that expectations are realistic. At the end of the appointment, the audiologist can easily demonstrate a quick Ling 6 Sound test to test this (oo, ah, ee, sh, s, m), either in the sound room using the audiometer, or informally in the testing room. This is especially important because some hearing aids (e.g. those with frequency transposing circuits) will give misleading results on real ear measurement, but will prove that they are doing their job with a Ling 6 sound test.
There are many ways that parents, teachers and others can find out how well a child responds to sound in different situations. The Ling 6 sound test can be done in a variety of different ways, in different environments, with different noise levels, with different speakers, at different distances, to get a sense of how well the child is able to detect speech sounds. The Early Listening Function (ELF) from this website is an excellent way to explore a child’s ability to hear in different situations. Assessments such as the Functional Listening Evaluation or the Functional Auditory Performance Indicators might be done by your child’s teacher of the deaf and hard of hearing, speech-language pathologist, auditory verbal therapist or early interventionist to gather more information about how the child is able to use auditory information. Many more examples of assessment tools can be found in the Tests section of this website. It is important for parents and teachers to have a good sense of how a child hears under different circumstances to set goals and expectations, and to know when a problem (like a malfunctioning hearing aid) arises.
For more detailed information on using DSL to understand how a child hears with hearing aids, go to Understanding Your Student’s Aided Hearing using the Desired Sensation Level (DSL) Approach.
Pam Millett, Ph.D. is an educational audiologist and associate professor in the Teacher of the Deaf and Hard of Hearing Education Program at York University in Toronto, Canada. She has over 25 years of experience working with students, teachers, parents, professionals and supportive personnel in schools. She teaches educational audiology, and language and literacy development at York University, and is currently conducting research projects in literacy outcomes for students with cochlear implants and the impact of sound field systems on development of phonological awareness skills for young children.
I read some information that Pam had written on this topic and am happy that she agreed to provide this important information to Supporting Success! Karen L. Anderson, PhD, Director
Published August, 2012