This post is a Reader’s Choice selection for 2013.
We all know hearing aids make things louder. But does it matter exactly how much louder and whether it makes it louder throughout the frequency spectrum? Everyone will say yes, it does. But how often do we check that hearing aids (or cochlear implants) are doing what we hope they are doing? Is it okay to hear well through 2000 Hz and not hear high frequencies at the level of speech? How about hearing to 1000 Hz and not well above 1000 Hz? How about hearing well only through 500 Hz? If we set hearing aids using real ear technology, do we know if the child is hearing at every frequency?
What speech information is available where?
Different speech information is available at different frequencies. When I was a relatively young audiologist, I worked at the New York League for the Hard of Hearing. Dorothy Noto Lewis was the director. She taught us that we needed to be able to predict a child’s hearing levels by listening to the child speak. We tried to anticipate the degree of hearing loss before testing and draw the audiogram based on talking to the child and observing response to sound. It was terrifying when I started doing this but it was a WONDERFUL learning experience. It made it very clear to me that I needed to know what a child heard and that I needed to fix what they did not hear.
Low frequencies
250 Hz provides voicing cues and the first formant of /n/,/m/ and /ng?. So if a child is having problems getting voicing cues we should check if they have enough information at 250 Hz. 500 Hz provides information for manner of production, first formant for most vowels, noise bursts for plosives, and information for semi-vowels and laterals /l/ and /r/.
If a child is having problems confusing manner of consonants, we should look to where the child is hearing at 500 Hz.
Mid Frequencies
1000 Hz provides additional cues of manner, nasal consonants, back and central vowels, noise bursts of most plosives and semi-vowels. 2000 Hz provides cues for place of consonant and additional information about manner, front vowels, noise bursts of most plosives and affricates and turbulent noises of fricatives /sh/, /f/, and /th/. So if a child is having problems hearing fricatives, we need to check how they hear at 2000 Hz.
High frequencies
4000 Hz provides more information on consonant production, third formant for vowels, noise bursts for plosives and affricates, turbulent noise of voiced and unvoiced fricatives. 6000 Hz provides information required for perception of /s/, and 8000 Hz provides information on turbulent noise of all fricatives and affricates. Let’s remember that you need /s/ to learn prepositions, possessives, etc. It is a critical frequency to hear.
Using this information
For the most part, the speech acoustics courses we took were viewed as theory. Unless you were involved in research, most audiologists do not think of this information as clinically useful. In fact, it is so very very critical. Dan Ling used to say “What they hear is what they say.” Dorothy Noto Lewis would definitely have agreed with that. We can check that a child is hearing throughout the frequency range by getting aided thresholds and we can also listen to what the child says. It is not unusual to have audiologists and teachers, etc., say that the child is not saying /s/ or some other phoneme because he has a hearing loss or an articulation problem. But maybe he is not saying it because he cannot hear it!!
Listen to the kids
If a therapist says that a child cannot hear /s/ don’t wait for the child to outgrow it. Check that the child is hearing high frequencies. If not, fix it. If it turns out he does have thresholds in the high frequencies, then we know something else is causing this, and the therapists know they need to fix it. DO NOT ASSUME. Test and find out what is real. I can tell you one thing for certain: if a child does not hear high frequencies, he will not be able to use that information for language learning.
