Audiologist Find The Right Mix
September 30, 2008--To help children at Tucker-Maxon and ultimately throughout the nation, on-site audiologist Kimberli Davenport recently completed a small study aimed at finding the best way to set the accessory mixing ratios on students’ cochlear implant systems. She recently presented her findings at the 10th International Conference on Cochlear Implants and Other Implantable Audiology Technologies in San Diego.
Technology has shaped opportunities for children who are deaf in profound ways. Students at Tucker-Maxon benefit from digital hearing aids, cochlear implants, and personal FM systems, so that they can hear as much as possible inside and outside of school.
However, determining the best way to combine assistive listening technologies to provide maximum benefit can be a real challenge—- especially when no formal recommendations exist for new products.
Her concern was that the Phonak Nucleus Freedom implant system’s default mixing ratio for the Microlink FM receiver would reduce the volume of classroom noise too much, hindering communication between children who are deaf and children with typical hearing.
FM systems function as miniature, personal radio stations that amplify the teacher’s voice and transmit it to a student’s cochlear implant or digital hearing aid at a constant volume. No matter where the teacher is in the room, the student hears her voice as if she were six inches away. “Cochlear implant processors and hearing aids don’t amplify speech sounds well enough for children to hear them at all times, especially in situations where there’s a lot of noise or distance from the speaker,” explains Davenport. “FM systems help overcome the effects of noise and distance by improving the signal-to-noise ratio.” The systems operate on special frequencies assigned by the Federal Communications Commission.A child receives the FM signal through a wireless receiver coupled to his or her sound processor. The processor can attenuateor “turn down”—environmental sounds coming from the classroom, allowing the FM signal from the teacher’s microphone to become more prominent. The term “accessory mixing ratio” refers to this decrease in environmental sound relative to the FM signal.
When the Phonak Nucleus Freedom processor and Microlink FM receiver came out, there was no formal recommendation on how the accessory mixing ratio should be set. The processor’s default mixing ratio was 3:1, effectively turning down the volume of classroom noises by 9 decibels (dB). Since Tucker-Maxon emphasizes everyday classroom interaction between children with hearing loss and children with typical hearing, Davenport was immediately reluctant to go with the default ratio. “Our classrooms are such language-rich, communicative environments, I didn’t want [students] to be missing out on other students.”
Davenport wondered whether students with hearing loss would still be able to understand the teacher at mixing ratios that allowed them to hear their classmates more easily. To find out, Davenport, who is working on a doctorate in audiology (AuD) at Pennsylvania College of Optometry, did a small study using Tucker-Maxon students and alumni who use the processor as research subjects.
To determine the best mixing ratio, Davenport simulated a noisy classroom and set up a loudspeaker to function as a pseudo-teacher. She then measured students’ ability to distinguish sentences spoken via the loudspeaker, using the Hearing in Noise Test for Children (HINT-C), at mixing ratios of 3:1, 2:1, and 1:1, which attenuate the environmental microphone by 9 dB, 6 dB, and 3 dB, respectively. (At the 1:1 ratio, sensitivity was also reduced from 12 to 10). She found that, while students could repeat the highest percentage of words with a mixing ratio of 3:1, they performed similarly with ratios of 2:1 and 1:1.
Davenport’s results also highlight an advantage of Tucker-Maxon’s highly personalized audiology services. She found significant differences from one child to another, with not all students performing best at the 3:1 ratio. To Davenport, these findings indicate a need to “test kids in different situations and adjust for each child. Tucker-Maxon is unique in that I’m on-site, and I can have those daily conversations with students and teachers about how things are working and make adjustments accordingly.”
RESULTS
• The figure below shows mean scores for the subjects in each listening condition. Not surprisingly, children performed best in the quiet condition with a mean score of 97.5%. Scores for the 3:1, 2:1 and 1:1 accessory mixing ratios were 73.8, 77.6 and 84.3%, respectively.
• Scores for Noise 3:1 were significantly better than for Noise 2:1 and Noise 1:1 (p<.05). However, HINT-C scores for the 1:1 and 2:1 conditions were not significantly different.
• Individual data showed great variation between subjects. Although all
children performed best in quiet, the following was noted:
• Some children showed little difference among listening conditions.
• Not all children performed best in the 3:1 condition.
• When children were asked which condition they preferred, all children
responded that the three conditions sounded very similar.
• When children were asked whether they preferred the CI microphone to be
attenuated in the classroom, 10/11 children reported that they wanted to be
able to hear their classmates as well as their teachers.—Jessica Johnson
This article is from the Fall 2008 issue of Now You're Talking.
