Effects of a directional mic on children's word recognition and novel-word learning

Abstract

Many hearing aid manufacturers have incorporated directional-microphone technology into their products to improve the signal-to-noise ratio (SNR) for the wearer. Although this technology has been shown to improve speech recognition in noise and user satisfaction for adults, there has been little research on its appropriateness in the pediatric population. Since children require greater audibility than adults to achieve equivalent performance on sentence recognition 1 and a greater SNR to recognize speech in multi-talker babble at a performance level equivalent to adults, 2,3 it seems logical that the technology would be useful for this population.Current data suggest that approximately 20% of audiologists typically fit directional hearing aids on children 0-6 months of age, 4 and 35% fit directional hearing aids to at least half of their pediatric clientele. 5 When speech is presented from the front and noise from the back, children have a 4.7-to 8-dB directional advantage in laboratory settings 6,7 and a 3-to 4-dB advantage in classroom environments 8 for microphone schemes providing the primary null in the back.Some have argued that directional microphones should be fitted with caution because, for safety reasons, children often need to be aware of sounds coming from all directions. 9 In addition, directional hearing aids may be disadvantageous to children as they may reduce the acoustic input necessary for learning localization and selective listening skills. This may ultimately reduce the number of incidental learning opportunities available to these children.The purpose of this study was to determine if off-axisplaced speech is negatively affected by fixed directionalmicrophone hearing aids in two situations that children face daily-recognizing old words and learning new words. We hypothesized that both recognizing familiar words and learning new words would be poorer in the directional condition (cardioid pattern) than in the omnidirectional condition for stimuli arriving from the rear sound field because sensitivity of the microphone may be reduced in the directional condition for signals arriving from the rear. STUDY OF WORD RECOGNITIONTwenty-four children (7 females; 17 males) between the ages of 6 and 10 years were enrolled in this component of the study, with the consent of their parents/guardians. They were screened for normal hearing and normal vision. MethodologyA GN Resound Metrix™ behind-the-ear hearing aid was programmed for a moderate, flat sensorineural hearing loss. Noise-reduction and feedback-management algorithms were disengaged. Using a custom-built standard earmold, the hearing aid was fitted to the right ear of a pediatric acoustic research manikin placed in the center of an anechoic space. A speaker was placed 1 meter from the manikin at the rear azimuth (180˚) for stimulus presentation. The output from the hearing aid was recorded to both left and right tracks using Adobe Audition 1.0 software, once with the hearing aid in omnidirectional mode and once with it in a fixed cardioid direc...