Is it possible to tell if a directional microphone system on a child's hearing aid is working by doing a listening check in the classroom, when no electroacoustic equipment is available?
In some cases, yes. As you mention, electroacoustic verification of directional microphones is valuable and should always be the preferred method of evaluation. There are, however, a few tips to help you perform listening checks of directionality in a classroom setting. It's best to do an A-B comparison with the omnidirectional and directional modes, but it may be difficult to tell which mode the hearing aid is in during a listening check in the classroom. Therefore, it's helpful to have the hearing aid programming software in the classroom with you, so you can select the microphone modes during your listening check.
When performing the listening check, you should begin by speaking or playing an audio sample toward the "front" of the hearing aid. With the hearing aid in omnidirectional mode, rotate the device such that the "front" of the device is 180° from the sound source. There should be little to no decrease in the level of the signal. Repeat this with the device in directional mode. You should notice a more substantial decrease in the level of the signal. However, if the hearing aid only switches microphone modes in select channels, the decrease in level between omnidirectional and directional modes may be lessened.
It can be difficult to verify the behavior of automatic directional microphone systems. These are directional systems that switch automatically between omnidirectional and directional microphone modes. Knowing the basic logic that the hearing aid uses to trigger the switch to the directional mode will help you to make predictions about the system's behavior in the classroom. Depending on the acoustic characteristics of a given classroom, the hearing aids may or may not switch into directional mode. Understanding the design of the manufacturer's directional system may help you to determine if a hearing aid is designed to switch into the directional mode in a given classroom environment.
I hope you find these tips helpful. For more information on considerations for prescribing directional hearing aids to school-age children and a review of research in this area, please refer to my AudiologyOnline recorded course, Directional Microphones in Classrooms.
Jason Galster, Ph.D., is Director of Audiology Communications with Starkey Laboratories. He is responsible for ensuring that all product claims are accurate and backed by supporting evidence. Dr. Galster has held a clinical position as a pediatric audiologist and worked as a research audiologist on topics that include digital signal processing, physical room acoustics, and amplification in hearing-impaired pediatric populations.