By Jennifer Groth
ReSound Global Audiology
My teenage daughter and I were visiting my father, who lives in rural Iowa. We decided to check out a new restaurant for dinner, and Dad drove, with me in the passenger seat and my daughter in the back. There are four miles of gravel road to get to the highway from his house, and, apart from kicking up a lot of dust, it’s pretty noisy. And you’ll usually meet a tractor or two on the road, not to mention cows and deer. So it’s pretty important to keep your eyes on the road if you want to stay out of the ditch.
“What have you been doing this summer?” Dad yells over his shoulder to my daughter.
“Hanging,” is her eloquent reply.
“What?” he says, straining to see her in the rearview mirror.
“She’s been hanging out with her friends,” I shout at the same time she repeats her answer.
“What?” he yells back at my daughter.
“Oh yeah, and Shannon and I went to see Oklahoma!” she says.
“Who has glaucoma?” he says, and cranes his neck around, swerving a little.
And so it goes the rest of the trip, and we manage to arrive in one piece. However, the challenge continues during dinner, as the restaurant turns out to be a huge, reverberant space. Even though there aren’t that many other guests, carrying on a conversation with Dad is tough. He wears hearing instruments with the latest technology, including directionality, and is a fervent advocate for the benefits of amplification. Even so, there are some situations that continue to be a struggle. Either the speech-to-noise ratio is so poor that directional hearing instrument processing cannot improve matters sufficiently, or it is not possible to look at the speaker to take advantage of visual and contextual cues, or some combination of these factors.
Hearing instrument fitters have long had to counsel their patients to manage expectations to the many and commonly encountered situations where hearing instruments don’t provide sufficient benefit. Happily, new technologies based on digital wireless transmission are changing that. As the name indicates, digital wireless is distinguished from analog wireless transmission by the digital encoding and decoding of the transmitted signal. The modes of transmission are similar to those of analog wireless in that current digital wireless hearing aids use either magnetic induction or radio frequencies to send and receive the signals. However, the fact that the transmitted information is in a digital format is attractive in that it opens up new possibilities for reducing interference, ensuring privacy of transmission, and enhancing audio quality, such as with stereo transmission.
One way digital wireless transmission is enhancing the usefulness of hearing aids in challenging listening environments is by means of a small companion microphone that can be placed near the sound source of interest. The microphone picks up the desired sound and streams it directly to the wearers’ hearing instruments, vastly improving the signal-to-noise ratio. The ReSound Unite Mini Microphone is an example of such as device. The ReSound digital wireless hearing instrument system is based on a robust and secure proprietary transmission protocol at 2.4 GHz, the same frequency band as the ubiquitous Bluetooth wireless technology.
The ReSound Unite Mini Microphone picks up sound at the source and transmits directly to the hearing instruments in the 2.4-GHz frequency band.
How much of a difference can a remotely placed microphone really make? It is well-established that dual microphone hearing instrument directionality can boost the signal-to-noise ratio compared with omnidirectionality by approximately 4 dB. It is also well-established that this benefit decreases with increasing distance and that it is based on spatial separation of the desired and competing sounds.
To illustrate the potential benefit of the Unite Mini Microphone in noisy situations where distance to the desired sound may not be ideal, the microphone was evaluated against adaptive directionality in a laboratory investigation with twenty hearing-impaired participants. Testing was performed in a setup with noise surrounding the listener, not just from the rear plane. Speech materials were presented via the artificial mouth of a head-and-torso simulator to which the Unite Mini Microphone was attached as a conversational partner to the hearing instrument user. Testing was carried out at distances of 1.5 meters, 3 meters, and 6 meters between the head-and-torso simulator and the listener.
The graph below shows the average signal-to-noise ratios at which 50 percent of the speech material could be correctly identified by listeners. Three findings were striking:
1. Streaming the speech via the Unite Mini Microphone resulted in a signal-to-noise ratio boost of more than 10 dB at the shortest distance compared with adaptive directionality.
2. The signal-to-noise ratio benefit was maintained with distance from the Mini Microphone, whereas benefit with adaptive directionality decreased as distance from the talker increased.
3. The signal-to-noise ratio benefit attained when the hearing instrument microphones were active during streaming from the Mini Microphone was equivalent to the benefit attained from the Mini Microphone alone.
Signal-to-noise ratio for 50-percent speech recognition with adaptive directionality versus the ReSound Unite Mini Microphone. Note how the benefit of the Mini Microphone remains constant with distance, while the benefit of directionality decreases markedly as the distance between the listener and the signal of interest is increased.
While directional microphone systems in hearing instruments are inarguably beneficial for users, the comparative benefit of a companion microphone that streams directly from the source of the desired sound to the hearing instruments is staggering. Furthermore, this benefit is achieved even if the hearing instrument microphones are on, meaning that the user can still hear other sounds in the environment while using the Mini Microphone. These results indicate how the ReSound Unite Mini Microphone can supplement conventional amplification and directional processing to overcome their limitations in particularly challenging situations. As illustrated at the beginning of this article, cars and noisy restaurants are examples of common and notoriously difficult listening situations. The Mini Microphone provides an easy and convenient solution to increase the number of environments in which hearing instruments are useful for wearers, which is a proven correlate of consumer satisfaction.