A review of the signal-to-noise ratio and convenience benefits of digital wireless hearing aids, the different types of technologies they employ, and a look at what new benefits these devices might offer in the future.
Not long ago, the author had a conversation with a sales rep from one of the major hearing aid manufacturers. He remarked that it was looking as if frequency-lowering technology (FLT) would become the “next big thing” in hearing aids, and I asked if the sales rep’s company had any plans to look at incorporating FLT in their hearing aids. His response was “Well…you know…the literature says it doesn’t work.” At first glance of the literature, this is not an unreasonable conclusion; however, when examined more closely, his statement becomes questionable.
Charlotte T. Jespersen,MA, is director of audiology development at GN ReSound A/S in Ballerup, Denmark. Correspondence can be addressed to HR or.
In one of our weekly telephone updates, my grandmother was telling me about one of her friends at church who had just purchased new hearing aids. As an audiologist, this topic is naturally of interest to me and I immediately perked up. “They’re called Bluetooth,” she informed me with great authority. Of course, I know there is no hearing aid on the market called this, nor are there currently any hearing aids that even contain Bluetooth technology. What struck me, though, was how impressed she was at the capabilities of these new devices and that hearing aids had come so far. Clearly, the advances in digital wireless technology are benefiting hearing instruments with some image boosting. But how significant are the actual benefits for the hearing aid user?
At present, the benefits of digital wireless technology in hearing instruments fall into two broad categories: improved signal-to-noise ratio (SNR) under certain conditions, and added convenience. There is some overlap in that the means of improving SNR also can be convenient, as when hearing instrument wearers stream sound from their cell phones to their hearing instruments via a wireless accessory. A developing area for the application of wireless technology is for bilaterally worn devices to share information for the purpose of enhancing sound processing algorithms, such as directionality and feedback cancellation.
This article will present an overview of current digital wireless hearing instrument technologies in the context of the advantages they provide for users.
Sensorineural hearing loss is a multifaceted disability affecting not just audibility, but also dynamic range, frequency resolution, and temporal resolution. This is often accompanied by decreased auditory processing, as well as difficulty interfacing with technologies that aid in specific listening environments.1 While hearing aids can significantly increase audibility and address the reduced dynamic range, they cannot alleviate decreased frequency and temporal resolution issues. Because of this, hearing-impaired individuals often have reduced speech intelligibility even after being fit appropriately with hearing aids. What this means is that these individuals need better listening conditions than normal-hearing individuals. Improving the SNR can have a significant positive impact when communicating in more challenging listening situations.
Typically, we think of SNR mainly in terms of whether background noise is present or not, but the distance between a speaker and listener, as well as the reverberation, are also key. SNR diminishes with distance because sound intensity is significantly lessened with increases in distance. It has been demonstrated that listeners should be no more than 1.8 meters away from the signal of interest for optimal speech intelligibility.2 Word recognition scores have been found to decrease systematically as the distance from the signal/speaker to the listener was increased. Crandell and Smaldino3 reported that, while students at a distance of 1.8 meters from the signal of interest/speaker achieved speech recognition scores of 95%, scores dropped to 60% when they were seated 7.3 meters away. These results were obtained in an advantageous acoustical environment.
Figure 1. Illustration of the triangle of problems in terms of speech understanding in background noise.
The intelligibility of a signal of interest is further diminished when environmental noise and reverberation are present3,4 and less optimal listening environments have been proven to result in increasing levels of difficulty, which especially affect hearing-impaired listeners.5 Noise, distance, and reverberation represent a triangle of problems in terms of speech intelligibility (Figure 1).
One proven solution to the noise, distance, and reverberation problem is to augment SNR by means of directionality in the hearing instruments. Directionality improves speech understanding in background noise by taking advantage of spatial separation of the signal of interest and competing noise. It works best in noisy near-field situations where the signal of interest is more powerful and thus less likely to be masked by background noise and reflected sounds that add delayed versions of the original sound to the direct sound.6-9 In other words, directionality is most effective when the signal of interest is near and the environment does not have a lot of reverberation. This is partly why the dramatic benefit demonstrated in laboratory investigations has been less striking in real-life situations.10-12 In addition, the basic premise of directionality is that the signal of interest is facing the hearing instrument user, which is not always the case in the real world.
An additional solution to directionality is to receive the signal of interest from the sound source directly where it is strongest and clearest. Then this strong and clear signal can be transmitted to the hearing instrument user. This dramatically increases the level of the desired sound compared to background noise and reverberant sound, and even overcomes many of the problems of ambient noise, distance from a sound source, and poorer acoustic listening environments that can limit directionality. This transmission can be either via a wire that plugs into the hearing instruments, or wireless, which offers obvious benefits in terms of convenience and discretion.
Analog wireless transmission from a sound source directly to hearing instruments has been used for decades in the form of induction loop systems, FM (frequency modulation) systems, and infrared systems. Digital wireless transmission of acoustic signals is a relatively recent advancement in hearing instruments. It provides more signal processing possibilities than analog wireless, similar to the trend of more signal processing possibilities seen in digital hearing instruments compared to analog. Additionally, this transmission offers overall improved SNR compared to analog processing, and depending on which technology is used, the transmitted signal is less susceptible to electrical interference or issues with encoding for privacy.13
Figure 2. SRT scores in the following test conditions: Adaptive Directionality Mode, ReSound Unite™ Mini Microphone, and ReSound Unite Mini Microphone with hearing instrument microphone active. A high negative SRT indicates that the test participants were able to understand 50% of the speech signal at levels where the speech was considerably lower than the background noise. Note how the benefit of the Unite 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.
To get an impression of the potential SNR boost afforded by digital wireless technology, a companion microphone is a highly illustrative example. Several manufacturers of digital wireless hearing instrument systems offer such a product. One example is the ReSound Unite™ Mini Microphone, which transmits sound and/or external audio sources directly to the hearing instruments of the user. It can be clipped to the clothing of a conversational partner in any situation where a dramatic SNR improvement is desired.
In a laboratory experiment, hearing-impaired participants fit with ReSound Alera hearing instruments completed speech recognition in noise testing under three conditions: hearing instrument directional microphones, ReSound Mini Microphone, and ReSound Mini Microphone with hearing instrument microphone active. The speech material was presented via the HATS (Head and Torso Simulator). For the Mini Microphone conditions, this device was clipped onto the HATS in the same way that a conversational partner to the hearing instrument user is intended to wear it. Test participants faced the HATS, and modulated speech-shaped noise was presented through six loudspeakers surrounding the test participant.
The study results are striking for three reasons:
- Large difference between the directional microphone and Mini Microphone conditions. The SNR at which 50% of the speech material could be accurately repeated was -9 dB with the hearing instrument directional microphone at the 1.5 m distance, while it was -21 dB with the Mini Microphone. In other words, test participants achieved an equivalent performance to directionality with the Mini Microphone at a 12 dB more-adverse SNR.
- Effect of distance on performance. As long as the Mini Microphone is within the transmission range of approximately 7 meters, performance is not degraded. In contrast, the negative effect of distance on performance when listening with the hearing instrument directional microphone is dramatic. In fact, it drops to the point where it may provide no additional benefit compared to an omnidirectional microphone in some situations, such as attending a lecture or worship service.
- Performance of Mini Microphone. Finally, it is interesting to note that activating the hearing instrument microphone during the test had a negligible impact on performance. This means that the hearing instrument wearer could enjoy the improved SNR offered by the Mini Microphone and still be able to maintain general awareness of surrounding sounds via the hearing instrument microphone.
Apart from companion microphones, streaming of sounds from other sources also improves the SNR for users. These sources can be anything from televisions and radios to iPads, music players, and PCs. This is an unequivocal benefit that is easy to demonstrate and explain to potential users, and that becomes obvious to them when they begin using the system.
Wireless hearing instrument features offer convenience to users in multiple ways. One is that they facilitate operation of the hearing instruments, either by synchronizing volume control and program choice settings between bilaterally worn devices, or by means of a remote control. As mentioned previously, there is also a convenience factor in being able to interface with other electronic devices and to receive the sound via one’s hearing instruments. This could include cell phones, music players, PCs, televisions and radios, or any audio source. Finally, wireless exchange of information between bilaterally worn devices can be used to automatically select and align the microphone mode and noise reduction settings most likely to be beneficial in the given listening environment. This reduces the need for users to recognize and manually select hearing instrument programs or features intended for particular listening situations.
Digital Wireless Hearing Aid Technologies
Today’s digital wireless hearing aid systems use one of two technology solutions: 1) Near-field magnetic induction (NFMI) combined with Bluetooth radio frequency (RF) transmission or proprietary RF transmission, or 2) Proprietary RF transmission alone.
NFMI combined with radio frequencies. Most digital wireless hearing instrument systems use near-field magnetic induction combined with Bluetooth. The NFMI transmission serves for communication between a gateway device and the hearing instruments, while Bluetooth RF transmission is used for communication between a streamer device and the gateway device.
Figure 3 illustrates how such a system works. A sound source, such as a television, is connected to an adaptor device. The adaptor device digitizes the signal and codes it into a protocol or language that can be understood by a radio. This protocol or language can be for Bluetooth communication, or it can be a proprietary protocol/language. The radio then transmits the signal via a 2.4 GHz antenna to a radio in a gateway device. In the gateway device, the protocol or language is decoded and sent into the NFMI system. The NFMI system has an induction transmitter coil in the gateway device. This transmitter coil produces a magnetic field, which is picked up by an induction receiver coil in the hearing instruments. The NFMI contains the transmission energy within a localized magnetic field, which is shaped almost like a magnetic bubble, and which doesn’t radiate into free space—it is called “near-field” and has a range of up to 1 meter. After being transmitted via the NFMI field, the signal is then treated by the hearing instrument itself and presented as an audio signal to the hearing instrument user.
Figure 3. Illustration of a digital wireless hearing aid system based on NFMI combined with radio frequencies.
The gateway device is typically worn around the neck or used as a remote control, as the hearing instruments need to be within the 1-meter NFMI field in order to pick up the signal.
NFMI is relatively easy to implement in hearing instruments due to existing radio frequency chips and the fact that the technology used is similar to the well-established telecoil technology. The current drain is low with NFMI systems, as it works on a lower frequency—generally 10 to 14 MHz. This provides the end user with longer battery life when using wireless features. Also, the NFMI will transmit through almost any physical object, so line-of-sight problems are minimized.
The NFMI wireless hearing instrument systems have the disadvantage of a short transmission distance (1 meter), which means that a gateway device needs to be close by or worn on the body; this might be perceived as stigmatizing by some end users. Sound quality can be affected by the orientation of the gateway device and hearing instrument radio receiver coil and any relay between components. Interference with magnetic sources (cochlear implants or induction stoves) also may be encountered, which can affect sound quality. Finally, for the NFMI systems that combine with Bluetooth for audio streaming, the delay associated with Bluetooth audio transmission can degrade sound quality when transmitted and presented with the direct sound mix in the user’s ear canal.
Proprietary radio frequency systems. More recently, digital wireless systems have been introduced that are based on proprietary RF transmission. These wireless hearing instrument systems use RF for direct communication between the streamer device and the hearing instruments.
Figure 4 shows how a wireless hearing instrument system based on proprietary RF works. A sound source, such as a television, is connected to an adaptor device similar to the NFMI-based system. The streamer digitizes the signal and codes it into a proprietary protocol/language. By using a proprietary protocol, rather than the power-hungry Bluetooth, power consumption is reduced to the point where it is possible to implement the receiving radio directly in the hearing instruments, thereby eliminating the need for a gateway device. In current systems, the RF transmission is either via the 2.4 GHz license-free ISM (Industry Science Medical) band, 900 MHz band, or 868 MHz band transmitted directly to an antenna in the hearing instruments where the protocol/language is decoded back into a digitalized signal, which is treated by the hearing instruments and presented as sound to the ears of the wearer.
Figure 4. Illustration of a digital wireless hearing aid system based on a proprietary radio frequency system.
The most immediate advantage of proprietary RF systems is that they can transmit over long ranges. This gives a benefit of convenience and discretion to the user in that no body-worn “gateway” device for media connectivity is required. In addition, these systems avoid the latency in transmission of audio that is associated with Bluetooth. This can markedly improve sound quality, as a delay between the streamed and direct sound in open fittings is avoided.
Proprietary RF systems require a specially designed antenna, which is a challenge for manufacturers to pack into small hearing instruments with the desired antenna efficiency. Assuming that this is accomplished, however, it is only a disadvantage for the manufacturer, as it requires more development effort. For users, the higher power consumption relative to NFMI systems can be viewed as a disadvantage.
The 2.4 GHz system is very robust and durable, avoiding interference in several ways. ISM bands tend to be congested. To ensure signal stability, RF systems avoid interference with other wireless products, such as a wireless mouse or a wireless network, by transmitting data in exceedingly small amounts, or “packets,” at a time. Another method of avoiding interference used with RF systems is transmission of data in very short time intervals across different channels.
This technique is called frequency hopping. Different channels can be used to avoid disturbance from other wireless devices. The ReSound technology makes use of 35 different channels to avoid disturbance from other wireless devices. Essentially, this means that each time a new piece of data is to be sent, a new channel out of the 35 possible channels is picked for the transmission. The hearing instrument mutually agrees with the wireless accessory on which channel to send the next data packet. As all the different devices in the band use a different selection strategy, they virtually always steer clear of each other.
A final way of avoiding interference is by sending every data packet with an “address.” This means that hearing instruments and the devices they communicate with must be paired. During pairing, addresses of the devices are exchanged. This solution allows coexistence of wireless devices in physical proximity, and prohibits “eavesdropping” by any other wireless devices.
Wireless in the Future
One avenue of development that manufacturers of wireless hearing instruments are pursuing is using the hearing instruments on both ears as a system rather than having them work independently of each other. Some advances in this area have already begun to appear, such as aligning microphone and noise reduction settings to suit the acoustic environment.
Other functionality that has emerged is to use inputs from both hearing instruments to further enhance directional microphone performance, or to improve identification of feedback oscillation. Loftier goals in this area are related to functionality that would emulate binaural auditory processing, with improvements in noise reduction and spatial hearing being obvious targets.
The lowest-hanging fruits and the ones likely to make the largest immediate impact for users, however, are related to connectivity. A major hurdle for manufacturers was to implement RF transmission direct to hearing instruments. Now that this has been accomplished, we are likely to see hearing instruments in the not-too-distant future connecting directly to compatible electronic devices without the need for a hearing instrument accessory. This would make possible direct communication from the hearing instrument to personal computers, phones, Wi-Fi, alert systems, and assistive listening devices. Connectivity directly to the Internet could give the user possibilities like upgrading the signal processing in their hearing instruments, or logging onto sound systems in theaters and in schools.
Who knows…perhaps the next pair of hearing instruments that my grandmother’s friend is fit with will allow her to take phone calls through her hearing instruments, Skype with her grandchildren on her tablet using her hearing instruments as the headset, and still let her listen to worship services in her looped church via the built-in induction coil.
Correspondence can be addressed to HR or Charlotte Jespersen, MA, at.
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Citation for this article:
Jespersen CT. A Review of Wireless Hearing Aid Advantages Hearing Review. 2012;19(02):48-55.