Over the last 10 years, wireless technology has been a driver of innovation in hearing aids and cochlear implants. Today, our counseling narratives almost universally include discussion of the features and benefits that are enabled by wireless connectivity—it’s been a short trip from novelty to normal. This article reviews the evolution of ear-level wireless technologies and summarizes the landscape across hearing aids and cochlear implants.
On-ear wireless hearing devices (hearing aids and cochlear implant sound processors) can be first classified as having two basic modes of wireless communication: Magnetic induction and radio frequency (RF).
Magnetic induction systems transmit and receive signals at lower wireless frequencies (e.g., 10 MHz) with antennas that consist of a small magnetic core wrapped in a copper coil. In contrast, RF systems transmit and receive higher wireless frequency signals (e.g., 2.4 GHz) with antennas that are formed as a loop or strand of copper.
In the case of cochlear implant systems, the sound-processor headpiece uses a wireless inductive link to transmit power and data through tissue to the implant. For this article, inductive power and data transfer will be considered a separate wireless application and not a focus.
Among audiologists, the concept of wireless communication through magnetic induction is a familiar one. Hearing aids and cochlear implants have offered telecoils that receive audio from compatible induction fields for decades, the benefits of which have been thoroughly documented (Atcherson, 2019).
The early use of these magnetic induction wireless systems was motivated by a combination of size, low-power demands, and available technology. Still, today, the telecoil represents the most universally accessible method for providing directly streamed audio to a hearing aid or cochlear implant. However, the telecoil is limited by the need for proper alignment between the telecoil and the inductive field, as well as a very limited capacity to transmit data for signal processing.
Introduction of NFMI
In the years between 2005 and 2010, hearing aid developers introduced near-field magnetic induction (NFMI) wireless systems designed specifically for the transmission of audio and data. The key benefits of NFMI were related to the magnetic principles of the wireless system. Firstly, these were low-power and low-frequency, which meant that data and audio could be transmitted for an acceptable period of time with a zinc-air hearing aid battery, and the low-frequency signal made transmission between ears (in a bilateral device pair) a possibility.
This contrasted with Bluetooth, which during these years was demanding of power, and the technical systems were sensitive to placement and orientation on the head. For this reason, hearing aids featuring NFMI as the primary mode of wireless communication also required an intermediate streaming device that was worn around the neck, contained its own larger battery, and acted as an intermediate between the hearing aid NFMI signal and Bluetooth connections available from mobile phones or other systems.
Integration of RF Wireless
The next stage of wireless development ushered in the retirement of neck-worn streamers in favor of integrating the RF wireless systems directly into the hearing aid and cochlear implant. In most cases, these integrated RF wireless systems operate in the 2.4 GHz international, scientific, and medical (ISM) frequency band. Note that 2.4 GHz is the same frequency band used by the Bluetooth protocol; however, there are many devices that transmit wireless information at 2.4 GHz, but do not use the Bluetooth protocol.
As examples, most WiFi routers transmit at 2.4 GHz, some wireless mice and keyboards use proprietary 2.4 GHz protocols, and many modern hearing aids and cochlear implants use proprietary 2.4 GHz protocols to transmit low-latency, low-power audio from the developer’s TV streamers and remote microphones. This wide variety of wireless communication and avoidance of interference across the 2.4 GHz band is managed by the signal-processing chipsets in each of the connected devices.
Bringing on Bluetooth
The integration of RF wireless systems into hearing aids and cochlear implants was enabled by the advancement of the Bluetooth protocol, developments on the part of Apple and Google, and the associated forward march of the necessary hardware. Today, the Bluetooth standard is a collection of different protocols, with each having a specific purpose.
The most used Bluetooth protocol is called Bluetooth Classic, which is responsible for managing most of the Bluetooth connections within our smartphones, computers, and cars. Bluetooth Classic allows for high-quality stereo audio transmission and secure high-bandwidth data transmission. Bluetooth Classic has proven to be a reliable and effective method of wireless communication for many of the devices we use every day. In the early 2010s, the realities of available technology made it nearly impossible to consider implementing Bluetooth Classic at the ear level, especially when 2010 wireless hardware would have consumed the power of a zinc-air battery in one to two hours.
During these same years, Bluetooth low energy (BLE) was introduced, allowing for wireless devices to transmit data (not audio) in an intentional and efficient manner that was not previously possible. Millions of activity-monitoring devices such as the FitBit and early Apple Watches, leveraged BLE to transmit activity and biometric data back to a smartphone.
Conceptually, BLE is like a water faucet: When data transmission is needed, you can open and close the faucet to the extent that allows for the data flow required. The cost is more or less power consumption with more or less data flow. While this was a solution for streaming data to and from hearing aids and opened the door for wireless programming, it was not a solution for streaming audio directly to hearing aids and cochlear implants.
After several years of development, the first made-for-iPhone hearing aids were introduced during 2013 and 2014. By this time, wireless hearing aids were well-established with ear-to-ear signal processing, remote programming, and neck-worn streamers for Bluetooth connectivity. The novelty of made-for-iPhone hearing aids was Apple’s Low-Energy Audio (LEA) protocol, a new approach to low-power audio streaming that made it possible to stream audio directly from an iPhone to a hearing aid while maintaining several days of zinc-air battery life. This was a noteworthy step forward in terms of usability for patients, and it planted the seeds of innovation that would grow with a nearly pervasive connection between hearing devices of all kinds and the internet (via a smartphone).
Google has since introduced the audio streaming for hearing aids (ASHA) low-power audio protocol to compete with Apple’s low-power audio protocol and is specific to smartphones running newer versions of the Android operating system. Both low-power audio streaming protocols are implemented in parallel to BLE, which remains the mechanism for wireless data transfer to and from the hearing devices (e.g., wireless programming or remote control from a smartphone application).
While not as flexible as Bluetooth Classic, the introduction of smartphone-specific low-power audio protocols provides many patients with access to directly streamed audio. However, a technical challenge remains, in that maintaining a wireless connection across the head at 2.4 GHz presents a substantial technical challenge, even today.
Through lessons in acoustics and psychoacoustics, we learn that low-frequency sounds have a longer wavelength than high-frequency sounds and that low-frequency sounds travel more efficiently through air than high-frequency sounds. These concepts extend to wireless signals as well, with lower frequency wireless signals having a longer wavelength that travels through and around objects more easily.
The NFMI systems in hearing devices transmit at lower frequencies that pass easily around the head and between a bilateral device pair. In contrast, signals transmitted at 2.4 GHz have a wavelength of four inches, which causes them to be impeded and dampened by the head.
Maintaining a data connection across the head becomes a delicate balance of power consumption versus the amount of data transmitted. For this reason, hearing aids and cochlear implants were developed to include both NFMI and RF wireless systems, with the NFMI system managing ear-to-ear communication and the RF system managing streaming of direct audio and data from other devices. The result is a layered package of electronics that leverages each system for its efficiency.
In the most complex implementation, a single hearing device also may include a telecoil for a total of three independent wireless systems—and a fourth when considering a cochlear implant headpiece that powers the cochlear implant electrode through a wireless inductive link.
When broken into the individual wireless communication protocols, hearing devices may include the following:
- An audio input for signals received via telecoil
- A proprietary method for ear-to-ear communication
- A proprietary protocol for low-latency direct audio streaming from accessory devices
- Apple’s made-for-iPhone protocol for direct audio streaming, Google’s ASHA protocol, or Bluetooth Classic
- Bluetooth low energy
Inroads are being made to simplify this technical complexity. At the time of this publication, hearing aids have been introduced that are developed on the first 2.4 GHz wireless hardware that is efficient enough to maintain a robust ear-to-ear connection, eliminating the need for NFMI hardware.
This trend will very likely continue across all the hearing device developers. Similarly, the need for numerous wireless protocols will be simplified through the implementation of new Bluetooth standards that eventually offer the convenient universal compatibility of Bluetooth Classic.
Today’s hearing aids and cochlear implants share similar wireless designs and features as those described here. The lifecycle of a cochlear implant sound processor will always be longer than that of a hearing aid, which means that eventual gaps in design and features should be expected. To ensure that these gaps in technological harmony are as brief as possible, close relationships among the companies developing tomorrow’s hearing aids and cochlear implants will ensure that the visions are closely aligned, both in terms of technical capabilities and audiological benefits.