Hearing aids have been around since the 17th century with the first use of an ear trumpet for those who were partially deaf. It wasn’t until the invention of the telephone and microphone that the creation of the modern day hearing aid began.
Large and bulky, these first devices were external. However, with the success of vacuum-tube hearing aids in the 1930s, hearing aids began to shrink, and as technology began to expand from transistor hearing aids to digital, the size continued to get smaller.
Physically, hearing aids come in different form factors. Those placed In-The-Ear canal are aptly called ITEs, whereas BTEs (Behind The Ear) are obviously worn behind the ear. BTE devices contain the receiver in the housing and have a hollow tube into the ear canal. Although BTEs are quite robust and simple to maintain, they are not as popular nowadays as their receiver-in-the-canal (RIC), aka receiver-in-the-ear (RITE) counterpart. RIC devices are also placed behind the ear, but are smaller and less visible. Their receiver is placed not in the housing but as the name implies in the ear canal, connected with tiny electrical wiring.
ITE shells (also known as “custom instruments” or “custom hearing aids”) are custom-shaped, using an ear impression taken by a hearing care professional. For over 12 years now, custom instruments have been manufactured using highly-advanced 3-D printing processes capable of mass production. While acrylic is the most common material used in 3-D printing, recently custom shells have been printed using titanium. Titanium allows for higher robustness with even thinner walls, thus enabling even smaller devices fitting into tighter ear canals.
Regardless of whether the hearing instrument is placed behind the ear or in the canal, all hearing instruments contain the same basic components: a battery, antenna, a microphone and amplifier, a receiver/loudspeaker, and an electronics package. Optional user interface (UI) elements, like pushbuttons, volume control switches, or rocker buttons, are sometimes added to the exterior to increase ease of use.
Let’s examine some of these components in more detail.
A disposable Zinc-Air battery is still utilized to power most hearing devices while enabling the amplification process. In 2016, rechargeable hearing aids based on Li-Ion technology appeared on the market, providing at least one day (a full 24 hours) of operating time including audio streaming through the antenna.
With continued advancements in hearing aid features, lithium-ion batteries are able to better support this new technology with enough power capacity for a full day of hearing in one charge. Also, lithium-ion batteries charge faster and last longer than its older counterpart, making them a very popular choice.
In hearing instruments, the antenna serves as an additional source for audio input. Traditional hearing aids utilize the 10.6 MHz bandwidth for near field magnetic induction (NFMI) communication. NFMI facilitates short range exchange of data and wireless binaural audio streaming to a pair of hearing aids or to remote control or streamer device worn around the neck or on the lapel with virtually no loss of fidelity. While serving various functions, the antenna often takes the place of the microphone for external input with very low battery consumption. In the latest hearing aids, a Bluetooth antenna is embedded within the micro-processing chip.
Microphone & Amplifier
In its simplest form, a hearing instrument can be viewed as an input/output device. Microphones —whether directional or omnidirectional — capture external audio signals (input) as acoustic sound waves. The strength of these signals is increased via the amplifier and then processed via a hybrid circuit utilizing analog to digital conversion (ADC). This enables digital sound processing (DSP). Once processed, the data is sent to the receiver for output.
The receiver converts the now digitally-processed signal back to an acoustic signal via a highly-sophisticated balanced armature driver. This helps create sound. In RIC devices, the receiver is attached to the end of a slim connecting tube, delivering audio into the ear canal via a dome or custom earmold in cases where higher levels of amplification is required.
The electronics package usually consists of three chips. An SOC-ASIC with ADC, DAC, DSP, uP, RAM/ROM and other functionalities is assisted by a non-volatile-memory. Hearing aids usually only use EEPROM and no FLASH memory as the latter draws far too high peak currents for the battery.
The third chip is a radio, connecting the device to the hearing aid on the other side of the head as well as the outside world. Wireless audio transmission using proprietary protocols, tailored for low power consumption, were first implemented 10 years ago.
Bluetooth chips like the Sonova Wireless One Radio Digital (SWORD) chip will become more and more common. These 2.4 GHz radio chips will be able to support audio streaming with music, television and car programs. Featuring 2.47 Mbit RAM and 5.83 Mbit ROM, it contains 42 million transistors on less than 6 sq. mm and can support several 2.4 GHz protocols including Bluetooth Classic, Bluetooth LE and various proprietary protocols.
The entire electronics package, including external components like caps, is about the size of two grains of rice: 1.7 x 4.9 x 2.9 mm3. All the signal processing, including the current needed to produce sound levels of 130dB SPL and more, draws a mere 1.5mA, thanks to a host of current-saving measures.
Not obvious, but equally important, is the protection of all components from environmental conditions: sweat, cerumen (ear wax), sun lotion, etc. can be highly corrosive. As hearing aids are worn all day, they have to last for numerous years, making reliability and customer satisfaction big factors during development. Tight production tolerances of the shells, grids, mechanical labyrinths, 2K sealings, as well as nanocoatings applied to the critical zones result in an IP67 rating or higher. Soft touch lacquering and other manufacturing processes provide an interleaved network of measures to protect the delicate components.