High energy efficiency is a critical design criterion for the electronic hearing aid placed in or around the ear to improve the hearing of those with hearing loss. Using battery power, the hearing aid must provide the appropriate amount of sound reproduction and information processing, with as little power consumption as possible. There are four main types of hearing aids. All must enhance hearing while minimizing power consumption:
- Behind the ear (BTE)
- In the ear (ITE)
- In the canal (ITC)
- Completely in the canal (CIC)
Some of the hearing aids include an ear mold, a plastic shape fitted into the ear. The BTE style sits behind the ear with a clear tube going to an ear mold in the ear that delivers the sound. The ITE style moves the hearing aid into the outer ear, where it becomes a single unit with an ear mold containing the electronic circuits. The ITC style moves the hearing aid electronics into the ear mold that fits in the ear canal. The CIC style is the smallest of them all, as its electronics fits completely inside the ear canal, thus nearly disappearing from view. Regardless of the type of hearing aid, virtually all use similar electronics.
The hearing aid that I am familiar with is the Starkey (Eden Prairie, MN) S Series, an ITC model. What is impressive is the extent to which the company designed the hearing aid for low power consumption. In the S Series, the battery voltage is applied directly to the hearing aid circuit — there is no internal regulated power supply. That means that all the electronic circuits in the hearing aid must operate efficiently with zinc-air batteries that start at 1.4V and can be used down to about 1.0V before requiring replacement. When battery voltage drops below a predetermined minimum value this hearing aid sends a voice message telling the user to replace the batteries. This hearing aid's power consumption is about 1 mA. With a new battery, the power consumption is less than 1.4 mW. Using a type 312 zinc-air battery eight hours a day, its useful life can range from a week to 10 days.
An important feature of the zinc-air batteries is its long shelf life, which can be several months. These batteries are small and light. They have a tag on them that is removed for use, allowing air to activate the battery. The battery is easily replaceable because it swings out as shown in Fig. 1. There is no power switch, it turns off when the battery swings out.
The Series S is a digital hearing aid that can be programmed during the fitting process with multiple patient-selectable listening profiles. The digitization of sound allows more advanced signal processing, such as noise reduction, filtering, and acoustic feedback (ringing) control. Compared to analog types, the majority of hearing aids sold today are digital because of better performance and flexibility.
Fig. 2 is a block diagram of the Series S digital hearing aid. This circuit employs three ICs: DSP, EEPROM, and RF (Radio). The three ICs are stacked one above the other to minimize space requirements. An ITC hearing aid has the electronics embedded within the earmold.
The audio-processing path has two microphones, preamplifiers and an audio codec that drives a power amplifier to produce sound output in the receiver (speaker). Receivers and microphones are typically supplied by Knowles and Sonion. Receivers can produce 115 to 140 dB sound pressure level (SPL). Typical microphone sensitivity is -34 dB relative to 1 V/Pascal (transfer factor). Typical audio frequency response is 100 — 8,000 Hz.
The power amplifier is a Class D type that exhibits low-power operation, low distortion, and small size compared with a Class A or B amplifier. The combination of the DSP and the audio codec provide the PWM (pulse width modulation) drive for the power amplifier. The PWM frequency is about 250 kHz. Shown in Fig. 3 is the Class D amplifier circuit that resembles a synchronous rectifier used in a switch-mode power supply.
Despite its circuit complexity, a class D amplifier offers the benefits of:
- Reduced size and weight
- Reduced power consumption and heat dissipation and hence smaller (or no) heat sinks
- Lower cost due to smaller (or no) heat sink and compact circuitry
High power conversion efficiency, usually above 90% at one-quarter of the amplifier's maximum power, and around 50% at low power levels.
The internal assembly of the hearing aid electronics. It fits in the earmold, which is approximately 2 × 1.5 × 1.5 mm, as shown in Fig. 4.
The digital signal processor (DSP) supports the functional benefits of a digital hearing aid. Usually, this includes multiple channels of compression and bands of amplification along with options such as special algorithms to enhance speech-in-noise, compress loud sounds, automatic feedback (whistling) cancellation and de-emphasis of low- level ambient noises. These features make wearing hearing aids much more comfortable to wear and hear in a wider range of environments. Also, automatic volume control works very well with this type of hearing instrument.
Using its wireless capability the audiologist can program this hearing aid while it is in the wearer's ear. Because of this, it can be finely tuned to personal likes and lifestyle. The DSP chip can be reprogrammed or changed, so this circuit is very adaptable to many hearing losses. Also, since hearing may change over time, the “prescription” in this hearing aid may also be readjusted. This easy adjustability allows better hearing over a longer period of time than lower classes of technology.
FEEDBACK CANCELLATION ALGORITHMS
Some hearing aids employ feedback cancellation (FBC) algorithms that address several of the most important improvements sought by hearing aid users — better sound quality, less whistling and buzzing, more audibility for soft sounds, better operation on the telephone, improved speech understanding in quiet, and better fit and comfort. In some cases, the annoyance associated with feedback from the hearing aids may be sufficient to negate the perceived benefits of amplification, resulting in the non-use of hearing aids by the wearer. The most obvious conclusion is that the whistling and buzzing associated with feedback must be minimized.
All hearing aids must pass IEC 61000-4-2 electrostatic discharge (ESD) requirements. Using electronics with built-in protection or adding ESD line protectors to exposed traces can help meet these safety requirements.