Gus Mueller, PhD
Ruth Bentler, PhD
University of Iowa
Iowa City, Iowa
How Hearing Instruments Work
While the process of hearing aid technology is complex and computer programming is required to make some of the adjustments, the basic components that make them work have not changed. The basic function of a hearing instrument is as follows:
- Sound waves enter through the microphone, which converts acoustic signals into electrical signals.
- The amplifier increases the strength of the electrical signal.
- From the amplifier, the signal is then transformed back to an acoustic signal by the receiver (a miniature loud speaker).
- From the receiver the signal is channeled in to the ear canal, either through a small tube or through an ear mold.
- A battery is required to power the hearing instrument and enable the amplification process.
Many hearing instruments also have user controls (e.g. toggle switch, volume control wheel, push button, or remote control) that enables the wearer to adjust to a variety of hearing aid parameters, including:
- Turning the hearing instrument “on” or “off”
- Changing the volume
- Switching to the telecoil
- Switching between omni- and directional-microphone settings
- Switching to a different pre-programmed memory
Styles of Hearing Instruments
Hearing aid technology is available in four styles:
- Behind-the-ear (BTE)
- In-the-ear (ITE) (Included in the category of ITE hearing instruments are in-the-canal (ITC) and completely-in-the-canal (CIC) styles)
Body & Eyeglass Style Hearing Instruments
While body and eyeglass style hearing instruments were regularly used 40-50 years ago, they comprise only about one percent of all hearing instruments marketed today. Instead, most individuals choose ITE (approximately 80%) or BTE (approximately 20%) style hearing instruments. This transition in style, use, and preference is occurring for a number of reasons, including the reduction in the size of the components, durability, and cosmetic concerns on the part of the consumer.
The ITE style hearing instrument fits directly into the external ear. The circuitry is housed primarily in the concha (external) portion of the ear. Due to the miniaturization of the component parts (including the microphone, receiver, and battery), it is possible to make hearing instruments small enough to fill only a portion of the concha (ITC) or fit deeply into the ear canal (CIC). All three of these styles have typically been considered to be more modern and cosmetically appealing.
Modern BTE hearing instruments have become smaller and at times are less noticeable than some ITC hearing instruments.
The BTE style hearing instrument is housed in a small curved case which fits behind the ear and is attached to a custom earpiece molded to the shape of your outer ear. Some BTE models do not use a custom earpiece, instead the rubber tubing is inserted directly into the ear. The case is typically flesh colored, but can be obtained in many colors and/or patterns. Other features include:
- BTEs may be the most appropriate choice for young children, as only the earmold needs to be replaced periodically as the child grows and the ear changes in dimension.
- Typically, BTEs are the most powerful form of hearing aid technology available, and may be the best option for persons with severe-to-profound hearing loss.
- FM and direct auditory input is routinely available as an optional or standard feature.
- Telecoil circuitry is often more powerful than with ITEs.
- Non-occluding earmold may be used with BTE hearing aids, if a medical condition exits or if the patient reports a “plugged” sensation when wearing other hearing aids styles.
- Directional microphone technology available with most BTE styles and models.
- Larger battery sizes used in BTEs may be easier to handle than smaller styles for those with limited manual dexterity or vision deficits.
Guide to Modern Hearing Aid Technology
In addition to the basic features of hearing instruments discussed earlier, there are many other features available in hearing aid technology – some of them are for convenience and ease of use, others are designed to improve speech understanding or listening comfort.
Adaptive Feedback Cancellation
Acoustic feedback (whistling from the hearing instrument) can be annoying, embarrassing, and in some cases, prevent the hearing instrument wearer from using the correct amount of gain. Many of today's hearing instruments have an automatic feature that quickly detects acoustic feedback and cancels it. This feature is designed to manage transitory feedback, (e.g. caused by placing one’s hand or a telephone next to the ear), and is not a solution to a poorly fitted earmold or hearing instrument.
Automatic Gain Control – Output (AGCo)
AGCo, or output compression, is used to put a “ceiling” on loud sounds. It handles the output after the amplifiers, and can be adjusted to correspond to the patient’s threshold of discomfort (maintaining sounds below this level).
Automatic Gain Control – Input (AGCi)
AGCi, or input compression, often referred to as wide dynamic range compression (WDRC) is used to “repackage” the speech signal (and other incoming sounds) to correspond to the reduced dynamic range of the hearing instrument user. That is, if the incoming sounds have a 60 dB range, and the patient only has a 30 dB range of useful hearing, the sounds might be “compressed” by 2:1 to fit into a useful auditory region. The notion is that most people with a hearing loss need more gain for soft sounds than for average, and more gain for average sounds than for loud.
Digital Noise Reduction
With digital hearing instruments, it is possible for the hearing instrument to analyze an incoming signal and differentiate speech from a broad-band noise signal. This can be accomplished simultaneously in several channels. If the dominant signal is believed to be noise in a gain channel, there is a reduction in the gain. Note, however, that what a typical hearing instrument user might consider to be “noise,” (background talkers at a party) might not be considered noise by the hearing instrument. While this feature has the potential to improve speech understanding in typical difficult listening situations, this has yet to be verified by research.
Digital Signal Processing
Until recently, the majority of hearing instruments utilized analog signal processing. This has changed rapidly over the past few years, and today, nearly all hearing instruments sold in the U.S. utilize digital signal processing. The advantage of digital signal processing is that less space is required, allowing manufacturers to include many more “programmable features” in a small package. Through the use of digital signal processing, the hearing instrument can conduct an analysis of an incoming signal, and make a reasonable classification of the content – speech versus broad-band noise versus acoustic feedback (whistling) versus music, for example. This classification can then be used to trigger automatic activation of other special features.
Directional Microphone Technology
Using special microphones or phase cancellation signal processing, it is possible to configure a hearing instrument so that sounds from the side, and especially the back of the hearing instrument user are amplified less than sounds originating from the front. It can serve as a type of "spatial" noise reduction if the user is correctly positioned. Directional technology is available on all hearing instrument styles except CICs (because of size constraints).
Hearing aid styles: A=BTE; B=ITE; C=ITC; D=CIC
Importantly, directional technology does not improve localization of sounds. Research has shown that many hearing instrument users prefer directional technology for listening in noise, usually when:
- The noise originated from behind the listener
- The talker is in front of the listener
- The listener is close to the talker
- The room has low reverberation
Some forms of hearing aid technology automatically switch to a directional mode when the signal type and/or input intensity are matched to the characteristics of the algorithm. Adaptive directional hearing instruments automatically track a dominant single noise source (e.g. a car passing by someone on a sidewalk), attempted to provide maximum reduction in gain toward the location of the source.
The majority of today's hearing instruments have multiple channels. Each channel represents a portion of the frequency range important for understanding speech. One advantage of multiple channels is that features such as gain and compression can be programmed differently to reflect changes in the patient's hearing across frequencies. Multiple channels also are useful for implementing other features such as digital noise reduction and feedback cancellation. There is no consensus regarding how many channels are enough (or how many are too many) - to some extent, this depends on the feature utilized within the channels.
A memory is a location to store hearing instrument settings that are designed for a particular listening situation. It is common for hearing instruments to have two or three memories. For example, in a hearing instrument with three memories, it is common that memory two will be used for listening in noise, and memory three will be used for telephone. On the other hand, many hearing instrument users find that single memory works in a variety of listening situations, and may only use one memory. Changing memories is accomplished by using a button (or toggle switch) on the hearing instrument, with a remote control device. In some digital hearing instruments, it happens automatically.
With the special circuit, electromagnetic signals can be picked up from the handset of the telephone and amplified in a manner similar to the amplifying function of the hearing instrument. Although many hearing instrument wearers report benefits with this circuit, there is a substantial variability across hearing instruments. Telecoils are not available in some smaller custom-made models due to space limitations. Often, hearing instruments with multiple memories will devote one memory to the telecoil. In these instruments, the telecoil can be accessed through a push button on the hearing instrument or by the use of a remote control device.