Audiology, Hearing Aids

Treatment and testing for hearing disorders can be traced back as far as the first century BC. However, nothing prior to the sixteenth century indicates that any consistent use or application of hearing assistance devices existed.

The fundamental function of early hearing aids was to amplify sound. Before electricity, the only way to do that was to filter out other noise by directing the desired sound straight into the ear with some kind of tube or trumpet. Ear trumpets were first used by sailors and others who needed to communicate over long distances, and later on were adopted for use by the hearing impaired.

Most early trumpets were custom made, and the real business of manufacturing and selling hearing aids only began around 1800. By the end of the nineteenth century a large variety of ear tubes and trumpets were available in many styles and designs, ranging from cheap devices made of tin or hard rubber to the more expensive ones constructed of more valuable materials.

The more expensive models were often treated like jewellery but did not necessarily work better. Cupping the hand behind the ear makes sounds 5 to 10 decibels (dB) louder; and depending on their size and shape, ear trumpets could amplify by about 10 to 20 dB, with most of this in the range of 500 to 1000 Hertz (Hz). As the range of human speech is 300 to 3000 Hz, ear trumpets could only help people with mild hearing impairments.

Auricles and cornets were developed as an alternative to the ear trumpet. It was hoped that the device would be less observable on the user. Other hearing assistance devices used through the early part of the 1800s were bone conduction aids. Sounds are transmitted to the ear by vibrations in the air, but also by vibration of the bones in the skull.

Bone conduction devices had been tested since the sixteenth century and were typically designed in two ways. The first design consisted of an apparatus held to the speaker’s mouth or throat while the opposite end was held in the listener’s teeth. The primary constraint of this model was the restriction on speaking distance and the number of persons involved in conversation.

Allowing a greater distance from the speaker, the second design involved an instrument that collected sound energy by means of a flat transmitting surface from the surrounding air, something comparable to an acoustic fan. Inventors and physicians however were not satisfied with these devices, and by the end of the 1800s efforts were being made to produce a hearing aid more powerful and effective than the ear trumpet or bone conduction aids.

Electrical hearing aids were introduced just after 1900 and brought useful amplification to a wider audience. It is uncertain who invented the first electric hearing aid, but it may have been Miller Reese Hutchinson in 1898. This first electric hearing aid came to market in 1901 and used the transmitting potential of carbon in order to amplify the sound.

There existed, however, substantial static noise and distortion caused by the carbon hearing aid. Such aids were also very large and impractical, but their effectiveness in amplifying sound surpassed any of the prior devices used. They offered the same amplification as ear trumpets, but covered a wider frequency range of 500 to 1800 Hz. All such devices consisted of the carbon microphone (technology borrowed from the telephone), processing unit, battery box and headpiece.

Batteries often did not last more than a few hours and were very expensive. Later models with multiple microphones provided 25 to 30 dB of amplification, and the introduction of amplifiers in the 1920s increased the range to 45 to 50 dB.

The development of the vacuum tube hearing aids in the 1920s offered a device with an even greater amount of power and reduced slightly the size of the processing components. Wearable multipart hearing aids were then developed and used in the 1930s and 1940s. The only drawback with these aids was that batteries remained large and still far from invisible.

A separate battery pack was needed to warm the vacuum tubes. In addition, most of the earlier large-size batteries did not last more than a day and had to be carried in special cases. This problem was resolved in 1947 with the invention of the transistor. As transistors got smaller, so did hearing aids, and concealment became an achievable and important goal.

By the late 1950s and early 1960s, models that were ‘‘at-the-ear’’ or ‘‘over-the-ear’’ combined a microphone with a battery and transistor in one unit and could amplify sounds within the range 400 to 4,000 Hz. These models were molded with custom-made ear tubes, and were easy to conceal behind the ear or under the hair. In the 1970s, batteries became even smaller, allowing ‘‘in-the- canal’’ aids to fill the ear canal without anything worn outside the ear. By the late 1980s, advanced circuitry and lithium batteries made possible ‘‘in-the-ear-canal’’ units that could be concealed completely in the ear canal.

Since the 1990s, most manufacturers have produced four basic styles of hearing aids:

Behind-the-Ear (BTE). The components are held in a case worn behind the ear and connected to a plastic earmold that fits inside the outer ear. Sound travels through the earmold into the ear. BTE aids are used by people of all ages for mild to profound hearing loss. Poorly fitting BTE earmolds may cause feedback, a whistle sound caused by the fit of the hearing aid or by the build up of earwax or fluid.

However, BTE aids can be as sophisticated as smaller hearing aids. In fact, they can hold more circuitry and amplify sounds to a greater degree than in-the-ear types. BTE aids can be more durable than other types and a few are even waterproof.

In-the-Ear (ITE). These devices house components in a custom- formed earmold that fits within the outer portion of the ear. ITE aids can accommodate added technical mechanisms such as a telecoil, a small magnetic coil contained in the hearing aid that improves sound transmission during telephone calls. ITE aids are used for mild to severe hearing loss but can be damaged by earwax and ear drainage.

Their small size can cause adjustment problems and feedback. Usually, children do not wear them because the casings need to be replaced as the ear grows. However, its size and easy-to-use controls may be helpful for those with limited manual dexterity.

Canal Aids. These fit into the ear canal and are available in two sizes. In-the-canal (ITC) hearing aids are smaller still, with an earmold that fits down into the ear canal, and a smaller portion facing out into the outer ear. They are discreet, yet still visible within the outer ear. The ITC hearing aid can also be customized to fit the size and shape of the ear canal and is used for mild or moderately severe hearing loss.

The newest generation of such hearing aids is those that fit completely in the canal (CIC). A CIC hearing aid is largely concealed in the ear canal and is used for mild to moderately severe hearing loss. In general, because of their small size, canal aids may be difficult for the user to adjust and remove, and they may not be able to hold additional devices such as a telecoil. Canal aids can also be damaged by earwax and ear drainage. They are not typically recommended for children.

Body Aids. Body aids are used by people with profound hearing loss. The aid is attached to a belt or a pocket and connected to the ear by a wire. Because of its large size, it can incorporate many signal processing options, but it is usually used only when other types of aids cannot be used.

Mechanisms of Action. The inside mechanisms of hearing aids vary among the different devices, even if they are of the same style. In general, every hearing aid is a miniature electronic circuitry encased in plastic. Every hearing aid consists of a microphone that picks up sound, an amplifier that boosts the sound, and a receiver that delivers the amplified sound into the ear.

All the parts are powered by replaceable batteries. However, due to microprocessor or computer chip technology, late twentieth century hearing aids far surpass the simplicity of this description. All hearing aid styles described above use three basic types of circuitry:

1. An analog hearing aid works much the same way as traditional high-fidelity audio systems. Sound is picked up by the microphone and is then converted to electrical signals. Once sound is turned from acoustic to electrical signal, it is fed into the amplifier of the hearing aid. The sound is then amplified overall and sent to the receiver of the hearing aid, and finally to the user’s ear. This type of sound processing has now been used over many years.

The biggest drawback of analog processing is that the amplified sound is over the full frequency range of hearing, so low frequency (background noise) would ‘‘mask’’ high frequency (speech) sounds. To alleviate this problem, ‘‘potentiometers,’’ which provide the ability to reduce or enhance the sounds needed by the end user, have been introduced to hearing aids to restore hearing to as ‘‘normal’’ a sound as possible. Analog circuitry is generally the least expensive.

2. Programmable hearing aids offer different (often customizable for the individual’s audiogram) settings for different listening situations such as the office or home, or a noisy rock concert. Programmable devices enable the audiologist to fine tune the hearing aid using a computer. Potentiometers are built onto the circuit within the programmable hearing aid, rather than sitting externally on the hearing aid.

The circuitry of analog and programmable hearing aids accommodates more than one setting. If the aid is equipped with a remote control device, the user can change the program to accommodate a given listening environment.

3. Hearing instruments incorporating digital signal processing (DSP) are widely known as digital hearing aids. The real difference between analog and digital hearing instruments is the ability of the DSP instruments to process more complex signal processing. Sound is still received into the DSP instrument by microphone and converted into ‘‘bits’’ of data. The circuitry within the ‘‘digital’’ hearing aid now acts like a very tiny computer.

The computer can sample the data and far more accurately fine tune to each individual’s requirements. These hearing aids are wearable minicomputers capable of monitoring the audiological environment. Digital hearing instruments compensate for different listening situations in a more flexible, accurate, and complex way than any analog circuit. Digital hearing aids use a microphone, receiver, battery, and computer chip. Digital circuitry can be used in all types of hearing aids and is typically the most expensive.

Implantable Hearing Aids. The latest twentieth century development in hearing aids is the implantable type. This type of hearing aid is placed under the skin surgically. Regarded as an extension of conventional hearing aid technology, an implantable hearing device is any electronic device completely or partially implanted to improve hearing. This allows for the hearing aid to be worn at or in the ear and still provide the user with a signal quality equal or superior to that of prior hearing aids.

Included in the category of implantable hearing aids is the cochlear implant, a prosthetic replacement for the inner ear or cochlea. In its most basic form, the cochlear implant is a transducer, which changes acoustic signals into electrical signals in order to stimulate the auditory nerve. The device is surgically implanted in the skull behind the ear, and it electronically stimulates the auditory nerve with small wires touching the cochlea.

External parts of the device include a microphone, a speech processor (for converting sounds into electrical impulses), connecting cables, and a battery. Unlike a hearing aid, which just makes sounds louder, this device selects information in the speech signal and then produces a pattern of electrical pulses in the user’s ear. It is impossible, however, to make sounds completely natural because a limited amount of electrodes are replacing the function of tens of thousands of hair cells in a normal hearing ear.

Hearing aid technology advanced greatly in the last few years of the twentieth century, thanks to the computer microchip and to digital circuitry. The majority of efforts in the research and further development of hearing aid technology is concentrated in the area of implantable hearing devices.