A Brief History of Hearing Aids
Hearing aids have been used to compensate for hearing loss for a long time.
Electronic hearing aids were developed after the invention of the telephone by Alexander Graham Bell in 1876. However, until the 1950s their use was limited due to their large size, heavy weight, and limited efficiency. After transistor hearing aids were introduced in the early 1950s, the use of hearing aids increased rapidly because smaller transistor 13 size and lower battery voltage requirements permitted dramatic miniaturization compared with earlier devices. According to American Hearing Aid Association data, in 1953 there were an estimated 225,000 hearing aids sold in the U.S. Of these, 100,000 were all transistor, 75,000 were hybrid, and 50,000 were vacuum-tube models. One year later, in 1954, an additional 335,000 hearing aids were sold, 325,000 of them all-transistor models (Bernard Becker Medical Library, 2005).
Before the mid-1980s, hearings aids did little more than amplify sound. These analog systems used a microphone to turn incoming sound waves into an electrical current to be amplified. A speaker then transformed the electrical current into sound waves in the ear canal. In 1996, the first digital hearing aid emerged. Instead of turning sound waves into electrical currents, they are converted into binary signals, processed by a speaker, and transformed into sound waves. The quality of sound is better in digital systems. In 1999, GN Danavox, Inc. of Minnetonka, Minnesota launched the first hearing aid consisting of a miniature computer with both hardware and software (Bernard Becker Medical Library, 2005).
Types of Modern Hearing Aids
There are four basic styles of hearing aids (NIDCD, 2005):
- In-the-Ear (ITE) hearing aids fit in the outer ear and are used for mild to severe hearing loss. The case that holds the components is made of hard plastic. These are suitable for adults; for children, the casing would need to be replaced as the ear grew.
- Canal Aids fit into the ear canal and are available in two sizes. The In-the-Canal (ITC) hearing aid is customized to fit the size and shape of the ear canal and is used for mild or moderately severe hearing loss. A Completely-in-Canal (CIC) hearing aid is largely concealed in the ear canal and is used for mild to moderately severe hearing loss. They are also recommended for adults, but not children.
- Behind-the-Ear (BTE) hearing aids are worn behind the ear and are connected to a plastic ear mold that fits inside the outer ear. BTE aids are used by people of all ages for mild to profound hearing loss. Poorly-fitting BTE ear molds may cause “feedback;” i.e., a whistle sound caused by the misfit of the hearing aid or by buildup of earwax or fluid.
- 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 their large size, they are able to incorporate many signal processing options but are used primarily when other types of aids are not effective.
Two of the above models – the ITC and CIC – represent 80% of the hearing aid market in North America, with BTE making up most of the other 20%. More recently, implantable hearing devices have been developed to overcome the inconvenience associated with use of external aids. These target people with sensory neural hearing loss who cannot be considered for cochlear implantation and otherwise would be assigned a traditional hearing aid.
Three types of sound processing are currently used (NIDCD, 2005):
- Analog/Adjustable: The audioprosthologist determines the volume and other specifications needed in an individual’s hearing aid, and a laboratory builds the aid to meet those specifications. The audioprosthologist retains some flexibility to make adjustments. This type of circuitry is generally the least expensive (below $1,000).
- Analog/Programmable: The audioprosthologist uses a computer to program the hearing aid. The circuitry of analog/programmable hearing aids will accommodate more than one program or setting. If the aid is equipped with a remote control device, the wearer can change the program to accommodate a given listening environment. Analog/ programmable circuitry can be used in all types of hearing aids.
- Digital/Programmable: Instead of converting sound waves into an electric current to be amplified, digital aids transform them into binary numbers that enable dramatically heightened auditory quality and allow people to hear better in noisy environments (by use of binary filters). The audioprosthologist programs the hearing aid with a computer and can adjust the sound quality and response time for an individual. Digital hearing aids use a microphone, receiver, battery, and computer chip. Digital circuitry provides the most flexibility for the audioprosthologist to make adjustments in the hearing aid. Digital circuitry can be used in all types of hearing aids and is typically the most expensive (approximately $2,000-3,000).
There are three commonly used hearing aid circuits:
- The Linear Peak Clipper (PC)
- The Compression Limiter (CL)
- The Wide Dynamic Range Compressor (WDRC)
A linear circuit is simply an amplifier; i.e., it makes everything louder. This circuit is the least sophisticated and can’t filter background noise. A compression circuit also amplifies everything, but with less distortion because of the way it processes sound.
This circuit is also better able to accommodate a patient’s range of hearing. Basically, the circuit makes the sound loud enough for the patient to hear, but never gets louder than a patient’s comfort level.