Hearing is something that people take for granted everyday. Until, we actually

lose it, we do not understand just how often we use hearing for simple tasks like

communicating with others, listening to music, and comprehending our environment.

Due to the advent of new technology in consumer goods such as iPods and surround

sound music players, researchers fully expect that each consequent generation to have

worse hearing than the previous. Therefore, it is important to understand one of the most

basic devices to help alleviate hearing problems, the hearing aid.

Resound Hearing aids are devices that make

listening to sounds easier for hard-of-hearing people.

The first type of a hearing aid was nothing like the one

seen today; it was a large metal funnel that would

amplify the sound. This type of device was typically

known as an “ear trumpet” or “ear horn”. It is difficult

to say who first invented this type of hearing aid since

similar aids made out of more natural materials (shells,

Figure 1. Common Hearing aid animal horns, etc) were probably made long before the

ear horn. Another type of hearing aid that was used is a desk

with a built-in amplifier. A microphone and earphones could

be plugged into the desk. While these hearing aids worked

better than passive ear trumpets, they are very inconvenient

compared to the small, portable electronic devices that fit

into the ear which we are familiar with today.

The first electrical hearing aid dates to some time

between the late 19th

century and early 20th

century. In 1895, Dr. Miller Reese

Hutchinson, an engineer, was working with a deaf friend whom he gave telephone-like

earpiece device that he manufactured. Dr. Hutchinson spoke a few words into this device

and, for the first time, his deaf friend was able to hear. After four years Hutchinson gave

the first public demonstration of this device which he called Acousticon. By 1901, a

company, Acouphone, began manufacturing and selling Acousticons.

Over the years, as new technologies developed, hearing aids became more

portable and clear. Hearing aids not only featured vacuum tubes, but they were the first

commercial devices to use a transistor. Transistors had been developed in Bell labs,

which at the time was part of AT&T. The company Sonotone initially put this type of

hearing aid onto the market for $229.50. Within a short period, competition appeared.

Maico offered another version, which had three transistors, and Acousticon was able to

pull the price down even further to $74.40 with a single transistor model.

Now, hearing aids have become advanced enough that some use digital signal

processing with microchips and are a fraction of the size of the earlier ones. Currently

the top manufacturer is GN Resound Hearing Aids. Hearing aids currently are priced

anywhere between $500 and $6000. Prices vary depending on the style of hearing aid

and its features. The programmable digital hearing aids are typically more expensive

than other hearing aids.

Before a hearing aid can be made, the patient must undergo a hearing test to

create an audiogram. Also during this screen stage, a mold of the customer’s ear is taken.

To prevent damage to the ear when the mold is being created (see Figure 3 below), an

autoblock is added inside of the canal prior to the mold filling. Once the audiogram is

Figure 2. Older Hearing Aids

completed, its data is entered into a computer to determine the optimal operating range

for the hearing aid; in other words, the level of amplification. This ensures that the level

of amplification will not be unsafe or uncomfortable for the consumer. Usually the

computer will also recommend the type of circuit to be used. An order card is then

created and sent to the production line with the ear impression.

Next, a reverse impression is taken of

the mold from before by placing a vinyl sheet

and then applying heat. This impression is then

removed and trimmed. After which, it is

dipped in wax to create a smooth film on the

outside. Next, a technician will taken the

impression and place it on a metal plate. Now

the technician can place rings around it so that

a colloid can filled into the rings and immerse

the impression. The liquid is cooled until it

reaches a silicon-like texture, and the

Figure 3. Mold of ear impression is pulled out of the gel.

Then, a shell is created out of this gel mold. First, this mold is heated to about

110 degrees Fahrenheit, and air is blown through to remove the impurities. A liquid

acrylic is prepared with the proper coloring to match the skin pigment. This liquid is then

poured into the gel mold and after ten seconds, and the excess liquid is removed. After

cooling, this process leaves a thin acrylic shell inside of the colloid. With ten minutes of

cooling, the shell is pulled out, and a perfect replica of the ear canal is produced. In order

to safely harden the acrylic, an ultraviolet light is used for hardening. Excess flanges and

stubbles are grinded off the shell.

With the shell ready, the technician now has to create a vent. Holes are drilled to

allow a silicon wire run through, and it is drawn out to act as a receiver tube. With the

holes in place, the outside is shined and buffed to smooth finish. The shell will be used to

size for a faceplate using the same vacuum mold from before and the plate’s angle is

adjusted to complement the user’s ears.

Due to the small size of the circuitry required, the electronics are typically created

on printed circuit board. The board has a ceramic base with layers of conductive and

insulating layers. The conductive layer is usually made up of gold and silver; whereas,

the insulating layers is made up of silicon-based compounds. The various electronic

components have to be bonded by hand to the silver and gold parts using gold wire of

about 0.001 inch thickness. When finished, the components are sealed with an epoxy

paste.

The technician takes the electronics and assembles it on the faceplate where holes

are located for mounting the microphone, amplifier, battery compartment, and volume

control. All of this has to be hand wired to ensure flexibility with simple soldering.

After mounting the equipment, the receiver is placed onto the shell, and the hearing check

is made.

There are many models of Resound hearing aids available. By far, the most

common hearing aid is the behind the ear (BTE) hearing aid (see Figure 1 for picture).

These are typically in a small plastic case that fit behind the ears and can be used for mild

to severe hearing loss. These are particularly useful for children because of their

durability and ability to connect to assistive learning devices. The colors of this range

from basic skin tones to bright colors and elaborate designs for children.

Another kind of Resound hearing aid is the inside the ear (ITE) aids. These

devices fit into the outer ear bowl and have to be custom fit to do so. These can be used

for mild to severe hearing loss. Often ITEs are not recommended for children because

their fit is not easily modifiable like the BTE. So, as children grow, ITEs become costly

to replace. However, newer, silicon-based ITEs though have greatly reduced the price,

making them much more affordable to replace.

Figure 4. Various Hearing Aids In the canal (ITC) aids, like ITE,

are fitted directly into the ear and have to

be custom made. These are intended for

people with mild to moderate hearing loss.

ITCs are not recommended for people with

good low frequency hearing due to a

resonance effect.

Many other but lesser known types

of hearing aids are available including

Open fit hearing aids. These use clear

tubes are often quite discreet compared to

the previously mentioned models. Eyeglass

aids integrate both eyeglasses and hearing

aids. This style was popular in the 70s but

soon lost favor due to the inflexibility of

the models. Style was hindered by the

limited number of frame styles and the consumer had to wear both the hearing aid and

glasses at once (never one or the other). Also, the hearing aid was only on one side of the

glasses. Bone anchored hearing aids (BAHA) are surgically implanted and use bones as a

pathway for sound to travel to the inner ear. This allows the external auditory canal and

middle ear to be completely bypassed. This is done by surgically implanting a small

titanium rod with a sound processor into the skull with a small piece external to the skin.

Sound is processed on the external portion of the rod and sound is transmitted by

vibrations to the inner ear. However, BAHA are the most expensive type of hearing aid.

Recently, a Dutch company, Varibel, created a model with a four-microphone system that

would allow the user to discern the direction of sounds much easier.

The future in Resound hearing aid technology will be focused on reducing the

size of the hearing aids even further and improving signal processing. Cosmetics are a

huge selling point of hearing aids, as potential buyers want the most discrete models

available. Also, since many of hearing aid consumers are of older age, less manual

control for the product is desired. Resound hearing aids in the process of are being

designed and programmed so that they can automatically switch modes depending on the

situation. Situations can range from outside in the park to a noisy pub. Doing so would

require less manual