Hearing Device Technology : Not Your Parent's Hearing Aids

Performance and Style…sounds like something you would hear in a car commercial. But today’s hearing devices are not your grandpa’s hearing devices. As you’ll see in the history of hearing aid technology listed below, backpacks and trumpets are no longer required as accessories to hear better.

The history of amplified sound is divided into 5 eras: the Acoustic Era, Carbon Era, Vacuum tube era, Transistor and Integrated Circuit era and the Digital era.
Acoustic Era:

• Began the first time someone cupped a hand to an ear.

• This produces about 5-10 dB of gain in the mid and high frequencies.

• It also shields the ear from sounds at the periphery, acting as a very effective noise reduction system for mid and high frequency sounds.

• Ear trumpets/horns or funnels were the next evolutionary step (1650 and 1673):

• A large open end draws in as much sound as possible.

• Had to be wide and long so that most of the collected sound energy was delivered to the ear and not reflected outwards.

• The speaking tube had the same purpose, but the speaker spoke into the horned end, providing a good amount of amplification and improved signal to noise ratio.

Carbon Era:

• First battery powered hearing aid.

• Consisted of a carbon microphone, a battery or 3-6 volts and a magnetic receiver.

• Was a ‘simple’ microphone, amplifier and speaker capable of 20 to 30 dB increase relative to the input from the microphone.

• The first carbon hearing aid appeared in 1899 which was a large table model.

• The first wearable model called the Akoulallion was invented in 1902.

• Continued to be worn through the 1940’s, but only for mild to moderate hearing losses.

• During the carbon era, was born the idea of amplifying different frequencies by different amounts.

Vacuum Tube Era:

• The vacuum tube electronic amplifier was first invented in 1907 and applied to hearing aids in the 1920's.

• By combining vacuums, a very powerful hearing aid could be made; 70 dB of gain and 130 dB SPL output.

• The overall size of the vacuum hearing aid was the biggest drawback. They required batteries housed separately from the microphone and amplifier.

• Later designs miniaturized them so the battery design was one piece, and microphones were concealed in pins/broaches, and receiver cords enclosed in strings of pearls.

Transistor and Integrated Circuit Era:

• Transistors were first commercially available in 1953.

• Main benefit was reduced battery requirements, and all new hearing aids were developed by 1953.

• The biggest impact was  reduced size.

• A hearing aid could now be worn at ear level. The first ones were head mounted barrette’s and eyeglass style hearing aids.

• Soon the stand alone behind-the-ear (BTE) was introduced

• In-the-ear (ITE’s) – mid to late 1950’s.

• 1960’s, two large leaps in performance were witnessed. In 1964, the first integrated circuit was applied to hearing aids so for the first time multiple transistors and resistors could be combined onto a single component. The second, in 1968, saw the introduction of the piezoelectric microphone, which was combined with a new type of resistor (field effect transistor) or FET.

• By the late 1980’s, in-the-canal (ITC’s) were introduced.

• The early 1990’s completely-in-the-canal (CIC’s) were unveiled.

• These are commonly referred to as conventional or analogue, non-programmable hearing aids.

• Advances from their predecessors include:

• Zinc-air batteries allowed a halving of battery volume for the same electrical capacity.

• Improved filtering for better frequency shaping.

• Miniature potentiometers to allow for some control over the sound characteristics.

• Class D amplifiers.

• Wireless transmission hearing aids (ALD’s).

• Improved understanding of earmolds/shell acoustics.

• Directional selectable hearing aids.

• The electronic screwdriver (digital programmable, analogue sound processing).

Digital Era:

• Began  in the 1960’s at Bell Labs, but limited due to slow computer processing speeds.

• Refined in the 1980’s.

• First commercially available hearing aids in 1996 (digi-focus).

• Huge advantages in areas of signal processing, flexibility in fitting frequency response curves, improved feedback systems, noise reduction, directional microphone performance, wireless technology, automatic gain control, telephone coupling.