Hearing Aids

First, some history!

The first electrical hearing aids used a carbon microphone, which was used to convert variations in acoustic pressure into changes in the electrical resistance of some granules of carbon located between two electrodes. In other words, the carbon granules were deformed under acoustic pressure and produced an electric current. By adding a battery and a headset, this system allowed an acoustic signal to be amplified in certain frequency regions.

It was Graham Bell who once again provided technical innovation. He was a teacher in a school for the hearing impaired and had married one of his students. He eventually invented the telephone whilst trying to find a solution to compensate for his wife's hearing loss. He failed in that particular endeavor, but at least the telephone was a success! Researchers of the time realised that there would be no futher progress without improvements in both electronics and the analog transformations of the current that they produced.

The invention of valve (or 'tube') amplifiers in the 1930s marked the beginning of a new period. Small variations in tension arising from a microphone could control the flow of electron across a lamp in a vacuum. By connecting a number of tubes together in series, the signal could be amplified by up to 70 dB! There were obvious drawbacks relating to the size of the system and its need for power. Tube amplifiers require two power sources: one to heat the filament and a direct high voltage source of around 30 Volts.

It wasn't until the beginning of the 1950s that transistors appeared in the first hearing aids. Because of this improvement, only a single, low voltage source (1.3V) was needed. As technology improves, the size of hearing aid components continues to decrease and signal processing becomes more and more sophisticated, offering increasingly effective devices.

This figure shows that there have been numerous technological eras, which have been spread quite evenly over time. There is always a delay between the arrival of these technical innovations and their use in clinical applications, mainly due to the specific requirements of hearing aids, which don't allow the use of standard components. Electronic engineers have to address all of the following issues: device miniaturisation, the need for the device to operate under low voltage (1.3V), managing background noise and creating specific functions (today these are the algorithms of digital devices). Figure: Cornet acoustique --> Ear Horn Appareils electriques --> Electrical devices Tubes Electroniques --> Valve amplifiers Transistors --> Transistors Circuit integres analogiques --> Analog integrated circuits Microprocesseurs --> Microprocessors AA Numeriques --> Digital Hearing Aids

Better Hearing...

Which reflex do we instictively use to hear better? We cup our ear with our hand! This has three functions:

• Focussing more sound energy into the ear canal
• Improving the signal to noise ratio by reducing the noise coming from behind us.
• Causing the person that we are speaking with to speak clearly (louder, slower, with greater articulation and concentration).

This principle also enables the resonance of the outer ear to change to favour certain frequencies. A hearing aid needs to emulate these characteristics.

Air conduction hearing aids

Currently, there are two main families of air conduction hearing aids: Behind the ear ('BTE') and in the ear ('ITE'). These families can be subdivided depending on their specifics (shape and technology).

A hearing aid mainly fulfils the three following functions:

• Conversion of sound into electrical signals
• Processing these electrical signals
• Reconversion of these electrical signals into sounds

A page dedicated to these functions can be found later in the subchapter: signal processing.

Behind the ear hearing aids

Historically, this is the most classic shape. All of the required components for signal processing are contained within a case that is worn behind the listener's pinna. This case is linked via the loudspeaker to a plastic tube which transmits the treated sound to an earmould that is custom-fitted to the  listener's ear canal. These devices can benefit from numerous microphones and controls on the top of the case (such as volume control, switches, etc...). In general, the greater the device's power, the larger it is (larger speakers require larger batteries!).

A variant of the BTE uses a thin acoustic tube and standard earpieces which replace the custom-fitted earmould. These are used mainly when the hearing impairment is not severe.

More compact models appeared in the 2000s. By placing the speaker inside the ear canal, the size of the external case can be reduced. These models, known as receiver in the canal (RIC) or receiver in the ear (RITE) hearing aids, reduce the likelihood of feedback because the microphone and the speaker are separated. These models can therefore be used to provide a greater sound pressure close to the eardrum.

In the ear hearing aids

These are the smallest hearing aids. All of their electrical components are housed within a resin case which is made specifically to fit the shape of the ear canal and pinna of the listener. Due to their small size, they are not appropriate for all patients: Good eyesight and dexterity are recommended so that they can be inserted without difficulty. They also use the smallest batteries available, which need to be changed after 5 to 6 days of use. Although they only have a single microphone, these devices can use the natural advantages provided by the subject's pinna (directionality, resonance, etc...).

ITE hearing aids exist in three types:

• Complete in the canal (CIC)
• In the canal (ITC), a little larger than the CIC, the external part containing the microphone and the battery is located at the edge of the tragus.
• Standard in the ear (ITE) aids spread further out into the pinna. Although less esthetically pleasing that CICs, they have larger batteries and can contain more components (such as induction coils).