At its basic level all a hearing aid has to do is to pick up sound, amplify it and deliver the end result to the wearer, however this is not as simple as it seems. An individual with a hearing impairment is not interested in having all sounds amplified, all that would achieve is to make everything noisy but would not increase the clarity of the spoken word.
The main objective of a hearing aid is to increase the Signal to Noise ratio 'SNR' (signal – that which is regarded as important e.g. speech and noise – competing sounds e.g. background sounds), so that the signal is not drowned out by the noise for example when in a restaurant.
The modern hearing aid has gone through a series of revolutionary developments designed to provide demonstrable improvements in the listening experience.
Schematic of a digital hearing aid's major components:-
A digital amplifier takes an analogue signal from the microphone and converts this to a numerical representation (binary code) via an analogue to digital converter (ADC). Any amplification, filtering and compression (smoothing) characteristics are applied/modified by arithmetic formulae via the Digital Signal Processor (DSP). The modified digital data is converted back into an acoustical signal via the digital to analogue converter (DAC) and then to the output amplifier and finally the receiver/speaker.
The characteristics that differentiate the various devices available fall into 2 main categories:-
The firmware is arguably where the main differentiation between hearing aid types takes place as it is the DSP that allows manufacturers to develop devices with enhanced processing capabilities and features.
The DSP allows for a number of processing strategies designed to provide the optimum benefit to a hearing aid wearer in a wide spectrum of listening environments. These strategies can either be fixed or adaptive where processing is automatically modified according to changes in pre-defined parameters.
Digital developments allow a number of analogue features and digital only attributes to be combined into a hearing aid rather than presented as a choice between aids and also offers the possibility of improvements not available with older analogue circuitry.
The DSP is one of the main aspects of development cited by the various manufacturers for improving the efficacy of hearing aids. Much of the hardware such as microphones and receivers are common across manufacturers but the circuitry and algorithms within the DSP are very much proprietary and account for a great deal of the research and development (R&D) currently being undertaken.
A more detailed explanation of the main strategies employed in hearing aids can be found in the following links:-