The technology and circuitry used in hearing aids vary according to the type of the device.
Conventional analogue devices are the least expensive variety. They are designed and adjusted individually by manufacturers. They essentially amplify all sounds (speech and noise) in the same way and are appropriate for many different types of hearing loss. Certain analogue devices can be individually programmed for different listening environments, such as quiet conversation at home, noisy situations like restaurants or large areas like theatres.
Digital devices, first introduced widely in the late 1990s/early 2000s, represent a major breakthrough in hearing aid technology and are increasingly widely adopted. They are more expensive than their analogue equivalents, but are usually self-adjusting, thanks to their use of DSP (digital sound processing), in which sound waves are converted into digital signals. They offer many benefits including improvement in programmability, greater precision in fitting, management of the discomfort that results from high sound volumes and their ability to reduce noise.
TC standardization work on hearing aids
IEC TC 29 work on hearing aids represents one of the group’s most significant activities. It has prepared 13 International Standards for these devices in the IEC 60118 series so far. They cover measurement of electroacoustical and performance characteristics, EMC (Electromagnetic compatibility) and many other technical (sound and physical) characteristics.
The basic safety of hearing aids, not specifically addressed as such until recently, became the object of an International Standards in October 2012.
Safety of medical electrical equipment
ME (Medical electrical) equipment and systems must meet specific safety requirements to ensure patients and staff can use and operate them safely and reliably. Safety requirements for these are covered by IEC 60601-1, Medical electrical equipment – Part 1: General requirements for safety and essential performance.
TC 29 developed IEC 60601-2-66: Particular requirements for the basic safety and essential performance of hearing instruments and hearing instrument systems, which supplements IEC 60601-1 and was published in October 2010.
A hearing instrument is defined as including "all detachable parts that are essential for the performance of its intended use"
This Standard does not apply to cochlear and other implanted hearing instruments or to some other devices, such as induction-loop systems and instruments using infra-red or radio.
IEC 60601-2-66 is a so-called particular (i.e. specific) Standard.
In the IEC 60601 series, particular Standards may modify, replace or delete requirements contained in the general Standard and collateral Standards as appropriate for the particular piece of ME equipment under consideration. They may add also other basic safety and essential performance obligations which take priority over those covered by the general Standard.
Many subclauses of the general Standard are not applicable to hearing aids – for instance they do not present electrical, fire or heat hazards owing to their low voltage and low energy characteristics.
However, protection against mechanical hazards is given special attention in the particular Standard and devices are submitted to drop tests. The Standard states that hearing instruments "shall have mechanical strength and shall not result in an unacceptable risk due to moulding stress or when subjected to mechanical stress caused by pushing, impact, dropping, and rough handling".
As a hearing instrument is frequently installed entirely or partially in ear canals "rough surfaces, sharp corners and edges that could result in an unacceptable risk shall be avoided or covered". A hearing instrument that can be worn in the ear canal "shall be safely retrievable by the patient" and it shall be designed in a way that parts do not come loose during use.
The Standard also sets out particular requirements that apply to detachable parts of devices intended for use by infants under 36 months.
As hearing instruments are usually quite small they rarely carry clearly visible graphical symbols, therefore the Standard gives specific guidelines regarding accompanying instructions, and these "must be clear and include warning and safety notices".
Ensuring safer devices
Since they operate at low voltages and are not connected to the mains, hearing instruments present, on the whole, significantly lower risks and hazards than do many other pieces of ME equipment and systems. However, as they are generally installed in ear canals, certain aspects of their design require to be given special attention. IEC 60601-2-66 ensures these are comprehensively addressed, so guaranteeing that these already intrinsically safe devices are made even safer for the millions of users who rely on them.