How to measure hearing loss

The ear is a complex sensory organ, responsible for both hearing and balance. It is divided into three parts – the outer, middle and inner ear. When sound waves enter the inner ear, hair cells respond by vibrating. This in turn moves the fluid that is contained in the cochlea, a snail-shaped organ, which sets 25 000 nerve endings into motion. These nerve endings send out electrical impulses which travel along the auditory nerve to the brain where they are interpreted.

There are several tests to measure hearing loss or absence of hearing, and some are more intrusive than others. The one which provokes the least disturbance is the otoacoustic emission (OAE) test. Through vibration in the inner ear, a gentle sound is produced that echoes back through the middle ear and into the outer ear. This sound is measured by the ear specialist through a probe covered with soft foam that is placed in the ear.

As IEC TC 29 expert Erik Nielsen describes, “The most important reason for performing otoacoustic emission testing is that it allows the hearing of a new-born child to be tested, even though the child cannot actively participate in the test. This method enables children to be evaluated within the first day or two after being born. Undetected and untreated hearing problems can affect speech and social development. The otoacoustic emission test can also be used on adults that are physically or mentally prevented from participating actively in a standard hearing test.”

The new edition of IEC 60645-6 defines minimum mandatory functions and characteristics for otoacoustic emission instruments to ensure that measurements, made under comparable test conditions with different instruments specified in the publication, will be consistent. In other words, says Nielsen, “We must ensure we get the same result, no matter which equipment is used, as long as the equipment complies with the standard. To achieve this goal, it is important to clearly define the technical features of such equipment, as well as the required accuracy of these features.”

Compared with the initial edition, which according to Nielsen “did a good job for several years”, the new standard defines new stimulus levels and frequency ranges to meet evolving requirements in the field. “Some sentences in the previous edition became ambiguous over time and needed to be clarified,” Nielsen adds.

The standard is targeted at many stakeholders, including companies specifying equipment to be used in hospitals and clinics, manufacturers of otoacoustic emission equipment and test or homologation laboratories that need to verify the performance of the equipment.

A redline version of the standard is available in English and provides users with a quick and easy way to compare all the changes between the official IEC Standard and its previous edition.

What’s in the future?

Several trends are likely to impact the evolution of this standard as well as many others produced by TC 29. According to Nielsen, technology advances are one of the most important ones: “Improvements in speed and accuracy of measurement will most certainly result of technology advances in the field. I also envisage that future otoacoustic emission equipment will be able to adapt the stimulus signals and the signal analysis algorithms to the acoustic properties of the individual patient’s ear, thus reducing some of the uncertainties that still exist today.”

One of the other more general developments is the increasing use of wearables, some of which will have specific medical purposes, notably for auditive requirements. “The population with hearing difficulties is much more used to wearable gadgets than the previous generation, and that is set to continue. People have become increasingly tech-savvy and have been using technical and smart equipment for a long time,” Nielsen explains.

Preparing for this tech-savvy future is part of the work of IEC TC 29, which is constantly adapting to new requirements to meet the demands of various markets, including an ageing worldwide population.